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Influence of Muscle Slack on High-Intensity Sport Performance: A Review

Van Hooren, Bas MSc; Bosch, Frans BSc

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Strength and Conditioning Journal: October 2016 - Volume 38 - Issue 5 - p 75-87
doi: 10.1519/SSC.0000000000000251



In the article that appeared on page 76 of volume 38, issue number 5, the text corrections to Table 1 in columns 3 and 4 were not applied correctly. The online version of this article has been corrected and reflects the most recent corrections.

Strength & Conditioning Journal. 39(1):91, February 2017.


In most sports, the time to develop force is limited. For example, a defending soccer player will try to prevent an attacking player from scoring by limiting the time available to perform the kick. In addition, during volleyball and basketball, some jumps are performed under time pressure, and in these situations jump height may be compromised (19). In many athletic movements such as sprinting, javelin throwing and shot putting, the time period in which force can be developed is only about 300 milliseconds and usually even much shorter (97). For example, during linear top speed sprinting, the ground contact time is only about 100 milliseconds, while it can take up to 900 milliseconds to develop maximum force (3). Therefore, in most sports, the capability to rapidly develop force is critical for maximizing sport performance. In addition, during some daily activities, the capability to rapidly develop force is also important. For example, falling (and associated injuries) in elderly people may be prevented by enhancing their capability to rapidly develop force after a sudden loss of balance.


Maximum force development takes time (3) and because time is limited during most sport actions, reducing the time to reach maximum force will likely improve performance. However, to effectively reduce this time, it is important to first understand the processes that limit rapid force development. If the limiting processes have been identified, a next step is then to investigate whether and how these processes can be altered by training. Therefore, the purpose of this review is to discuss these processes and to elucidate on whether and how they can be altered by training.


During force development, 6 consecutive mechanisms can be distinguished (Table, Figure 1). We will first briefly define these mechanisms and then expand on their influence.

Process of force development in 6 chronological steps
Figure 1.
Figure 1.:
Schematic representation of the time course of different processes during a squat jump in reaction to a simple visual stimulus. The duration of the premotor time and mechanical processes is very much related to the context in which the movement is performed. EMD = electromechanical delay.

First, a relevant stimulus is detected. This stimulus contains perception or is processed into perception. Subsequently, the central nervous system sends a gross signal to activate the muscles. These 2 processes will be described as the premotor reaction time (steps 1 and 2). The signal arrives at the neuromuscular junction and propagates across the muscle membrane to activate chemical processes which lead to shortening of the contractile element (CE) of the muscle. The delay associated with these processes is termed the electrochemical delay (ECD; step 3). The contraction of the CE aligns, or straightens, the muscle-tendon unit (MTU) that before activation hung in a relaxed position between the attachment points of the muscle. The time required for straightening the muscle is known as the mechanical delay (step 4). In the scientific literature, steps 3 and 4 are usually combined and described as the electromechanical delay (EMD). As soon as the MTU is aligned, the series elastic element (SEE, the tendon and aponeurosis) begins to stretch. SEE stretching will be defined as compliance (step 5). Once CE force production diminishes, the SEE recoils and this is termed the catapult effect (step 6). In this review, the delay between the start of CE contraction (step 4) and SEE recoil (step 5) will be termed muscle slack, and the processes involved in this delay will be described.

Reducing the time to move through these steps will benefit high-intensity sport performance. For example, reducing the premotor reaction time and the associated degree of muscle slack will result in a faster action, or, when total movement time remains equal, provide more time to apply force. Unfortunately, most strength and conditioning research has focused on the isolated force development capabilities of the muscle CE, neglecting the influence of muscle slack and limiting application of the findings to actual sport performance. This limited application will be evident in closed and especially open motor skills, where rapid force development is crucial. This review though will start the discussion at the EMD.


