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Strength is Specific By Chris Beardsley version 2.0 (draft)

Strength and Conditioning Research

CONTENTS CONTENTS.............................................................................................................................. 2 1. WHY ARE STRENGTH GAINS SPECIFIC?.................................................................. 3 2. MUSCLE ACTION............................................................................................................ 18 3. VELOCITY.........................................................................................................................48 4. RANGE OF MOTION....................................................................................................... 83 5. EXTERNAL LOAD TYPE................................................................................................99 6. LOAD.................................................................................................................................119 7. STABILITY...................................................................................................................... 146 8. FORCE VECTOR............................................................................................................ 171 9. FINAL WORD.................................................................................................................. 184

DRAFT Strength is Specific v2.0, Copyright Strength and Conditioning Research Limited, 2017

Strength and Conditioning Research

1. WHY ARE STRENGTH GAINS SPECIFIC?

DRAFT Strength is Specific v2.0, Copyright Strength and Conditioning Research Limited, 2017

Getting strong is really, really important. In fact, strength is probably the single most important thing for many athletes. But strength is very hard to measure with a single test. Some people score very highly on one test of strength, but less highly on another similar test, even when it involves the same muscle groups. Similarly, training for one particular strength test (such as a 1RM back squat) will make you stronger at the test exercise, but not necessarily increase your force production in another exercise, or during a sporting movement. This is because strength is specific.

How is strength specific? Strength gains are greater when tested under the same conditions as performed in training. There are 8 ways in which strength is specific: 1. Muscle action (eccentric or concentric) 2. Velocity (fast or slow) 3. Repetition range (maximum strength or muscular endurance) 4. Range of motion (full or partial) 5. Degree of stability (stable or unstable) 6. External load type (constant load or accommodating resistance) 7. Force vector (vertical or horizontal) 8. Muscle group Maximizing the effectiveness of a strength training program means designing it to fit the specific goal you want to achieve.

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How much difference does specificity make? The impact of specificity on strength training is not a small detail. It is huge. Compare the effects of training with eccentric muscle actions, compared to training with concentric muscle actions. Using elbow flexion exercise, Vikne et al. (2006) showed that gains in eccentric 1RM after eccentric training are more than twice as large as those after concentric training.

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Or compare the effects of training with fast or slow velocities. Both high and low velocity training can produce gains in high-velocity and low-velocity strength, but the gains are velocity-specific (Coyle et al. 1981).

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Or compare the effects of training with heavy or light loads. Both heavy and light loads can increase muscular strength, but the gains in maximum strength are almost always much greater when using heavy loads. Similarly, the gains in repetition strength (muscular endurance) are usually much greater when using lighter loads (Schoenfeld et al. 2015).

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Or compare the effects of training with partial or full ranges of motion. When partials do improve full range of motion strength, it is almost never as much as full range of motion training. On the other hand, they usually produce larger gains in partial range of motion strength (Rhea et al. 2016).

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Or compare the effects of training under different stability conditions. An easy way to explore the effects of stability is to look at strength gains after training on machines that use fixed bar paths or on machines that use cables to allow freedom of movement (Cacchio et al. 2008). Strength gains are totally different between the two types of machine.

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Or compare the effects of training with different external load types. Pneumatic resistance provides a constant external force, while free weights involve a force that is greatest at the start of the exercise and smallest at the end, because of inertia. Training with pneumatic resistance leads to greater gains in pneumatic resistance strength, than in free weight strength, while training with free weights leads to greater gains in free weight strength, than in pneumatic resistance strength (Frost et al. 2016).

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Why are strength gains specific (part 1)? People get confused about the relationship between muscle size and strength, and this leads them to make extreme statements in one direction or another. Some people think that muscle size and strength have little connection with each other. They believe that it is possible to increase strength substantially and over a long-term period of time without large gains in muscle size. Other people argue that strength is almost entirely a function of size, and that it is impossible to increase strength without hypertrophy.

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In fact, the truth is halfway between these extremes. Although muscle size is the single most important predictor of strength, other factors also have an impact. This is apparent even in cross-sectional studies (Trezise et al. 2016). The remainder of the difference is probably accounted for by many different peripheral and central factors (not just neural ones, like you might read in an old textbook), including: 1. muscle fascicle length 2. muscle pennation angle 3. moment arm length 4. single fiber contractile function 5. lateral force transmission 6. neural drive to the agonists 7. coactivation of antagonists 8. coordination And these factors are also what makes strength gains specific.

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Why are strength gains specific (part 2)? The specific nature of strength gains is produced by lots of small peripheral and central adaptations that interact with each other. Across the fitness industry, you will find that most people simply refer to vague “neural adaptations” whenever specificity or the Specific Adaptation to Imposed Demand (SAID) is raised. The reality is much more complex. Specific gains in strength can occur through many different mechanisms, some of which are neural and some of which are due to changes within the muscle itself. Let’s take a look at the mechanisms underlying the 5 most important ways in which strength is specific.

#1. Muscle action Strength gains are specific to the muscle action (contraction mode) you use. Eccentric training produces greater gains in eccentric strength than in concentric strength. Although many people will argue back and forth about the hypertrophic potential of eccentric training, the specificity effect is not caused by differences in the amount of hypertrophy produced. In fact, it probably happens partly because of increased extracellular matrix and titin content, which increase passive force production, and partly because of neural adaptations that are specific to lengthening muscle actions.

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#2. Velocity Strength gains are specific to the velocity that you use. High-velocity (light load) training produces greater gains in strength at high speeds than at low speeds. Velocity-specificity is confusing, because some very influential research suggested that “intent” was the main factor driving velocity-specific strength gains, and not actual bar speed. However, this is probably not true, and both “intent” and actual speed likely contribute to velocity-specificity. Velocity-specificity probably happens for many reasons, including greater increases in muscle fascicle length, larger increases in single fiber velocity, greater increases in early phase neural drive, more suppressed co-activation, and bigger improvements in coordination, compared to low-velocity (heavy load) training.

#3. Range of motion Strength gains are specific to the range of motion you use. Partial range of motion exercises produce greater gains in strength at partial ranges of motion than at full ranges of motion. Partial range of motion exercises probably improve strength at short muscle lengths because of joint-angle specific increases in neural drive. In contrast, full range of motion exercises likely improve strength at long muscle lengths because of specific gains in regional hypertrophy, which may be related to greater increases in muscle fascicle length.

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#4. Load Strength gains are specific to the load that you use. Heavy loads lead to greater gains in maximum strength, while lighter loads (performed to failure) lead to greater gains in repetition strength (muscular endurance). Greater gains in maximum strength with heavy loads likely occur because of more relevant increases in inter-muscular coordination in multi-joint exercises, greater changes in single fiber contractile properties, greater increases in lateral force transmission, and larger increases in neural drive. Greater gains in repetition strength likely occur with lighter loads because of larger improvements in capillarization, and changes buffering capacity, and in the rate of ion (Na+, K+, Ca²+) transport.

