The role of elastic energy in force, work, and power has been studied for decades. Cavagna et al. (11) investigated the amount of power generated during sprinting from start to peak running velocity (9.4 m·s−1). It was found that power output increased up to an average of 5 m·s−1 through intrinsic properties of muscle contraction. However, the remaining increases in power output necessary to obtain maximal velocity were attributed to the storage and subsequent release of elastic energy in relevant leg musculature.
More than 3 decades have passed since the Cavagna et al. (11) study. During this time, extensive research has explored the role of elastic energy in human movement. Due to the extensive nature of this research, investigators have constructed substantial reviews spanning the general contribution of elastic energy to normal human movement (47), movements such as running that require higher physical prowess (4), how elastic energy principles can be applied to clinical situations (6), and finally possible mechanisms that mediate these contributions (1). What is still needed, however, is an in-depth analysis of research on elastic energy as it directly and practically applies to the strength and conditioning domain. Therefore, the purpose of this article is to provide a critical analysis of the literature, particularly as it pertains to the enhancement of activities with relatively greater force and power requirements and from this to construct a coherent set of principles that can be applied to both athletes and coaches alike. According to Knuttgen and Komi (39) force can be defined as the quantity of that which tends to change the motion or state of rest of matter, while work refers to force expressed through a displacement. Finally, power is defined as the rate at which a quantity of work is performed (28,39). Using these definitions as a framework, this article specifically covers 1) the nature of elasticity and its application to human participants, 2) the role of elastic energy in performance during activities requiring a stretch-shorten cycle such as the depth jump and bench press, 3) the role of muscle stiffness in the use of elastic energy, 4) the control of muscle stiffness through feedforward and feedback mechanisms, and 5) factors affecting muscle stiffness. Throughout the article, practical examples are provided to illustrate the importance of these scientific principles to the domain of strength and conditioning.
The Nature of Elasticity and Its Application to Human Performance
Elasticity can be defined as a measure of how readily a body will reform after it has been deformed by being stretched, compressed, or twisted (18). Elastic energy is the capacity of a body to do work during reformation (18). Hill (33) provided a 3-component model of musculotendinous behavior that explains the role of elastic energy in human movement. The model consists of a contractile element (CE) that exerts active force during shortening, a series elastic component (SEC) that serves to store and latter release elastic energy, and a parallel elastic component (PEC) that stores elastic energy in parallel to the contractile components of a muscle (Figure 1).
The CE is considered to be a function of the formation of cross bridges between the thick and thin myosin and actin filaments (18). In this context the rationale behind the typical force velocity relationship is that greater shortening velocities of sarcomeres require faster cycling rates of the cross bridges, leading to fewer cross bridges that can be attached and contribute to force production (18).
Traditionally, the SEC has been thought of as lying within the tendinous tissues (31). However, a more recent model (31) suggests that the SEC also lies within the muscle fiber itself. When cross bridges are formed between filaments and a stretch is applied, elastic energy is suggested to be stored within the cross bridges. The PEC is composed mainly of the connective tissue surrounding and binding muscle tissue (18).
The Hill model makes a number of predictions relevant to movement. During tetanus, for example, a whole muscle can exert greater force than a single muscle twitch (66). According to the Hill model, the contractile component is enhanced by a greater release of calcium and subsequent build-up of cross bridge formation. In terms of the SEC, the model predicts that with a single muscle twitch, submaximal force will first be transmitted to taking slack out of the SEC, which acts as a spring (18). However, with a sustained maximal contraction, the SEC is stretched and force can be transmitted externally (18). These interactions are expanded on throughout the remainder of the article.
Elastic Energy Use During the Stretch-Shorten Cycle
The phenomenon that occurs when muscle tissue is stretched to cause eccentric tension immediately before concentric contraction is known as the stretch-shorten cycle (SSC). Cavagna et al. (12) investigated the effect of stretching a muscle immediately before contraction on work output in a frog gastrocnemius muscle and found a nearly 3-fold difference in the prestretched condition (Figure 2). In a follow-up investigation, Cavagna et al. (10) found similar results when prestretching the human forearm flexors. Cavagna et al. (12) suggested that the differences in force output experienced were related to the differences in potential energy in the SEC before concentric contraction. More recently, Zatsiorsky (66) and Van Ingen et al. (60) suggested that the increased work output seen after a countermovement can be explained by the time to build up force development, storage, and reuse of elastic energy, potentiation of contractile machinery, and reflex contributions.
