How do plyometrics improve stretch-shortening cycle function?

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Plyometrics are often used by strength coaches and sports coaches to enhance high-velocity athletic performance. In addition to improving the high-velocity force production and maximum theoretical velocity of muscles, plyometrics are assumed to produce additional adaptations that alter the stretch-shortening cycle in a useful way. Yet, exactly how they might alter stretch-shortening cycle function has not previously been explained.

What are plyometrics?

Plyometrics can be defined most accurately as unloaded, high-velocity, high effort exercises that involve a definite landing phase and a stretch-shortening cycle (SSC). Consequently, all plyometrics are SSC exercises, but not all SSC exercises are plyometrics. In other words, plyometrics are basically a subset of SSC exercises.

Indeed, many high effort exercises that are in common use among athletes are excluded from this definition because they only involve one or two of the characteristics.

What is a stretch-shortening cycle (SSC)?

An SSC is a concentric contraction that is immediately preceded by an eccentric contraction. Since the placement of an eccentric contraction immediately before a concentric contraction typically increases the force and velocity of the concentric contraction, this leads to a phenomenon known as the SSC effect. The SSC effect can be quantified by the extent to which either concentric speed, power, or force are increased when the concentric phase is preceded by an eccentric, compared to when the same concentric phase is performed from a standing start.

Although the SSC is often described as a single phenomenon, it is known to be produced by at least five different mechanisms, all of which work in different ways and experience different adaptations after training:

By considering these mechanisms carefully, we can identify which of them are the most important during plyometrics, and subsequently also identify what adaptations they might experience such that plyometrics performance can be improved by training.

Which SSC mechanisms are most important during plyometrics?

Introduction

Some SSC mechanisms are more important in some types of muscular contraction, while other SSC mechanisms are more important in other types. To understand more about plyometrics, it is helpful to consider which of the SSC mechanisms are most important for high-velocity, high effort movements with a pronounced SSC.

The stretch reflex

While the stretch reflex is probably the most famous mechanism that underpins the SSC effect, it is unlikely to contribute much to the SSC effect during a plyometric exercise. When a movement is performed with a high effort, it is already employing a high level of motor unit recruitment. Thus, the stretch reflex, which works by increasing motor unit recruitment, has little scope to enhance force production in a SSC action (compared to a similar concentric-only action), because there is little capacity for increasing motor unit recruitment beyond a level that is already very high (and in some cases may already be maximal).

Preactivation

The preactivation effect is likely a key SSC mechanism during plyometrics, since the relatively high eccentric force produced in the landing phase means that the muscle is strongly activated before the start of the concentric phase. In this way, preactivation is also an SSC mechanism during heavy strength training. For this reason, preactivation is not that interesting for understanding plyometrics, since it occurs similarly in most high-effort contractions.

Residual force enhancement

The residual force enhancement (RFE) effect has been carefully described by many studies. When titin is stretched inside an activated muscle fiber, it produces a very high force. In this way, titin also explains why eccentric contractions produce much higher forces than either isometric or concentric contractions. Nevertheless, the force that titin produces is caused by its stretch, in the same way as an elastic band. Thus, as the muscle fibers begin the concentric phase, they still experience this elastic force pulling them back to their starting length. Consequently, titin can contribute to force production in the concentric phase of SSC movements, and this is called the RFE effect.

The magnitude of the RFE effect is dependent upon two factors.

Firstly, it is dependent upon the number of muscle fibers that are activated in the eccentric phase, because titin only produces this high force when it is inside an activated muscle fiber. Secondly, it is dependent upon the extent to which the muscle fibers are lengthened while they are activated, in the same way as an elastic band produces force in proportion to the extent to which it is elongated. Consequently, it is easy to see that the RFE effect is an important contributor to the SSC effect during heavy strength training exercises.

During heavy strength training, a large number of muscle fibers are activated for a large proportion of the lowering phase of the exercise, which means that the RFE effect can contribute substantially to the force production in the subsequent lifting phase.

During plyometrics, there is also a large number of muscle fibers activated in the lowering phase, due to the relatively high eccentric forces that are exerted upon landing. Yet, the RFE effect is limited by the extent to which muscle fibers lengthen in this phase, because this lengthening is much smaller than in heavy strength training, for two reasons. Firstly, plyometrics typically involve a shorter joint angle range of motion than strength training exercises. Secondly, tendons are viscoelastic, and this means that they tend to lengthen very little during heavy strength training. This means that the muscle fibers that are in series with the tendons can lengthen a long way (which produces a large RFE effect). In contrast, tendons lengthen a lot during plyometrics, which means that the muscle fibers cannot lengthen anywhere near as far (which produces a limited RFE effect). Thus, while the RFE effect probably does contribute to the SSC effect during plyometrics, it does not contribute to the same extent as during heavy strength training.

Elastic energy storage in tendons

Whether the storage of elastic energy in tendons during the eccentric phase of an exercise can enhance velocity, power, or force during the subsequent concentric phase is currently unclear, although it does seem to play an important role in increasing energy efficiency, which makes it a very important mechanism during aerobic exercise.

Changes in the force-velocity relationship

The final SSC mechanism involves an alteration in the force-velocity relationship of the working muscle fibers. This occurs due to the lengthening and shortening of the tendons that are in series with the muscle. When a tendon lengthens in the eccentric phase of an SSC movement, it allows the muscle fibers to lengthen less. When the tendon shortens in the subsequent concentric phase, it allows the muscle fibers to shorten less. When muscle fibers shorten less (over the same joint angle range of motion), they shorten slower for the same joint angular velocity. This means that they produce a higher force, due to the force-velocity relationship. The greater the tendon lengthening in the eccentric phase, the greater the tendon shortening in the concentric phase, and the less that the muscle fibers lengthen and shorten. Thus, the more that the tendons lengthen and shorten during the SSC, the greater force that the muscle fibers produce.