#5. Stability Strength gains are specific to the amount of stability you use. Free weight strength training leads to greater strength gains on free weight exercises than on machine exercises. Stability exists on a continuum with machines at one end, and lifting free weights while balancing on a stability ball at the other end. Lifting free weights while standing on the ground or lying on a bench sits somewhere in the middle. Strength gains are specific to the type of stability used in training. This is because the need to balance in any less-than-perfectly stable environment affects the co-ordination patterns of muscles in multi-joint exercises, increasing both synergist and antagonist activation. Training in an unstable environment leads to reduced antagonist activation and increased synergist activation, as the more complex nature of the movement is learned. These changes leads to a more efficient pattern of muscular contractions for those exact conditions of stability. And this increases strength in a stability-specific way.

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What about the other ways in which strength is specific? Strength is specific in at least 8 ways. However, the other 3 ways are specific mostly because of the way in which they produce changes in the big 5 ways. External load type (constant load or accommodating resistance) produces specific strength gains because of changes in joint angle-specific (range of motion-specific) strength and velocity-specific strength. Force vector (vertical or horizontal) also produces specific strength gains because of changes in joint angle-specific (range of motion-specific) strength, and because it tends to develop certain muscle groups more than others. Muscle group specific strength gains arise where an exercise trains a muscle group in a non-specific way, which then transfer to completely different joint actions involving the same muscles. A good example of this might be training the hamstrings in knee flexor exercises (such as Nordic hamstring curls or lying leg curls), which then transfers to hip extension strength.

What does this mean in practice? Getting strong for sport means analyzing the requirements of a sporting movement, and figuring out how force is produced in terms of muscle action (eccentric or concentric), speed (high or low velocity), range of motion (point of peak contraction), load (maximum or repetition strength), and stability (stable or less stable). Matching the features of sporting movement with the goals of your strength training program will lead to optimal sport-specific strength gains. Many coaches have already figured this out. The importance of eccentric strength for sprinting and change of direction is why eccentric training with flywheels is suddenly so popular. The success of velocity-based training points to the superior transfer of high-velocity strength gains to sporting movement. And partial squats are having a comeback for developing sprinting and jumping abilities. It is only a matter of time before the rest of the industry catches up.

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References 1.

Bloomquist, K., Langberg, H., Karlsen, S., Madsgaard, S., Boesen, M., & Raastad, T. (2013). Effect of range of motion in heavy load squatting on muscle and tendon adaptations. European Journal of Applied Physiology, 113(8), 2133-2142.

2.

Cacchio, A., Don, R., Ranavolo, A., Guerra, E., McCaw, S. T., Procaccianti, R., & Santilli, V. (2008). Effects of 8-week strength training with two models of chest press machines on muscular activity pattern and strength. Journal of Electromyography and Kinesiology, 18(4), 618.

3.

Coyle, E. F., Feiring, D. C., Rotkis, T. C., Cote, R. W., Roby, F. B., Lee, W., & Wilmore, J. H. (1981). Specificity of power improvements through slow and fast isokinetic training. Journal of Applied Physiology, 51(6), 1437-1442.

4.

Frost, D. M., Bronson, S., Cronin, J. B., & Newton, R. U. (2016). Changes in Maximal Strength, Velocity, and Power After 8 Weeks of Training With Pneumatic or Free Weight Resistance. The Journal of Strength & Conditioning Research, 30(4), 934-944.

5.

Hartmann, H., Wirth, K., Klusemann, M., Dalic, J., Matuschek, C., & Schmidtbleicher, D. (2012). Influence of squatting depth on jumping performance. Journal of Strength & Conditioning Research, 26(12), 3243.

6.

Rhea, M. R., Kenn, J. G., Peterson, M. D., Massey, D., Simão, R., Marin, P. J. & Krein, D. (2016). Joint-Angle Specific Strength Adaptations Influence Improvements in Power in Highly Trained Athletes. Human Movement, 17(1), 43-49.

7.

Schoenfeld, B. J., Peterson, M. D., Ogborn, D., Contreras, B., & Sonmez, G. T. (2015). Effects of low-vs. high-load resistance training on muscle strength and hypertrophy in well-trained men. The Journal of Strength & Conditioning Research, 29(10), 2954-2963.

8.

Trezise, J., Collier, N., & Blazevich, A. J. (2016). Anatomical and neuromuscular variables strongly predict maximum knee extension torque in healthy men. European Journal of Applied Physiology, 116(6), 1159-1177.

9.

Vikne, H., Refsnes, P. E., Ekmark, M., Medbø, J. I., Gundersen, V., & Gundersen, K. (2006). Muscular performance after concentric and eccentric exercise in trained men. Medicine & Science in Sports & Exercise, 38(10), 1770-1781.

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2. MUSCLE ACTION

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Over the last decade, there has been a surge of interest in eccentric training. Eccentric training is strength training using only the lowering phase of an exercise. It is most commonly used for preventing or rehabilitating muscle strain injuries or tendinopathies. From a strength and conditioning point of view, eccentric training is also very interesting because the strength gains are eccentric-specific. In other words, strength gains after eccentric training are greater when measured in an eccentric test of strength, compared to in a concentric test of strength. But why does this happen?

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What do “eccentric” and “concentric” training mean? When you lower a weight in an exercise, it lengthens the prime mover muscles. This is the eccentric phase. When you lift a weight, it shortens the prime mover muscles. This is the concentric phase. Normal strength training involves both lowering and lifting a weight. This means lengthening and then shortening muscles under tension in sequence. This sequence is called the stretch-shortening cycle (SSC). In contrast, eccentric training involves just performing the lowering phase. A popular example of an exercise often used for eccentric training is the Nordic hamstring curl. In the Nordic hamstring curl, the lifter lowers themselves forwards to the ground by extending the knee, slowing their descent by trying to contract the hamstrings muscles. When they reach the bottom, the lifter places their hands on the ground, and returns to the top by flexing the hip and pushing backwards with the hands. This means that there is very little effort exerted in the concentric phase. This is why eccentric training is often called “eccentric-only training” because it involves very little loading in the concentric phase compared to the eccentric phase. Eccentric training differs from accentuated eccentric (also called eccentric overload) training, which involves working hard in both concentric and eccentric phases, but with an even greater load in the eccentric phase. As we will see shortly, eccentric training has some unique effects, and consequently many review articles have been published about it (e.g. Brughelli & Cronin, 2007; Roig et al. 2009; Butterfield, 2012; Isner-Horobeti et al. 2013; Herzog, 2014; Vogt & Hoppeler, 2014; Kjaer & Heinemeier, 2014; Gluchowski et al. 2015; Mike et a. 2015; Duchateau & Enoka, 2016; Douglas et al. 2016a; 2016b; Ruas et al. 2016). To understand these unique effects, it helps to recall that muscles are structured of both active and passive elements. Eccentric and concentric phases differ how they make use of both of these elements.