Shortening of the EMD can potentially enhance high-intensity sporting performance (40,52,90,95). Primarily in older studies (8,78) the term motor time is also used to describe EMD. The EMD is the time interval between activation of the muscle fibers (i.e., arrival of the action potential at the neuromuscular junction) and the onset of force production as detectable on for example a force platform (9). The duration of the EMD is influenced by electrochemical processes (i.e., the propagation of the action potential across the muscle membrane and the excitation-contraction coupling) and mechanical processes (e.g., alignment of the MTU). The EMD is a concept that lacks a clear definition because it contains several processes that are to a certain extend independent of each other. Measurements of the EMD therefore do not provide information about the relative contribution of each of the processes involved. To understand the overall process, it is important to distinguish between the subprocesses. For example, the influence of the mechanical processes can be large or small, depending on the situation in which the measurement is made (e.g., fatigued or nonfatigued, length of the MTU, the movement pattern executed and measuring equipment) (12,76,84,98). Therefore, this review will divide the EMD into the electrochemical processes; the ECD and mechanical processes; the mechanical delay.

The electrochemical delay

The ECD starts when the action potential arrives at the end plates of the motor neuron and ends when the CE begins to contract. The duration of the ECD is very brief, with most studies reporting a duration of approximately 3–6 milliseconds (41,58,70,76), although a longer delay of approximately 20 milliseconds has also been reported (2). These discrepancies may be related to different methodological approaches. Because of their brevity, only small performance gains can be made by reducing the duration of these processes.

Fiber type distribution is usually considered very important for rapid force production. Indeed, type II fibers typically have a shorter ECD and a higher contraction speed compared with type I fibers due to a faster excitation-contraction coupling (77).

The mechanical delay

The processes that occur during the mechanical delay are poorly understood. Some processes that may occur are the uptake of slack in the CE and SEE, alignment of the MTU, and changes in 3-dimensional muscle shape (Figure 2). The order in which these mechanisms occur may overlap and differ between an active CE contraction and passive lengthening of the MTU. Moreover, during some movements, the CE can be shortening, while large external forces simultaneously stretch the MTU, for example, the gastrocnemius medialis during the ground contact in high-speed running. In this case, the order in which these mechanisms occur may differ yet again. The following order may occur during an active CE contraction:

  • The take up of slack in the contractile element

Figure 2.
Figure 2.:
Schematic representation of the muscle action during a push-off (left) and the time course of force production (right). The horizontal double arrows indicate where muscle slack dominates the push-off. The increasing size of the vertical arrow in the left images represents the increase in force production and force application.

In relaxed muscles, both the CE and the SEE can be slack (37,38), which means they produce no passive elastic force (42). This slack has to be taken up before force can be transmitted to the bones. CE slack is probably taken up as it begins to contract.

  • Alignment of the muscle-tendon unit

The MTU includes both the in series arranged passive and active tissues between the attachment points. Slack of the SEE and CE is the slack that usually is measured or modeled for a muscle that is aligned in a straight line between the attachment points. However, the MTU may initially hang in a relaxed arched position between the attachment points of the muscle, and therefore the length of the total MTU will be more than the distance between the attachment points. As a result, total MTU slack may be larger than the combined slack of the CE and SEE measured in isolation (Figure 3). After CE slack is taken up by CE contraction, the MTU is aligned between the proximal and distal MTU attachment points.

  • The take up of slack in the series elastic element

Figure 3.
Figure 3.:
Schematic representation of the dangling position of the muscle-tendon unit and slack in the contractile element (CE) and series elastic elements (SEEs).

As mentioned previously, the SEE can be slack in relaxed muscles. When the MTU is aligned between the attachment points, further contraction of the CE will reduce SEE slack. In computational musculoskeletal modeling studies, slack is usually modeled by setting tendon slack length (96). This parameter represents the length at which the tendon begins to generate force. Most of these studies assume that the tendon falls slack at the same length as the entire MTU, and therefore these models have used only tendon slack length to represent MTU slack length. However, the length at which the SEE and CE fall slack can differ. For example, a recent study found that slack length of the gastrocnemius medialis fascicles and Achilles tendon occurred at different joint angles (42), possibly because subcutaneous adipose tissue and/or the parallel elastic elements first take up slack in the tendon (39,42). Therefore, tendon slack length may not give a good indication of total MTU slack length. Assuming that the MTU falls slack at the same length as the SEE may lead to errors in these computational models. In addition, recent research has shown that CE slack length differs between synergistic muscles (39), making generalizations of slack lengths even more limited. Also computational musculoskeletal modeling studies have been found to be very sensitive to tendon slack lengths, but the value used to represent tendon slack length is based on arbitrary estimates of which the accuracy cannot be determined (18,61). This is because experimental data, for example, based on ultrasound measures, have only recently become available for some muscles (37,38). Finally, some models require a minimum amount of activation, which prevents the MTU from going slack.