Use of Elastic Energy and Time Delays to Enhance Work Output
In a comprehensive review, Van Ingen et al. (60) suggested that elastic energy stored during negative work did not contribute to the increased force output seen during a countermovement. Instead, the countermovement maximizes torque and therefore vertical velocity over the entire concentric range of motion by allowing individuals more time to stimulate relevant muscle tissue (stimulation dynamics) and increase cross-bridge formation (activation dynamics). In support of this, Bobbert et al. (5) found a direct (r = 0.88) relationship between the time to stimulate the gluteus maximus and vertical ground reaction force in a noncountermovement jumping task, while their work in simulation models suggests that time to build force could explain enhanced work output during the vertical jump without regard to storage of elastic energy (4). Another line of evidence put forth by Van Ingen et al. (60) was that individuals with predominantly slow-twitch muscle fiber makeup benefit to a greater extent from the countermovement than individuals with predominantly fast-twitch muscle fiber makeup. The rationale given was that fast-twitch fibers have faster cross-bridge cycling rates than slow-twitch fibers and can build maximal force at a greater rate than slow-twitch fibers.
A number of investigators have disputed the van Ingen et al. (59) article as overly dogmatic and not entirely explanatory (31,67). For example, Herzog (31) suggested that greater benefits seen in predominantly slow-fiber makeup individuals is a factor of the capacity of muscle fibers to store elastic energy between cross bridges. If an SSC occurs before cross bridges detach, then the energy stored within can be used to perform work. Slower coupling rates would favor maintenance of stored elastic energy. In support of this, Bosco et al. (7) found no difference between individuals with predominantly fast and slow twitch muscle fiber composition for rapid SSCs. However, with longer SSCs, individuals with predominantly slow-twitch muscle fibers benefited most. Zatsiorsky (66) was also in disagreement and stated that “the mechanism suggested by the authors does not explain how athletes perform take offs of much shorter duration than 300-500 ms, for instance, sprinting (t = 100 ms), long jumping (t = 120 ms), and high jumping (t = 180 ms)” (p. 480). In each of these cases, Zatsiorsky (66) suggests that during the breaking or countermovement stage, negative work is being done by the body parts on the SEC and as such is stored in the SEC. The enhanced force output seen is in part a consequence of the release of this stored energy. Finally, Fini et al. (20) found that individuals allowed to perform a countermovement before a vertical jump had greater jumping performance compared to a condition allowed to build maximal isometric force before a vertical jump, indicating that other factors such as elastic energy are contributing to enhanced performance following a prestretch.
In conclusion, while experts are in sharp disagreement in terms of the mechanisms involved in enhanced work output due to prestretching, the above evidence appears to support both increased time to enhance force output (particularly in longer movements such as the vertical jump) and the storage of elastic energy for enhanced force output. The major determining factor governing the relative contribution of these factors appears to be the length of time taken to complete a stretch shorten cycle. Generally, it is found that shorter stretch-shorten cycles rely more heavily on the reuse of elastic energy than slow stretch-shorten cycles.
Additionally experts appear to be in agreement with the capacity for the SSC to enhance power output. Evidence suggests that energy can be stored relatively slowly in the SEC and released at a more rapid rate to increase power output during movements that incorporate an SSC (60).
Neural Mechanisms Involved in Force Output During Stretch-Shorten Cycles
Imbedded within and parallel to muscle fibers are spindle fibers, innervated by specialized afferent neurons. During rapid stretching, the equatorial region of the spindle fibers is distended, and their mechanical deformation stimulates a number of reflex mechanisms. Monosynaptic stretch reflex (M1) occurs with a latency of 30-40 ms (55) and involves communication from the muscle afferent nerves directly with motor neurons of the same muscle tissue. A second reflex with a latency of 50-80 ms is known as functional stretch reflex (M2) and is thought to involve higher structures in both the spinal cord and brain (55). One of the proposed mechanisms behind the force potentiation seen in countermovements has been attributed to reflexes stimulated during the prestretching phase. However, in the previously mentioned study, Finni et al. (20) found no differences in electromyographic (EMG) activity between SSC and isometric conditions in the concentric portion of a vertical jump, indicating that reflex activity was not involved in the increased torque seen. These findings have led a number of scientists to suggest that reflex activity is not involved in increased force output during the SSC (60).