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How do eccentric and concentric phases differ? (passive elements) When performed from a standing start (and ignoring exactly how titin gets involved for a moment), concentric contractions can only make use of the active element within muscle fibers. In contrast, eccentric contractions can make use of both active and passive elements. This is because the passive elements resist the lengthening of muscle tissue that occurs in the eccentric phase. The passive elements comprise the cytoskeleton, which is the structure of the inside of the muscle fiber and includes titin, and the elements of the extracellular matrix (ECM). There are three main layers of ECM: •

the endomysium, which surrounds the muscle fiber

•

the perimysium, which surrounds the muscle fascicle

•

the epimysium, which surrounds the whole muscle

Although many parts of the ECM and cytoskeleton are involved in passive force production, there is evidence that titin is particularly important during active lengthening of muscles. Indeed, titin it is known to be activated by the presence of calcium ions (Labeit et al. 2003; Cornachione et al. 2016), and seems to be responsible for many of the unique behaviors of eccentric contractions, including residual force enhancement (Shalabi et al. 2016). Force production during muscle lengthening is driven only partly by chemical energy being converted into kinetic energy inside the sarcomeres. It is also partly supported by the passive elements, which require little or no chemical energy in order to function (Herzog, 2014). This makes the eccentric phase stronger (and more efficient) than the concentric phase.

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How do eccentric and concentric phases differ? (active elements) Eccentric and concentric phases also differ in how the active elements produce force. Shortening contractions are caused (almost) entirely by chemical energy being converted into kinetic energy within individual sarcomeres. Muscle fibers contain chains of sarcomeres in series with one another, arranged in long molecular structures called myofibrils. When calcium ions are released, this causes the sarcomeres to contract. The shortening of the individual sarcomeres in the chain causes the shortening of the myofibrils, and therefore of the muscle fiber itself. In the concentric phase, the thin (actin) and thick (myosin) myofilaments of the sarcomeres slide past one another. This is called a crossbridge cycle. It is the myosin myofilament that drives this process, by detaching from actin, releasing ADP and rebinding with ATP, and it then binds to actin again further along the myofilament (Månsson et al. 2015). In the eccentric phase, the myosin crossbridges of the active elements are forcibly broken, detaching the myosin head from actin. This occurs without the release of ADP and subsequent rebinding of ATP (Månsson et al. 2015). Force production from the active elements during muscle lengthening therefore requires less energy than muscle shortening. This also makes the eccentric phase more efficient than the concentric phase.

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Why is this important? (more force) Maximal force producing capability in lengthening contractions is greater than in shortening contractions, because they can make much better use of the passive elements in the eccentric phase than in the concentric phase. Force is greater by around 50 – 80% when measuring single fibers in vitro, and by around 25 – 50% when measuring strength in living humans (Kelly et al. 2015; Duchateau & Enoka, 2016). Maximal force is greater in lengthening contractions, because both active and passive elements can be made to contribute to their full extent at the same time, and the sum is much greater than just the active elements on their own.

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Why is this important? (more endurance) In addition to being able to produce more maximal force, lengthening contractions are more efficient, partly because of the involvement of passive elements and partly because of the different function of the active elements. These differences lead to greater muscular endurance, which is also called “repetition strength”. Interestingly, this superior repetition strength can be observed both in absolute terms (number of reps with the same weight) and in relative terms (number of reps with the same percentage of 1RM).

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For example, Kelly et al. (2015) found that untrained males performed 4.5 concentriconly reps with 90% of concentric 1RM, but 7.7 eccentric-only reps with 90% of eccentric 1RM. This shows that we are more efficient when using the eccentric phase, even when accounting for our greater strength. Importantly, this greater efficiency translates to lower muscle activation (measured by EMG), as well as a smaller metabolic cost in the eccentric phase compared to the concentric phase, when using the same external force (Bigland-Ritchie & Woods, 1976; Duchateau & Enoka, 2016).

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Does eccentric training display specific strength gains? Originally, Komi & Buskirk (1972) found that eccentric training improved eccentric strength more than concentric training did. In contrast, both eccentric and concentric training caused similar gains in concentric and isometric strength. Later studies confirmed that gains in eccentric strength are greater after eccentric training than after concentric training (Higbie et al. 1996; Hortobágyi et al. 1996; 2000; Miller et al. 2006; Vikne et al. 2006; Nickols-Richardson et al. 2007). And although not quite as consistently, concentric training can also produce greater gains in concentric strength, compared to eccentric training (Vikne et al. 2006).

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How can we measure eccentric-specific strength gains? Eccentric training almost always produces greater gains in eccentric strength than concentric training. And concentric training sometimes produces greater gains in concentric strength than eccentric training. However, eccentric training sometimes produces greater gains in strength across all measures (concentric, isometric, and eccentric). Eccentric training probably has a superior transfer effect because of the greater mechanical loading, which benefits maximal efforts of both concentric strength and eccentric strength. However, the difference in absolute absolute gains in strength across all measures between eccentric and concentric training makes it hard to see the specific nature of the strength gains. So the best way to analyze the specificity of the strength gains is to look at the ratios between eccentric and concentric strength, which almost always changes in the direction used during training. For example, after elbow flexion training in resistance-trained subjects, the ratio of eccentric to concentric 1RM decreased from 1.30 to 1.20 after concentric training, but increased from 1.21 to 1.33 after eccentric training (Vikne et al. 2006). But why does this ratio change?

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Why are strength gains after eccentric training specific? So why are the gains in eccentric strength after eccentric training so much larger than gains in other forms of strength? Although it has been extensively discussed whether eccentric training is superior to concentric training for hypertrophy (Roig et al. 2009), the answer probably lies in some of the other adaptations that occur after strength training, as hypertrophy should affect strength gains in all contraction modes in a relatively similar way. On the other hand, that does not mean that we can immediately identify neural adaptations as responsible (pace: Douglas et al. 2016a), as changes in peripheral features (especially in the passive elements) could just as easily be responsible for eccentric-specific behaviors. Indeed, there are many features of the musculoskeletal system that change differently after eccentric training compared to after concentric training: 1. Muscle architecture 2. Muscle fiber type 3. Regional hypertrophy 4. Extracellular matrix and cytoskeleton 5. Tendon stiffness 6. Neural adaptations Let’s take a look at each of these in turn.

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#1. Muscle architecture Strength training produces alterations in muscle architecture. Muscle architecture is measured in three ways: i.

muscle fascicle length

ii. pennation angle iii. physiological cross-sectional area Most of the research into the effects of eccentric training on muscle architecture have focused on the changes in muscle fascicle length.

#i. Muscle fascicle length Muscle fascicle length increases more after eccentric training than after concentric training (Ema et al. 2016), probably through a larger increase in the number of sarcomere in series within the myofibrils of a muscle fiber (Brughelli & Cronin, 2007; Butterfield, 2012). The increases in muscle fascicle seem to be partly dependent upon the amount of stretch during training, as some research indicates that training at longer muscle lengths leads to greater adaptations (Guex et al. 2016). However, they are also likely dependent on the mechanical load incurred by the prime mover, as knee flexion (hamstring only) exercise seems to lead to greater adaptations in the hamstrings than hip extension (hamstring, gluteus maximus, and adductor magnus) exercise, even when muscle length at peak contraction is shorter (Bourne et al. 2016). Such adaptations doubtless have advantages for high-speed movements, such as sprinting and jumping. Longer fascicle lengths likely allow superior contraction velocities, as all the sarcomeres in a myofibril contract at the same time. Indeed, longer fascicle lengths have been observed in faster high-level sprinters (Kumagai et al. 2000; Abe et al. 2001). Increasing muscle fascicle length through eccentric training could therefore be a valuable method for improving athletic performance in high-velocity movements, such as sprinting. Indeed, even though sprinting ability is notoriously difficult to improve (especially in well-trained athletes), eccentric leg curls have been shown to increase sprinting ability in elite soccer players (Askling et al. 2003).