  • Changes in the 3-dimensional muscle shape

Changes in 3-dimensional muscle shape include mechanisms linked to the muscle bulging out, variable fascial curvature and changing pennation angle. This article will not describe these mechanisms in detail.

  • Stretch of the series elastic element

The SEE is stretched until the force required to bring about more stretch is higher than the force needed to move the joints.

These 5 mechanisms can differ greatly between movements, and they are therefore very difficult to model, especially for high-intensity sport movements. Their influence on performance can also differ greatly, and this makes it difficult to interpret the findings of studies that have investigated the duration of the EMD. For example, if the mechanical delay is 100 milliseconds, it is not known how much of this time is needed to take up slack in the CE and SEE and how much of this time represents compliance of the SEE, although slack may have a large influence on the duration of the mechanical delay at very short MTU lengths (58,68,76). For example, Sasaki et al. (76) found the mechanical delay of the elbow flexors to be approximately 8 milliseconds at the most extended position (40° joint angle), whereas it increased to approximately 20 milliseconds at the most flexed position (i.e., 130° joint angle). In addition, there was no significant change in the duration of the mechanical delay when the MTU was lengthened beyond slack length, indicating that more slack lead to an increase in mechanical delay. Furthermore, it has been shown that the way in which the EMD is measured can have a large influence on the duration (12). For example, the time of the mechanical delay is very much related to the context in which the movement is performed, and therefore large variations in EMD duration have been found, ranging from about 6 milliseconds (41,47,70) to more than 100 milliseconds (11,84,85). However, in the latter studies, the duration of ECD and mechanical delay was not directly measured, and the duration of the mechanical delay is therefore based on the assumption that the ECD is approximately 6 milliseconds (41,70).

Because ECD is very brief, it is usually assumed that the uptake of slack and SEE alignment in vivo is responsible for the majority of EMD (2,9,35,68,98). Indeed, a large contribution of the SEE (i.e., tendon and aponeurosis of the gastrocnemius medialis) to the EMD has recently been confirmed using very high–speed ultrasound (4 kHz) in vivo (70). However, another study did not find any difference between the influence of the electrochemical and mechanical processes during biceps brachii actions (41). These contrasting outcomes are likely due to differences in the muscle and/or tendon structure between the studied muscles (58), or they might be attributed to differences in mechanisms 1, 2, and 3 of the mechanical delay.

Because the structure of the MTU and the joint angles (76) have a large influence on muscle slack, the results of studies of isolated muscles have limited transfer to movement patterns that involve multiple muscles and joint angles. Nevertheless, since the duration of EMD, and especially the uptake of slack and MTU alignment, can be long, it is likely that this limits high-intensity sports performance in both relatively untrained and elite athletes. Because several studies found a simultaneous reduction in the duration of the EMD and improvements in rapid force development (40,52,90) or vertical jump height (95), the duration of ECD and mechanical delay may be reduced to enhance performance. However, because ECD is very brief, only small performance gains can be made by reducing the duration of these processes. In contrast, because of the sometimes large duration of the delay between muscular contraction and SEE elongation, significant time can potentially be gained by reducing this duration.