We might expect that this final mechanism would be more important for plyometrics than for heavy strength training, because tendons tend not to lengthen and shorten very much during heavy strength training, while they do lengthen and shorten quite a lot during plyometrics, and other light load exercises.

Comparing SSCs in heavy strength training and plyometrics

When comparing heavy strength training and plyometrics, we can identify that the working mechanisms in both activities are: [A] preactivation, [B] the residual force enhancement effect, and [C] changes in the force-velocity relationship due to tendon lengthening and shortening.

Moreover, we can see that the contribution of preactivation will probably be fairly similar in both types of exercise, while the residual force enhancement effect will be greater during heavy strength training, and the effect of changes in the force-velocity relationship due to tendon lengthening and shortening will be greater during plyometrics.

The magnitude of the SSC effect can be measured by comparing the athletic performance, force or bar speed of a non-SSC exercise variation with the athletic performance, force or bar speed of an SSC exercise variation. For example, we might compare jump heights during squat and countermovement jumps. Alternatively, we might compare concentric bar speeds in bench presses or back squats involving both eccentric and concentric phases with those same exercises performed as concentric-only exercises from a static starting point. The percentage increase in performance that occurs as a result of using a SSC exercise variation instead of a concentric-only exercise variation can be defined as the SSC effect.

The magnitude of the SSC effect is greater when performing light load exercise (like plyometrics), compared to when doing heavy load exercise (like heavy strength training). This suggests that the overall contribution of changes in the force-velocity relationship due to tendon lengthening and shortening is greater than the overall contribution of the residual force enhancement effect, and also that the SSC effect is a very important aspect of plyometrics.

What causes the SSC effect to increase after plyometrics?

The SSC effect is a good way to assess SSC function, or the extent to which the SSC can enhance performance. It can be quantified as the extent to which either concentric speed, power, or force are increased when the concentric phase is preceded by an eccentric, compared to when the same concentric phase is performed from a standing start.

To the extent that the SSC effect increases after plyometric training, then this would be a very important contributor to increased athletic performance. Moreover, given that the main contributor to the SSC effect during plyometrics seems to be the changes in the force-velocity relationship due to tendon lengthening and shortening, it seems likely that any increase in the SSC effect would involve an improvement in this mechanism.

For tendon lengthening during the eccentric phase of a plyometric movement to increase, the muscle would have to increase its eccentric strength relative to the stiffness of the tendon. In this way, the muscle would be better able to resist lengthening during the eccentric phase, and the tendon would have to lengthen more instead.

While it has traditionally been assumed that increases in tendon stiffness contribute to the increases in athletic performance that occur after plyometric training, this is probably not true. In fact, not all studies have revealed increases in tendon stiffness after plyometric training. Moreover, when plyometric training is compared with other types of exercise, such as heavy strength training and isometric training, it almost always produces smaller increases in tendon stiffness. Consequently, if the mechanism by which plyometrics improved SSC function was an increase in tendon stiffness, then heavy strength training would be superior at producing this effect, and plyometrics would be largely unnecessary. Since this is not true, it is likely that other adaptations are involved.

In contrast to the effects on tendon stiffness, there is good evidence that plyometrics improve the eccentric strength of the muscle to a greater extent than other types of exercise, including isometric training. In the context of plyometrics research, eccentric strength is sometimes measured and described as “active muscle stiffness” to make it clear that the muscle is working in series with the tendon, and that the muscle must overcome the stiffness of the tendon.

By increasing eccentric strength to a greater extent than tendon stiffness, plyometrics enhance the size of the SSC effect. After plyometric training, the tendons tend to lengthen more and the muscle fascicles tend to lengthen less in a high-velocity, SSC contraction. Thus, the traditional idea that plyometrics cause an improvement in athletic performance by means of increases in tendon stiffness is not correct. Rather, plyometrics improve athletic performance by increasing eccentric strength more than tendon stiffness. In contrast, isometric training (and heavy strength training) cause greater increases in tendon stiffness than in eccentric strength (since eccentric strength often lags concentric strength these types of training, while increases in tendon stiffness are quite pronounced. Thus, the SSC effect is likely reduced after these types of training, which is likely one of the reasons that these types of training are much less effective for improving athletic performance than plyometrics.

What is the takeaway?

In addition to improving high-velocity force production and speed, plyometrics improve SSC function, leading to an increase in the size of the SSC effect. This probably occurs by means of a greater increase in eccentric muscle strength than in tendon stiffness, such that muscles are able to lengthen less during landing (eccentric) phases and tendons are forced to lengthen more. By lengthening less in the eccentric phase, muscles necessarily shorten less in the following concentric phase. By shortening less, they shorten less quickly for the same joint angular velocity, and this allows them to produce higher forces owing to the force-velocity relationship.

In this way, plyometrics differs from heavy strength training, which likely produces greater increase in tendon stiffness than in eccentric muscle strength, such that muscles are forced to lengthen more during landing (eccentric) phases and tendons are able to lengthen less. It seems likely that extended periods of time spent performing heavy strength training likely reduce SSC, but that subsequent blocks of plyometrics may be able to reverse this adverse effect. In this way, the success of traditional periodization models involving blocks of heavy strength training followed by blocks of plyometrics can be explained.

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