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Longer fascicle lengths also seem to increase the joint angle for force production to longer muscle lengths (Brughelli & Cronin, 2007; Brughelli et al. 2010). Increasing muscle fascicle length through eccentric training could therefore be a valuable method for improving athletic performance in movements that have peak contractions at long muscle lengths, such as the terminal swing phase of sprinting, or the ground contact phase of sharp change of direction (COD) maneuvers. On the other hand, increasing muscle fascicle length through eccentric training seems to be a disadvantage for changes in rate of force development (RFD), probably because it causes a decrease in muscle stiffness (Kay et al. 2016). Essentially, the longer fibers require more time to go from slack to taut, at the onset of a muscle contraction (Blazevich et al. 2008; 2009). It seems likely that the increases in muscle fascicle length that happen as a result of eccentric training lead to greater increases in high-velocity strength, smaller increases in RFD, and greater increases in strength at long muscle lengths (by a shift in the optimum angle). So eccentric training could produce greater high-velocity strength gains, as well as superior gains in full range of motion strength. However, the effect of changing muscle fascicle length on eccentric-specific strength is less clear.

#ii. Pennation angle Muscle pennation angle seems to increase by more after concentric training, than after eccentric training (Ema et al. 2016; Franchi et al. 2016). Increases in muscle pennation angle seem to be mainly a way to accommodate increases in muscle size, by packing more muscle tissue into the same space (Fukunaga et al. 1996). And since the angle of force production becomes less advantageous with increasing pennation angle, this involves a trade-off between more muscle tissue and a smaller component of force. An emphasis on increasing muscle fascicle length rather than pennation angle may therefore be beneficial for both eccentric and concentric strength, in comparison with concentric training. However, the effect of changing pennation angle on eccentric-specific strength is less clear.

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#2. Muscle fiber type Muscle fiber type is probably one of the most misunderstood areas in strength and conditioning science. We say that muscle fiber type changes either when there is a shift in the isoforms observed in individual fibers (fiber type proportion), or when there is an change in the relative area occupied by each group of fibers. Strength training causes preferential hypertrophy in type II muscle fiber area in comparison with type I muscle fiber area. And almost all types of activity, including strength training and aerobic exercise, cause a shift in type IIX fiber type to IIA fiber type of individual fibers (Andersen & Aagaard, 2000; Farup et al. 2014). In comparison with concentric training, it has been proposed that eccentric training could cause either: i.

a preferential increase in type II muscle fiber area, or

ii. a preferentially smaller reduction in type IIX fiber type proportion.

#i. Preferential type II fiber hypertrophy Strength training in general produces preferential type II muscle fiber growth, compared to type I muscle fibers. As explained above, this does not involve a shift in muscle fiber type from type I to type II, but simply a difference in the change in size of the individual muscle fibers. There are indications that eccentric training could produce even more preferential hypertrophy in type II muscle fiber area, compared to concentric training (Hortobágyi et al. 2000; Friedmann-Bette et al. 2010), but not all studies have reported the same findings (Mayhew et al. 1995; Seger et al. 1998). More importantly, it is hard to tease apart the differences in overall hypertrophy and preferential type II muscle fiber area hypertrophy. It is particularly difficult because type II muscle fiber area tends to increase by more than type I muscle fiber area anyway. One explanation for why such fiber type differences might occur is that the size principle is breached. According to this idea, eccentric training produces earlier recruitment of high threshold motor units, which are suggested to correspond to type II muscle fibers (McHugh et al. 2002).

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But this explanation has two flaws. •

A review of studies using careful methods has shown that the size principle is almost certainly not breached during eccentric training (Chalmers, 2008).

•

High threshold motor units do not actually correspond directly to type II muscle fibers (Enoka & Duchateau, 2015).

Given the conflicting research, the difficulty in identifying whether a preferential increase in type II fiber area actually occurs (rather than just a normally greater increase), and the weakness in the biological mechanism, we should be cautious about accepting this proposal.

#ii. Preferential retention of type IIX muscle fiber proportion In contrast, it seems much more likely that eccentric training leads to a smaller reductions in the proportion of type IIX fibers, compared to concentric training (Colliander & Tesch, 1990; Hortobágyi et al. 1996; Raue et al. 2005; Vikne et al. 2006). Type IIX muscle fibers are more common in untrained individuals than in trained individuals, and their proportion tends to increase with detraining (Hortobágyi et al. 2000). This suggests that type IIX muscle fibers are the “natural state” before starting resistance training, and that the move from the very glycolytic type IIX to the more oxidative type IIA occurs in response to forceful but also fatiguing contractions (Staron & Johnson, 1993; Douglas et al. 2016a). Since eccentric training involves far less fatigue for the same or greater force levels, this could reduce the stimulus for increasing type IIX muscle fiber proportion, while maintaining the stimulus for overall hypertrophy at the same level. Indeed, the trigger for muscle fiber type shifts seems to be muscle activation, which is dependent the involvement of the active elements, while the trigger for muscular hypertrophy seems to be mechanical loading, irrespective of the level of muscle activation (Eftestøl et al. 2016). What is more, there are other indicators suggesting that fatigue is a key driver of the change from type IIX to type IIA muscle fiber proportion. Velocity-based training preserves type IIX muscle fiber proportion (Pareja-Blanco et al. 2016), while endurance exercise causes a shift from type IIX to type IIA.

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Effects on muscle strength Ultimately, however, any preferentially greater increase in type II muscle fiber area or any preferentially greater retention of type IIX muscle fiber type will probably not cause major changes in maximum strength. This is because there is actually only a very small difference in force production between fiber types, so fiber type shifts probably do not contribute much to the changes in either concentric or eccentric strength after training. However, it is possible that preferential retention of type IIX muscle fiber proportion could lead to superior improvements in high-velocity strength (Pareja-Blanco et al. 2016). Either way, shifts in muscle fiber type are unlikely to explain any eccentric-specific strength gains, as the force production associated with different muscle fiber types is not thought to be contraction mode-specific.

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#3. Regional hypertrophy Regional hypertrophy is a normal aspect of resistance training, and has been observed after various different programs, and in many muscles. It has been argued that eccentric training might produce differences in regional hypertrophy from concentric training (Hedayatpour & Falla, 2012). However, while differences in regional hypertrophy have been observed after eccentric and concentric training (Franchi et al. 2014), there are also contrary reports (Smith & Rutherford, 1995; Blazevich et al. 2007). Exactly why eccentric training might produce differences in regional hypertrophy from concentric training is uncertain. Some researchers have proposed that it could occur because of differences in the activation of different regions of a muscle between lengthening and shortening contractions (Hedayatpour & Falla, 2012), although the exact nature of the differences in neural control between eccentric and concentric muscle actions is unclear (Duchateau & Enoka, 2016). It seems more likely that differences in regional hypertrophy occur because of different changes in muscle architecture. The addition of sarcomeres in series (and muscle fascicle length) after eccentric training may lead to greater increases in distal muscle size. The addition of sarcomeres in parallel after concentric training (because of increases in pennation angle) after concentric training may lead to greater increases in muscle size at the mid-point of the muscle (Franchi et al. 2014). Whether this might then lead to differences in eccentric-specific strength is unclear. Perhaps at high velocities, differences in the location of the muscle mass at different points along the limb may produce an effect because of differences in angular momentum, with distal increases in muscle size being less helpful than more proximal increases. So although there might be small differences in regional hypertrophy between concentric and eccentric training (because of the differences in the muscle architecture adaptations), it is still unclear whether this phenomenon is responsible for the specificity of strength gains after eccentric training.