Because the processes involved in the mechanical delay and stretching of the SEE partly overlap, they have a combined effect within muscle slack. Aligning and elongating the SEE can be compared to using an elastic tow cable to pull a car. However, it should be noted that this comparison is a very simplified representation that does not include 3-dimensional effects such as muscle gearing ratio (1) and lattice spacing (94). If the car doing the pulling starts moving, the other car will not instantly move. First, the elastic tow cable will be straightened. Thereafter, the cable will stretch until the force required to stretch the cable is higher than the force needed to pull the other car. It is only at this point that the car will start moving. Similarly, initially only a small amount of force is needed to take slack out of the MTU and to align the MTU (or elastic tow cable in the example). However, as the SEE gets stretched, an increasing amount of force will be needed to bring about more stretch (Figure 4).

Figure 4.
Figure 4.:
Compliance and stiffness. As the length of the series elastic elements (SEEs) increases (more stretch), an increasing amount of force will be needed to stretch the SEE further. SEE b has a higher stiffness than SEE a as a result of training and as a consequence, the force transmission to the bone will be faster.

An increase in the amount of stretch will result in an increased stiffness of the SEE. Therefore, this stiffness is a consequence of the interaction between the CE and SEE. In addition, the speed of force production will also influence the SEE stiffness. A rapid increase in force will lead to a higher stiffness because of the viscoelastic properties of the SEE (51). Therefore, muscle slack and the capability of the CE to rapidly develop force are interdependent. An increased SEE stiffness will enhance rapid performance because the transmission of force from the CE to the attachment points of the muscle will be faster.

SEE stiffness can be altered by training because of exercise-induced adaptations in the mechanical, material, and morphological properties (6). Although a decrease in SEE stiffness following training has been reported (31), most studies among untrained or recreationally trained individuals found an increased SEE stiffness as a result of training both with and without external load (25,31,52,82,91,95). Therefore, training (also with the addition of external load) may be beneficial for SEE stiffness as a result of structural changes. However, these structural adaptations occur over time, and therefore these cannot immediately reduce muscle slack.


The SEE has several functions. One of these functions is to increase the power output beyond what can be reached by the CE in isolation. If the force produced by the CE diminishes, the SEE will recoil and provide extra power (54,72). This SEE recoil is called the catapult effect. It is often thought that a stretch and recoil of the SEE only occurs during movements with a countermovement (CM) (e.g., a countermovement jump) or where large external forces stretch the SEE (e.g., ground contact during sprinting). However, during movements such as a squat jump, the SEE is initially stretched before recoiling as the force in the CE decreases (24). Moreover, even in isokinetic strength measurement, there is a dynamic interaction between the CE and SEE (36).


In the remaining part of this review, we will discuss the acute and long-term effects of 3 strategies that may be used to reduce the influence of muscle slack on high-intensity sport performance:

  • The use of pretension through cocontractions.
  • Using a CM.
  • Using external load.

Furthermore, the way in which these 3 strategies influence each other will be outlined. This mutual influence is important in identifying training interventions that have a positive influence on high-intensity sport performance.


The first possibility for reducing muscle slack is creating pretension by simultaneously contracting agonistic and antagonistic muscles around a joint (i.e., coactivation and cocontraction).

Acute effects of cocontractions on muscle slack

By cocontracting muscles before joint motion starts, a certain amount of slack is taken out of the MTU, the MTU is aligned, and possibly some of the compliance is overcome. For example, after all slack is taken out of the MTU, further contraction of the CE may stretch the SEE, which increases SEE stiffness and reduces the effect of SEE compliance. When the antagonist muscle relaxes, the agonist will be able to produce force with a reduced influence of muscle slack. As a consequence, force production at the attachment points starts from a high plateau and performance will benefit (87). For example, it is well possible that the MTU of the gastrocnemius would be slack right before ground contact in high-speed running or right before ground contact in a drop jump if there was no contraction of the CE before ground contact (37,38). In this case, the ground contact would initially serve to take out slack and only when all slack is taken out, force could be applied to accelerate the body. Fortunately, the CE is activated before ground contact, and therefore less slack has to be overcome during the initial ground contact and more time can be used to accelerate the body. In addition to the possibly beneficial effects on performance, cocontractions may also offer protection against injuries (34). For example, cocontractions may result in a shorter mechanical delay and more rapid force development, which results in a faster correction after an unexpected perturbation. For example, an unexpected inversion in the ankle joint during the ground contact of high-speed running may be corrected faster when the muscles are pretensed because of the preflex capabilities of the MTU.

However, there may be a trade-off between cocontractions and rapid force development. Milner et al. (66) found less muscle activation during cocontractions compared with the sum of agonist and antagonist activation alone, which they attributed to reciprocal inhibition. Therefore, excessive cocontractions may hamper rapid force development. In addition, asynchronous cocontractions may result in a loss of energy and a lower net force production. For example, although muscles reached maximum torque faster after an unexpected inversion of the ankle joint when the subjects used pretension compared with no pretension (48), another study among untrained individuals found that pretension before a ballistic action resulted not only in a shorter EMD but also in a lower rate of torque development (83). Therefore, when used effectively, cocontractions may be an appropriate strategy to reduce the influence of muscle slack. However, when used inappropriately, they may hamper performance.

Cocontractions in endurance sports

Creating cocontractions may not seem important for all sporting actions. One could suppose that an athlete participating in an endurance sport like cycling may have enough time to develop force. However, reducing muscle slack in endurance sports like cycling is important as cycling with a high cadence requires activation of the muscles before force application on the pedals to prevent a slow force development due to muscle slack. For example, counterintuitively, the quadriceps have to be activated when the pedal is moving upward, so that muscle slack is minimized when the downward stroke is initiated. If the muscles are not activated at the initiation of the downward stroke, the result will be a brief moment where no force is applied (69,86) and as a consequence, cycling speed will slow down. This problem increases as cadence rises. Activating (cocontracting) involved muscles earlier at higher pedaling frequency will reduce EMD to a certain extent. Sarre and Lepers (75) speculated that a strategy of even earlier activation of the muscles would perhaps further decrease the EMD, but because muscle activation costs energy, it would also be detrimental to the efficiency of the movement.

It is well possible that cocontractions can never completely take up all muscle slack and hence, without the use of an external load providing a stretching force to the MTU, muscle slack will always limit performance to some extent. For endurance sports, there will always be a trade-off between developing sufficient cocontractions to minimize muscle slack while simultaneously minimizing the associated energy costs.

Long-term effects of cocontractions on muscle slack

As cocontractions may reduce muscle slack and hence enhance rapid force development, it would be interesting to know which training methods and/or exercises can improve the capability to create pretension by cocontractions. Several studies have investigated how pretension can be trained (10,53,56,57). In addition, longitudinal studies have investigated the effect of training on the EMD during an involuntary (17,31–33,43,60,82) and voluntary contraction (7,21,40,43,52,79,80,82,90,91,95,98). Although several studies showed the EMD to be shorter following training, they did not clearly differentiate between the mechanisms affecting the change in performance and hence cannot be used to determine the impact of training on the capability to create pretension. Therefore, the training interventions effective for creating optimal cocontractions and pretension remain unknown.


The second possibility that may be used to reduce the impact of muscle slack is the performance of a CM.

Acute effects of a countermovement on muscle slack

When the attachment points of a muscle move closer together, muscle slack will increase, and therefore the duration of the EMD will increase (37,58,76). By performing a CM, for example during the downward phase of CM jump, the attachment points of the quadriceps will move away from each other. This will take up slack, line up the MTU, and stretch the SEE, hereby reducing muscle slack (23,24,38,45,55). Therefore, immediately after the CM, force can be transmitted directly to the bone. As a consequence, some researchers and strength and conditioning professionals suggest that using a CM is a good strategy to reduce the negative influence of muscle slack on the high-intensity sport performance. In addition, it is usually thought that a CM improves rapid force production because a reflex is triggered by stretching the muscle fibers. The effectiveness of a CM may however be the result of (slow) take up of slack rather than stretch of the muscle fibers. However, an action with CM takes longer than one without CM (Figure 5). For instance, a throw using a light medicine ball takes about 310 milliseconds without CM and 500 milliseconds with CM (81). Furthermore, data from several studies indicate that CM jump times range from 500 to 1,000 milliseconds (measured from the initiation of the downward movement until toe-off), whereas squat jump movement times range from 300 to 430 milliseconds (measured from the initiation of the upward phase until toe-off). During most sporting actions, there is not enough time to perform a CM, and therefore it is not a very useful strategy to reduce the effect of muscle slack.