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#4. Extracellular matrix and cytoskeletal adaptations Extracellular matrix (part 1) Around muscles, around muscle fascicles, and around muscles themselves is an extracellular matrix (ECM) made of different types of collagen. The layer surrounding the muscle fiber itself (the endomysium) contacts with the sarcolemma at the basement membrane. The endomysium is the point at which laterally-directed force is transmitted outwards from the muscle fiber, into the ECM, through which it is transmitted onwards to the tendon (Grounds et al. 2005). Although most people think of contracting muscles as transmitting force longitudinally, they actually transmit the majority of their force laterally, through structures called costameres, which link the myofilaments of the muscle fiber to the endomysium (Bloch & Gonzalez-Serratos, 2003; Grounds et al. 2005; Hughes et al. 2015). Increases in the capacity for lateral force transmission probably partly explain how strength is increased to a greater extent than muscle size after strength training (Erskine et al. 2011). Exactly what is involved when lateral force transmission is increased is currently unclear, but might be reasonably expected to involve changes to the muscle fiber, to the costameres, and to the endomysium. Indeed, muscle protein synthesis and collagen synthesis responses after strength training do appear to occur in parallel with one another, indicating a coordinated effect (Miller et al. 2005). We know that training with heavy loads (whether eccentric or concentric) leads to greater gains in strength than training with moderate (Schoenfeld et al. 2016) or light (Schoenfeld et al. 2015) loads, even when volume loads are not matched. In addition, this greater gain in strength occurs alongside similar hypertrophy, which suggests that specific tension has also increased. Logically, if the superior effect of heavy loads occurs because of greater increases in specific tension, and if specific tension is caused by increases in lateral force transmission capability, then heavier loads must lead to greater changes in lateral force transmission. This suggests that changes in lateral force transmission capability are triggered by the magnitude of the mechanical loading. And this would explain why eccentric training tends to produce greater gains in strength overall, because eccentric training typically involves greater absolute loads.

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Extracellular matrix (part 2) The collagen that makes up the ECM is very stiff. Adding more collagen around the myofibrils increases the stiffness of the individual muscle fibers (Gillies & Lieber, 2011). Since it forms part of the passive elements within muscles, it seems very likely that the stiffness of the ECM contributes to enhanced force production during lengthening contractions. Several long-term training studies have shown that the amount of certain types of collagen within a muscle can increase as a result of exercise and strength training, including eccentric training (Kjaer, 2004; Willems et al. 2010; Wisdom et al. 2015; Jakobsen et al. 2016), although others have not (Roman et al. 1993). Moreover, acute studies show that collagen synthesis is increased after strength training, just like muscle protein synthesis (Moore et al. 2005; Holm et al. 2017). Eccentric training produces comparable increases in collagen synthesis when the workload is matched (Moore et al. 2005), but a greater increase when the volume (sets x reps) is matched (Holm et al. 2017). This suggests that increases in collagen may occur after eccentric training, primarily as a result of the greater mechanical loading stimulus. Since there are loads that can be used during eccentric training that cannot be employed during concentric training, this could still be a mechanism by which eccentric-specific strength gains occur. Moreover, increases in collagen also seems to play a key role in the repeated bout effect after workouts (particularly those involving eccentric contractions), and may therefore be important for protecting the muscle from future injury (Heinemeier et al. 2007; Hyldahl et al. 2015; Takagi et al. 2016). Therefore, although the research is still at an early stage, it seems like a fairly safe bet that the specific gains in eccentric strength that are observed after programs of eccentric training are caused at least partly by changes in the ECM, probably by increases in certain types of collagen.

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Cytoskeleton Within each muscle fiber are myofibrils, supported by scaffolding-type structure, called the cytoskeleton. This cytoskeleton has longitudinal (parallel) and transverse (perpendicular) elements, and the most important longitudinal element is titin, which is also called connectin in some older studies. Similarly, the structure and content of the cytoskeleton within a muscle fiber can increase with training. Titin in particular could be affected, although the evidence is still weak. Rodent studies show that the number of titin filaments that surround each myosin filament can increase from 3 to 5 with training (Hidalgo et al. 2014; Krüger & Kötter, 2016), and this would be expected to improve force generation specifically during lengthening contractions (Lindstedt et al. 2001). Even so, cross-sectional studies in humans show that residual force enhancement (one of the features of eccentric muscle actions that is almost certainly explained by titin) is not increased in weightlifters compared to untrained controls (Siebert et al. 2016). Eccentric exercise is particularly good at damaging both the ECM and the cytoskeleton, including titin (Friden & Lieber, 2001), and it triggers cellular signaling processes that interact with titin (Krüger & Kötter, 2016). This is almost certainly because eccentric training naturally relies more on these passive elements during contractions, and this greater loading in turn leads to more damage. Therefore, it is logical that eccentric exercise might produce greater adaptations in the passive elements than concentric training. Consequently, although it remains to be demonstrated, it seems possible that the specific gains in eccentric strength that are observed after programs of eccentric training are caused at least partly by changes in the cytoskeleton, probably in titin.

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#5. Tendon stiffness, and muscle stiffness Stiffness is the extent to which an object resists being lengthened. A stiff spring only lengthens a little when you attach a weight to it. On the other hand, a compliant spring lengthens quite a long way. Muscle-tendon units have both muscles and tendons in series (one after the other). So while we can look at the overall stiffness of the whole muscle-tendon unit, we can also assess the individual stiffness of both the muscle and tendon, separately. Strength training leads to increased tendon stiffness, and although the effects are affected by load (higher loads are better), they do not differ between eccentric and concentric training (Bohm et al. 2015). In contrast, while muscle stiffness probably increases slightly after concentric training, most likely because of increases in ECM and titin content (Gillies & Lieber, 2011) it actually decreases after eccentric training (Kay et al. 2016). This seems strange. If anything, we might expect that increases in muscle stiffness should be superior after eccentric training. Indeed, older research in animals using accentuated eccentric training, such as downhill running, has reported contrary results (Lindstedt et al. 2001). One possible explanation for this discrepancy could be the large increases in fascicle length that are produced by eccentric-only training. Stiffness is the gradient of the stress-strain curve, which can be calculated as stress (force per unit area) divided by strain (relative length change). •

Stress is the force per unit area, which is easiest to think of as just force applied. This force is usually applied on the body by the ground, when absorbing an impact during running or jumping.

•

Strain is the relative length change, which is how much the muscle fascicle is stretched in comparison with its starting length. Strain is normally best thought of in percentage terms, where the percentage refers to the increase in length, relative to the resting length.