Figure 5.
Figure 5.:
Time course of the vertical ground reaction force (y axis) during a countermovement jump (CMJ) and a squat jump (SJ). Although a CMJ results in a higher jump, it also takes more time to perform (x axis).

Long-term effects of countermovement training on muscle slack

Although the difference between a CM jump and squat jump can be small (4), an action with CM will almost always result in a better performance than one performed without CM. For example, jump heights reached during a squat jump are lower compared with a CM jump (5), and a ball throw without CM results in a lower ball speed and hence a smaller throwing distance than a throw with CM (assuming an equal throwing angle) (71,81). These findings often make researchers and practitioners jump to the conclusion that producing better results using a CM will automatically lead to better results during competition. However, this may not be the case because it is possible that practicing CMs leads to an increase in muscle slack because the athlete's ability to perform cocontractions may be reduced as a consequence of the supporting effect of CM. The athlete gets used to the CM reducing muscle slack and hence does not create pretension to minimize the muscle slack (i.e., the central nervous system becomes lazy). Although direct evidence to support this reasoning is lacking, some indirect evidence supports it. For example, untrained and recreationally trained individuals have been found to increase the amplitude of the CM as a result of CM jump training (13,63–65). This larger amplitude increases the duration of the CM (49,64,73) and because the available time during most sporting actions is only very brief, this training approach may actually be detrimental to performance.

If CM indeed negatively alters the capability to develop pretension, then this will likely affect highly trained individuals more than untrained or recreationally trained individuals because untrained or recreationally trained individuals may experience more positive adaptations as a consequence of training (e.g., increased neural drive and cross-sectional area (16)), whereas these adaptations may already be well developed in highly trained individuals. As a consequence, for untrained or recreationally trained individuals, potentially negative adaptations resulting from CM training will be masked by the positive adaptations. This masking effect might lead to the incorrect conclusion that untrained individuals benefit from CM training also in the long run, while it actually may be detrimental to their long-term performance. Indeed, hardly any athlete (untrained or highly trained) will benefit from more muscle slack. Every athlete can perform a large CM, but all athletes have to learn to minimize the amplitude and duration of the CM.

Therefore, we recommend minimizing CM within training for those sporting movements where a CM will always have a negative influence on the performance (e.g., a swim start, sprint start, and rugby scrum). During other sporting actions, there may be plenty of time to perform a CM. For example, not all jumps in volleyball are performed under time pressure, and during tennis a player can sometimes make a larger amplitude backswing. However, minimizing CM during training for these activities is suggested as making a bigger CM is never a problem and reducing CM always is.

Cocontractions and countermovements in running

Sprinting is characterized by elastic muscle activity, which means that for instance, the SEE in calf and other muscles is stretched at foot strike. Foot strike effectively triggers a CM (i.e., ankle dorsiflexion), which decreases the amount of muscle slack that needs to be overcome, and therefore one may think that cocontractions are not important during sprinting. However, because the ground contact time, and hence time to apply force is very brief, muscles have to develop pretension before initial ground contact. This pretension is created partly by CM during the flight phase. For example, knee extension may act as a CM to reduce the amount of muscle slack in the hamstrings and hip extension may act as a CM for the rectus femoris. These CM may not create enough stiffness on ground contact, and therefore cocontractions in the involved muscles need to be built into the movement pattern. A proper technical execution of this tensing action before ground contact is complex and requires a great deal of practice. Better sprinters are able to produce more force during the short ground contact (67,92,93), probably, at least partly as a result of less muscle slack and better stiffness. The major problem in sprinting technique may therefore be in the flight phase rather than in the stance phase.

The ground contact during middle- and long-distance running is longer when compared with sprinting, but still too short to build up maximum force. Therefore, during endurance running, it is also important to create cocontractions. However, there is again a trade-off between minimizing muscle slack and minimizing energy costs. As a consequence, one might expect elite runners to develop more cocontractions (i.e., more electromyographic activity) when compared with nonelite runners just before ground contact. However, elite Kenyan distance runners have been found to have less activity of both the agonist (gastrocnemius medialis) and antagonist (tibialis anterior) when compared with national-level Japanese distance runners 100 milliseconds before ground contact and during the ground contact phase in which the SEE is stretched (74). These, perhaps contradictory, findings may be explained by a higher SEE stiffness and greater isometric muscle actions, making retraction of swing leg before ground contact more effective and requiring less muscle activation.


The use of external load such as barbells, dumbbells, and elastic bands is a third possibility for reducing muscle slack.

Acute effects of external load on muscle slack

Generally, when resistance is added to a movement such as a jump, slack will be taken up, the MTU will be aligned, and the SEE will be stretched by the extra gravitational forces (20). This will reduce the influence of muscle slack (Figure 6). If no external load is used, or if the body weight is reduced by assisting the movement with elastic bands, the athlete needs to create more cocontractions to reduce muscle slack. For example, several studies have shown that the addition of external load to a CM jump resulted in a smaller amplitude of the CM, whereas a larger amplitude was observed when elastic bands assisted during the CM jump (22,62,88,89). In contrast, other studies found larger or similar amplitude during the CM jump when external load was added compared with no load (44,59). However, in one of these studies (59), the participants were allowed to use an arm swing during unloaded CM jump, which may explain the contradictory findings. Another possible reason for the larger CM with the addition of external load is that a (too) fast downward movement resulted in a greater inertia which had to be overcome, and this forced the subjects to perform a larger CM.

Figure 6.
Figure 6.:
Effect of external load on muscle slack. Left: the vastus lateralis is still slack. Therefore, first slack has to be taken out of the muscle-tendon unit (MTU), the MTU has to be aligned, and the series elastic elements have to be stretched before force can be transmitted. Right: there is less muscle slack in the MTU of the vastus lateralis because the external load stretches the MTU.

Long-term effect of external load on muscle slack

Numerous sport actions have no or very low external resistance that can reduce muscle slack at the start of CE contraction. For example, at the start of a stroke during rowing, the water provides only very low resistance because it moves in the opposite direction of the blade. Therefore, claims that training with external load (i.e., traditional resistance training) will automatically improve high-intensity sport performance should be questioned. Because the addition of external load to a movement may decrease muscle slack, it may not teach the athlete to develop proper pretension by cocontracting muscles. This reasoning also lacks direct evidence but some indirect evidence supports it. It is possible that athletes become accustomed to external load taking up the muscle slack and as a consequence, the athletes create fewer cocontractions during a CM jump, and this will cause the amplitude of the CM to increase. However, studies investigating the effect of external load on the amplitude of the CM during a CM jump show inconsistent results. This may be a consequence of differences in the used training protocol and/or the training status of the participants.

Several studies among recreationally trained individuals found an increased CM amplitude following training with external load (14,63,64), although the increase in the CM amplitude was not always the largest in the group training with external load. Other studies among recreationally trained individuals (15,46) or elite female rugby players (26) did not find increases in CM amplitude or duration following training with external load.

The small amount of indirect evidence leads to the careful conclusion that the use of external load may negatively impact on the ability to develop pretension. However, it should be noted that resistance training and CM (or loaded jump squats) were combined in all studies (14,15,26,46,63,64), and therefore the potentially negative effect of external load on the ability to develop pretension may have been caused by the CM. In addition, changes in the CM amplitude do not necessarily indicate changes in actual sporting performance. For example, it is possible that resistance training results in a smaller CM, but still negatively impacts on sporting movements without a CM because both CM and external load reduce muscle slack, whereas muscle slack cannot be reduced by a CM or external load during most sporting movements. Therefore, future research should investigate the separate effects of external load and CMs on actual sport performance using highly trained individuals.