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Applying a stress to a long muscle fiber will result in a larger relative length change than the same stress applied to a shorter muscle fiber, all other things being equal. Thus, increasing muscle fascicle length will mean that you record a lower value of stiffness, even if the individual muscle fibers are themselves now made of stiffer material. What this means is that while muscle-tendon stiffness often increases with normal strength training or with concentric exercise, it does not necessarily increase after eccentric exercise (Kay et al. 2016). Increased muscle-tendon stiffness probably translates to greater joint stiffness, which could be desirable or not, depending on the goals of the athletes (Brazier et al. 2014). Ultimately, what we can say is that since changes in tendon stiffness do not seem to differ between concentric and eccentric training, that changes in tendon stiffness are not responsible for the specificity of strength gains after eccentric training. The changes in muscle stiffness are less clear, but this might be because they are produced by a combination of variables.

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#6. Neural adaptations Introduction Eccentric and concentric muscle actions involve different neural control (Ruas et al. 2016; Douglas et al. 2016b), and this may lead to differences in the adaptations that arise after strength training. Contentiously, a case was once made that the neural control of lengthening contractions was very different from shortening contractions, such that high threshold motor units were recruited earlier (Enoka, 1996). However, it is now generally accepted that the size principle is maintained in both lengthening and shortening contractions, and the difference in the neural control of eccentric and concentric muscle actions is thought to lie elsewhere (Duchateau & Enoka, 2016). Yet, it is still possible that other aspects of neural control (that do not violate the size principle) could differ after training with either lengthening or shortening contractions. Such changes could be measured by: i.

changes in agonist neural drive

ii. changes in antagonist co-activation iii. alterations at the corticospinal level

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#i. Agonist muscle activation There are several (limited) methods available for examining the changes in agonist neural drive after strength training. These include EMG amplitudes, voluntary activation, and (less clearly) changes in the H reflex. •

EMG amplitudes – the EMG amplitudes measured during maximal strength tests are electrical potentials detected across parts of the muscle during contractions, and therefore provide an indicator of the neural drive from the central nervous system, but also reflect changes in the muscle structure, which affect the conduction of the signal.

•

Voluntary activation – voluntary activation is the difference between the levels of force produced during voluntary contractions, compared with the levels of force produced during similar, involuntary contractions (when stimulated by electrical impulses).

•

The H reflex (the Hoffman reflex) – the H reflex is an electrically-stimulated version of the stretch reflex. The electrical stimulus is detected by sensory Ia afferent nerves, which then send signals to the spinal cord. At the spinal cord, they produce excitatory postsynaptic potentials, which then send action potentials down efferent nerves back to the muscle (Palmieri et al. 2004). Increases in Hreflex amplitude during maximal strength tests after training have therefore been proposed to reflect either increases in the excitability of efferent motor nerves or the inhibition of Ia peripheral afferent nerves (Vangsgaard et al. 2014)

EMG amplitudes are typically lower during eccentric muscle actions, compared to during concentric muscle actions (Douglas et al. 2016b). And eccentric training may well increase EMG amplitudes to a greater extent than concentric training, in both isometric and eccentric strength tests (Komi & Buskirk, 1972; Hortobágyi et al. 1997; Aagaard et al. 2000), but this is by no means a uniform finding (Higbie et al. 1996). Yet, there is evidence that the increases in EMG amplitudes after eccentric training are greater when tested in eccentric strength tests, which is a promising sign for explaining eccentric-specific strength.

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Voluntary activation is lower during eccentric muscle actions, compared to during concentric muscle actions (Westing et al. 1990; Amiridis et al. 1996; Beltman et al. 2004) but whether voluntary activation can be preferentially increased during eccentric muscle actions after eccentric training (compared to during concentric muscle actions) is currently unknown. Some studies have reported increases in H-reflex measurements after eccentric training (Duclay et al. 2008; Vangsgaard et al. 2014), but this not a uniform finding (Ekblom, 2010; Barrué-Belou et al. 2016), and given the compound nature of the H reflex signal, the correct interpretation of these results is hard to assess.

#ii. Antagonist co-activation Like agonist activation, antagonist co-activation is generally measured using EMG amplitudes. Methods such as voluntary activation and the H reflex are less relevant in this case. Currently, it is unknown whether eccentric training affects changes in antagonist coactivation differently from concentric training or standard strength training, although there is evidence that it can cause reductions (Pensini et al. 2002), as has been reported after some (mostly high-velocity) conventional strength training programs.

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#iii. Corticospinal adaptations Some studies have indicated that electroencephalography-derived movement-related cortical potential signals differ between concentric and eccentric muscle actions (Fang et al. 2001; 2004). These differences indicate greater signal magnitudes during eccentric muscle actions for (1) cortical preparation, planning and execution of movements, and (2) feedback information processing, as well as a longer onset time associated with the cortex sending the movement control strategies. This may indicate that lengthening muscle actions are much harder to perform under control, at least when studied in untrained people. This could imply that eccentric training then leads to quick improvements in the control of such movements, which might logically then produce large gains in eccentric-specific strength. However, this has yet to be studied in detail. In addition, it is interesting to observe that after programs of unilateral exercise, eccentric training produces a greater cross-over of strength gains from the trained limb to the untrained limb than concentric training (Hortobágyi et al. 1997; Seger et al. 1998; Nickols-Richardson et al. 2007; Kidgell et al. 2015). And Kidgell et al. (2015) suggested that this occurs because of greater reductions in corticospinal inhibition following eccentric training, compared to after concentric training.

Summary of neural adaptations There is certainly evidence for specific and/or greater neural adaptations after eccentric training, compared to after concentric training, which may explain the gains in eccentricspecific strength. However, exactly what those adaptations are is much less clear (Duchateau & Baudry, 2014; Duchateau & Enoka, 2016). Given the results of the acute comparisons between the corticospinal control of eccentric and concentric muscle actions, in combination with the long-term cross-education studies, it seems most likely that specific gains in eccentric strength after programs of eccentric training are caused at least partly by greater reductions in corticospinal inhibition.

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Conclusions We are much stronger and more efficient when lowering a weight (eccentrically) than we are when lifting a weight (concentrically). This is partly because of the involvement of passive elements within the muscle during lengthening contractions, and partly because of the difference in the way the active elements function. During eccentric training, we can use a much heavier weight, for more reps, and create a much greater mechanical loading on the muscle, at the same time as producing far lower muscle activation. As a result of these differences, compared with concentric training or conventional stretch-shortening cycle training, eccentric training seems to cause: •

Greater increases in muscle fascicle length

•

Smaller increases in pennation angle

•

Smaller reductions in type IIX fiber proportion

•

Different regional hypertrophy

•

Greater increases in lateral force transmission

•

Greater increases in ECM content (and possibly changes in titin)

•

Smaller increases in muscle stiffness

•

Greater reductions in corticospinal inhibition (and possibly other neural adaptations)

The greater increases in muscle fascicle length and smaller reductions in type IIX muscle fiber type proportion may be beneficial for high-velocity strength; and the greater increases in lateral force transmission are likely what produce the high degree of transfer between eccentric training and maximum concentric strength. Gains in eccentric-specific strength after eccentric training are probably caused by two factors: (1) increased ECM (and possibly titin) content, which increase passive force production, and (2) neural adaptations that are specific to the eccentric phase.