Nevertheless, the statement “strength training makes athletes slower” could very well be based on the intuitive feeling that (incorrect and too much) strength training may increase muscle slack and as a consequence makes the athlete slower. In addition, “athleticism” could very well encompass the capability to move with a minimum amount of muscle slack and the capability to rapidly develop force.


In the scientific literature, it is not always properly defined what a concept means. A good example of this is the concept of the “stretch-shortening cycle” (SSC). Often, the SSC is simply described as a stretch of the muscle followed by a shortening. However, it is not described which components of the muscle stretch and shorten. This can lead to wrong interpretations. For example, it is often assumed that there is an eccentric action of the CE of the leg muscles during the downward movement of the CM jump. Although some studies show the CE to lengthen during the downward movement (23,24), other studies show the CE to shorten (55) or work isometrically (50). Therefore, the downward movement does not necessarily present an eccentric action of the CE. Perhaps only during slowly executed submaximal and large amplitude CM jumps, the CE lengthens, whereas it works isometrically or concentrically during faster maximum effort and small amplitude jumps. Future research should therefore refer to the downward and upward phases rather than the eccentric and concentric phases of the CM jump.

Another probably incorrect assumption is that during sprinting, the knee extension of the front leg during the flight phase causes an eccentric action of the CE of the hamstrings. As a consequence, several researchers and strength and conditioning professionals use exercises thought to produce an eccentric muscle action (e.g., the Nordic hamstring exercise) as a core exercise for “functional” strengthening of the hamstrings. However, there is an increasing body of evidence suggesting that there is no eccentric action, but rather an isometric action of the CE during the swing phase in high-speed running (27–30). The knee extension during the swing phase will first take slack out of the MTU, align the MTU, and then stretch the SEE before the SEE recoils. Functional training of the hamstrings should therefore not be done through eccentric training, but in an elastic-isometric way, reflecting hamstring functioning during sprinting.


The time to develop force is limited in most high-intensity sporting actions. We propose the initial part of muscle slack, which is the delay between contraction of the muscle fibers and the start of SEE stretching as an important performance limiting factor, especially for highly trained individuals. Therefore, both athletes and strength and conditioning professionals should continuously be searching for strategies to minimize muscle slack. We concluded that a CM was not an appropriate strategy to reduce muscle slack because of the extra time requirements associated with performing a CM and because of the probable increase in resultant muscle slack as a result of the reduced capability to create cocontractions. Using external load in training is also an inefficient strategy for improving the rapid force development for those sporting activities that are performed without significant external load. The effect of resistance training on muscle slack will be especially problematic in highly trained individuals because most positive adaptations may already be well developed. The only effective direct way to reduce muscle slack in movements performed against low resistance may be through creating pretension by cocontractions. An effective and efficient technique of cocontractions may be complex from a coordinative point of view and therefore requires a great deal of practice.

For the strength and conditioning professional, it is important to examine which aspects of high-intensity performance require improvement. For highly trained individuals, it is important, when using external loads during training, to search for a balance between the possible negative effects on the capability to create pretension and the positive effects such as an increased motor unit firing frequency, motor unit synchronization, and SEE stiffness.

Scientific research is unaware of the mechanisms of muscle slack and the influence of CM and external load during the exercises and activities described above. Also the difference in training experience between untrained/poorly trained athletes and highly trained individuals are often not taken into account when drawing conclusions. Therefore, when reading research studies, appraisal of the following 3 aspects is recommended:

  • Have the study participants been described with sufficient detail?
  • Has the tested movement been described with sufficient detail?
  • Has the resistance used in training and testing been described with sufficient detail?

In the near future, the capability to rapidly develop force may prove to be an important battlefield for discussions on training transfer between the more classic “mechanistic” approach and a more motor control-based approach to resistance training.


The authors thank Kenneth Meijer for his comments on a preliminary version of the manuscript and Craig Ranson for his feedback on the final version of the manuscript.


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rate of force development; electromechanical delay; resistance training; explosive sport performance; high-speed running; muscle-tendon unit

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