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3. VELOCITY

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For developing the ability to produce force at high speeds, most coaches make use of ballistic exercises that involve moving quickly under load, like jump squats and Olympic weightlifting derivatives. On the other hand, when asked, many people will tell you it is the intention to move quickly that makes athletes stronger at high speeds. But if that was true, then ballistic training would be unnecessary, and everything could be accomplished purely through standard, heavy resistance training. So are strength gains velocity-specific or not?

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What are “velocity-specific” strength gains? Velocity-specific strength gains occur when strength gains are largest at the same speed as is used during training. If we train using a fast speed, we should see the greatest gains in strength when we test strength at a high velocity, and the smallest gains in strength when we test at a low velocity. Similarly, if we train using a slow speed, we should see the greatest gains in strength when we test strength at a low velocity, and the smallest gains in strength when we test at a high velocity. This all sounds fairly straightforward. However, understanding the impact of strength training at different speeds is complex, because: •

there are two forces acting against you when you lift a barbell,

•

there are two ways in which you can change velocity, and

•

muscles and tendons behave differently when performing the same exercises at different velocities.

Let’s look at each of those issues before going any further.

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What forces are involved in lifting a barbell? When you try and lift a barbell, the external force that you must overcome is made up of two different parts: •

weight (force due to gravity), and

•

inertia (force required to accelerate mass)

If you are using free weights, then the weight of the barbell stays the same throughout the exercise range of motion. However, the inertia changes during the exercise range of motion, according to the acceleration of the barbell.

At the beginning, when you are working to get the barbell moving, inertia is large and positive (it works against you) so this is where total force is greatest. At the end, when you are riding the momentum of the barbell to lockout, inertia is large and negative (it works for you), so this is where total force is least. When using a heavy load, inertia will have a comparatively small impact on the overall force, because the weight is large and the peak velocity is small. Additionally, the above curve will be flat for a long period of time in the middle, and have relatively shorter curves at either end.

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In contrast, when using lighter loads, inertia will have a larger impact on the overall force, because weight is small and peak velocity is high. And the above curve will have be flat for a shorter period of time in the middle, and have more pronounced curves at either end. This could lead to differences in the joint angles at which the muscle is subjected to the greatest loading, and thus joint angle-specific strength gains. Of course, this all changes when using a different type of external load, but that is a topic for another time.

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How can you alter bar speed? There are two ways in which you can alter bar speed, and each of these affect weight and inertia differently. Firstly, you can simply add plates to the bar. That will slow you down even if you are using maximal effort for both the light and heavy loads. You cannot move heavy loads as quickly as light loads, it is just not possible. This is called the force-velocity relationship.

Secondly, you can alter your intent towards how you perform the rep. You can choose to perform it with maximal effort, or you can perform it with sub-maximal effort. If you take the first approach, and add plates to the bar to reduce your speed, you will increase the weight of the barbell, because the extra plates are heavier. However, you will not really alter inertia that much. Overall, external force increases, but velocity decreases. If you take the second approach, and keep the same plates on the bar but deliberately change your tempo to reduce bar speed, the barbell weight stays the same, but inertia decreases, because you have chosen to accelerate the barbell more slowly. Overall, external force decreases, and velocity decreases (Bentley et al. 2010).

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In the first approach, force and velocity are matched to the underlying maximum capabilities of the muscle as determined by the force-velocity relationship, and the profile of the external force is strongly curved at the beginning and end of the movement. In the second approach, force is lower than it could be, and the profile of the external force is flatter, with a smaller curve at the beginning and end, and a longer period of time in the middle when force is constant.

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How do muscle and tendon behaviors differ with load and speed (part 1)? Tendon behavior is affected by both load and speed. Essentially, this is because tendons are not simple connecting structures that link the muscle to the bones that make up a joint. Tendons are viscoelastic tissues, and this benefits force production during high-velocity movements, by allowing power amplification, such as in the take-off phase of a jump.

At rest, before the jump commences, the muscle-tendon unit, muscle, and tendons are all at their normal lengths. In preparation for the jump, before the joint angles change, the calf muscles activate, produce force, and shorten. Since the muscle-tendon unit remains at its normal length, the ankle tendons must lengthen. During the take-off from jump, the calf muscle remains short, but the tendon shortens by recoiling from its lengthened state, releasing its stored elastic energy, and the muscle-tendon unit overall also shortens.

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Overall, this process makes the movement much more efficient, as the muscle can contract more slowly before the movement happens (and therefore produce more force, because of the force-velocity relationship). The muscle does work on the tendon, which it stores as elastic energy, and this is then released during the jump. A similar effect can be observed upon landing, which is called power attenuation.

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How do muscle and tendon behaviors differ with load and speed (part 2)? When comparing light and heavy loads (performed with maximal effort), tendons seem to lengthen less with heavy loads compared to with lighter loads (despite the greater forces with heavy loads). When working against heavy loads, the tendon is more rigid, and acts more like a force transducer. In contrast, with lighter loads, tendons lengthen more. They are more pliable, and act more as a power amplifier, storing more elastic energy in the eccentric phase and releasing it in the concentric phase (Earp et al. 2014).

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Even so, it is high forces produced against heavy loads that produce tendon adaptations, and not low forces produced against lighter loads (Malliaras et al. 2013; Bohm et al. 2014). Ballistic and plyometric training does not seem to be as helpful for increasing tendon stiffness as conventional, heavy load strength training. Similarly, there are differences in muscle and tendon behavior between slow and fast movements performed with the same load. Faster movements lead to greater peak tendon lengths just prior to recoil in the concentric phase, suggesting that the faster speed makes tendons behave more elastically, and involves more power amplification (Earp et al. 2016).

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Ultimately, this suggests that both velocity and intent lead to differences in muscle and tendon behavior during a strength training exercise. Specifically, both light loads and maximal intent lead to greater tendon elongation and power amplification than heavy loads and sub-maximal intent.

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Does the weight on the bar produce velocity-specific strength gains? If the weight on the bar produces velocity-specific gains in strength, then we should find that training with low forces (and therefore light weights) produces greater strength gains at faster speeds than when using high forces (and therefore heavy weights). Essentially, we should get something roughly like this, when low-velocity and highvelocity groups test their strength at the same speeds, before and after programs of strength training:

Many researchers have explored this question, going back nearly 50 years. They have usually found velocity-specific results after (maximal effort) isokinetic strength training when comparing two or more groups, where one group used a slow angular velocity, and the other a fast angular velocity. Typically, higher velocity training leads to greater gains in strength when tested at high isokinetic velocities (Moffroid & Whipple, 1970; Caiozzo et al. 1981; Coyle et al. 1981; Jenkins et al. 1984; Garnica, 1986; Thomeé et al. 1987; Petersen et al. 1989; Bell et al. 1989; Ewing Jr et al. 1990), although not always (Farthing & Chilibeck, 2003).

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The velocity-specific effect of training with different loads is less consistently observed when using free weights, but is still apparent. Interestingly, there tends to be a clearer velocity-specific pattern when the subjects use single-joint exercises (Kaneko et al. 1983; Aaagaard et al. 1994; 1996; Moss et al. 1997; Ingebrigtsen et al. 2009), than when they use multi-joint exercises (Almåsbakk & Hoff, 1996; McBride et al. 2002; Mora-Custodio et al. 2016). In summary, high velocity-specific strength gains are observed after training with light loads.

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Does changing intention also produce velocity-specific strength gains? It is hard to figure out whether intent produces velocity-specific strength gains. On the one hand, it is easy to find studies showing that groups training using free weights with maximal speeds tend to experience greater strength gains measured at high velocities compared with similar groups that train at sub-maximal speeds (Jones et al. 1999; Morrissey et al. 1998; Ingebrigtsen et al. 2009; González-Badillo et al. 2014; Pareja-Blanco et al. 2014). The problem with this approach is that it does not measure intent alone. It measures both intent and actual muscle contraction velocity. Some researchers have tried to isolate the effects of intent by controlling velocity. They do this by setting velocity to zero, which means using isometric contractions. However, muscle contraction velocity cannot be totally controlled for in isometric contractions. Isometric contractions involve muscle shortening, even when the joint angle does not change. When a muscle contracts, it shortens, even during isometric contractions, but the tendon lengthens so that there is no net change in the length of the total muscle-tendon unit. So the velocity is not zero, and the muscle shortens more with increasing force production (Narici et al. 1996). Even so, training with maximal speed-intent and sub-maximal speed-intent isometric contractions do in fact produce different results, with maximal speed-intent training leading to greater gains in higher-velocity strength measures (Tillin et al. 2012b; Tillin & Folland, 2014; Balshaw et al. 2016). In summary, high velocity-specific strength gains are probably observed after training with maximum intent.

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Is maximal intent the only factor that causes high velocity strength gains? (part 1) In perhaps the most famous velocity-specificity study, Behm & Sale (1993) tasked subjects to perform ankle dorsiflexion training in two ways (isometric and isokinetic), where both conditions required the subjects to “move as rapidly as possible regardless of the imposed resistance.” The isometric training was performed with maximal speed-intent but without moving, while the isokinetic training was performed with maximal speed-intent, and at a relatively high angular velocity (300 degrees/s). Strength was tested at a range of angular velocities (0 – 300 degrees/s). However, there was no difference between the two groups in respect of the changes in strength at any velocity. So is intent the only important factor?

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Is maximal intent the only factor that causes high velocity strength gains? (part 2) Although very influential, the study by Behm & Sale (1993) had some very important limitations, and given that there is also a great deal of evidence supporting velocityspecificity, this make it difficult to accept as the final word on the matter. It used a within-subject design, where one leg was trained using the isometric training program, and the other used the isokinetic training program. This means that the strength gains from the ipsilateral limb could easily have produced a cross-over effect of strength to the contralateral limb, which would completely confuse the results. The joint angle of peak contraction differed between muscles, as both started at 30 degrees of plantar flexion, but the isokinetic training program had to accelerate without load until it reached the target velocity. so the isometric training program involved a joint angle of peak contraction at longer muscle lengths, which would be expected to produce greater hypertrophy (and therefore strength). Even though 500ms was allowed for each contraction (meaning that maximum force was almost certainly reached), there was at least one outcome where there was a reduction in maximum isometric force after training. This is odd, because isometric training normally produces large increases in maximal isometric force (Del Balso & Cafarelli, 2007). Even when the intent is explosive, gains in maximum isometric force are usually seen (Maffiuletti & Martin, 2001; Tillin et al. 2012b; Tillin & Folland, 2014; Balshaw et al. 2016). There were no differences in the gains in maximum isometric force between the two conditions. However, gains in maximum isometric force after training with isometric contractions are usually larger than gains in maximum isometric force after dynamic contractions (Jones et al. 1987; Folland et al. 2005). Essentially, it seems premature to suggest that maximal intent is the only factor that leads to high velocity strength gains.

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Why are gains in strength velocity-specific? (part 1) So having settled that velocity-specific strength gains probably happen both in response to actual movement velocity and the intent to move quickly, what is responsible for the adaptation anyway? Well, in theory, muscles can either increase their rate of force development, or increase their maximal contraction velocity while producing a certain level of tension (or they can do both at the same time).

As you can see, within this model, the point in time where you measure force impacts whether you will expect to observe an increase or decrease in force expressed at high velocities after training. If you measure force in the early phase (say around 100ms), while force is still rising (e.g. Tillin & Folland, 2014), then you will see a change in force only if rate of force development has increased. On the other hand, if you measure force in the late phase (say around 300ms), after force has reached its peak (e.g. Behm & Sale, 1993), then you will see a change in peak force only if force production at high velocities has altered.

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Life, however, is not as simple as this theoretical model, and there are two complications. Firstly, it is widely accepted that the motor program for explosive contractions is preprogrammed (Duchateau & Hainaut, 2008). Once triggered, it goes all the way through to the end before you get to stop running it. So many types of explosive or ballistic training may well develop both rate of force development and high-velocity strength at the same time. Secondly, contraction type affects how quickly we reach peak force. Although it is widely accepted that peak force is only reached after around 250 – 300ms, the reality is that this only applies to isometric and eccentric contractions. Peak force in concentric contractions can be reached in type IIA > type I (e.g. Trappe et al. 2006; Harber & Trappe, 2008). Indeed, some studies have reported velocity-specific strength gains in conjunction with shifts in muscle fiber type or in fiber type distribution (Liu et al. 2003; Zaras et al. 2013), but others have found no changes in fiber type distribution, while still reporting velocity-specific strength gains (Coyle et al. 1981; Thomeé et al. 1987; Ewing Jr et al. 1990; Malisoux et al. 2006; Vissing et al. 2008). Additionally, while there are indications of preferential increases in type II muscle fiber type area after training at faster speeds (Coyle et al. 1981; Thomeé et al. 1987; Zaras et al. 2013; Pareja-Blanco et al. 2016a), this is also by no means a uniform finding (Ewing Jr et al. 1990; Malisoux et al. 2006; Vissing et al. 2008; Lamas et al. 2012).

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Effects of velocity-based training on fiber type One interesting new development is the comparison of fast and slow bar speeds averaged over a set, where the slower average bar speed is achieved by training closer to failure. This approach is called “velocity-based training” and seems to produce routinely greater gains in high-velocity strength and transfer to fast sporting movements (Pareja-Blanco et al. 2014; 2016a; 2016b), and the effects might result from differences in the hypertrophy of the various muscle fiber types.

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Explaining conflicting results Leaving aside the new developments from velocity-based training, the lack of consistency in the results of the other studies might be because of a trade-off in the effects of changing fiber type, with early phase effects being disadvantageous and late phase effects being advantageous. Maximal force production is affected by muscle size, and changes in muscle size occur more in type IIA fibers, in parallel with a reduction in the proportion of type IIX fibers. So we might expect there to be a trade-off in early phase velocity-specificity and late phase velocity-specificity, because the loss of type IIX reduces rate of force development in the early phase, and gains in type IIA increase rate of force development and force production in the late phase. This is more or less what we find, although the research is fairly limited. For example, Häkkinen et al. (2003) found an increase in rate of force development over early and late phases combined (500ms), while type IIX fiber area reduced, and type IIA fiber area increased. Aagaard et al. (2010) reported no increase in rate of force development in the early phase (