Schmidtbleicher (56) divides the SSC into 2 classifications, slow and fast, with classification of these SSCs dependent on the eccentric-concentric contraction times associated with the respective movements. A slow SSC such as a jump shot in basketball is characterized by large angular displacements at the hip, knee, and ankle joints and by longer contraction times. These have been classified by Schmidtbleicher (56) as being evidenced by ground contact times of >0.25 second. Fast SSCs (ground contact phases during sprinting) consist of smaller angular displacements and quicker eccentric-concentric coupling, evidenced by ground contact times of <0.25 second.
Komi et al. (40) suggest that the vertical jump does not serve as an adequate model of the fast SSC nor its effect on reflex potentiation of muscular force. The authors suggest that an effective SSC requires 3 critical elements, including “a well timed preactivation of the muscle(s) before the eccentric phase, a short and fast eccentric phase, and an immediate transition (short delay) between stretch (eccentric) and shortening (concentric phase)”(p. 452). The countermovement jump does not fulfill each of these criteria, as would running, hopping, and depth jumps, which demonstrate a relatively rapid transition from stretch to shortening phases as well as a greater stretch velocity, which is critical for activation of the stretch reflex. It is important that strength and conditioning practitioners are aware of these principles. Where a training stimulus is desired to improve performance in fast SSC activities or to assess fast SSC function, the vertical jump is not an appropriate modality.
Ultrasound Technology for Assessment of the Role of Elastic Energy in Stretch-Shorten Cycles
With the use of ultrasound technology, researchers have gleaned a great deal of insight as to the role of elastic energy in the SSC, particularly in the gastrocnemius muscle. This is due to the ratio of its tendon length to fascicle lengths. The gastrocnemius has relatively shorter fascicle and longer tendon lengths than other muscles involved in typical locomotion (Figure 3) (25).
In this context, Kubo et al. (42) investigated the in vivo dynamics of the human medial gastrocnemius muscle-tendon complex during an ankle bending movement. The movement consisted of dorsiflexion followed by plantar flexion at 2 different speeds of lengthening in the dorsiflexion stage. It was found that the tendon length increased to a greater extent in the fast lengthening condition than the slow lengthening condition. In the fast condition, no significant change was found in the fascicle length during the first half of plantar flexion, whereas a rapid change in shortening occurred in the tendinous tissue. Both the tendon and muscle rapidly shortened in the second half, particularly the final 10° of plantar flexion. This suggests that during the transition from countermovement to shortening, the muscle is contracting relatively isometrically. This occurrence allows individuals to avoid the lowered force output that occurs with increasing velocity and is evidenced in a number of studies, which indicate that isometric contraction force can far exceed concentric force output (66).
In a study investigating the tendon interaction with the medial gastrocnemius muscle during walking, Fukunaga et al. (25) found that during the stance phase, participants' medial gastrocnemius muscle contracted isometrically, whereas the tendon lengthened by 7 mm. During the push-off phase, both the tendon and muscle rapidly shortened. This was confirmed by Ishikawa et al. (37) who found that the Achilles tendon during walking lengthened slowly throughout the single-stance phase and then recoiled rapidly close to the end of the ground contact phase. Also of interest was that the medial gastrocnemius was lengthened during the early stance phase, whereas the soleus continued to lengthen until the end of the stance phase. Finally, both muscles rapidly shortened on the toe off. The authors suggest that this action can be likened to catapult like mechanical behavior. The results also indicate that muscle tissue may be lengthened closer to optimal filament overlap, followed by an isometric contraction (26).
In contrast, Kurokawa et al. (44) found that without a countermovement during the vertical jump, the fascicles of the gastrocnemius shortened by 26%, whereas the tendons lengthened by 6%. This suggests that a great deal of the energy produced by the contraction was first delivered to the tendinous tissues and that the muscle tissue had greater shortening, which may have reduced its capacity to generate force due suboptimal filament overlap
In summary, the above studies indicate that during the countermovement, tendinous and muscle tissues are stretched, whereas during the contraction phase, tendons rapidly shorten as muscle tissue is able to contract in a quasi-isometric state while in a more optimal position to produce contractile force.
How Stiffness is Defined in Exercise Literature
An important subject when analyzing elasticity is the concept of stiffness. Stiffness has its origin in physics and can be defined as “the property of a system to resist an applied stretch” (46, p. 653). Stiffness can be described by Hooke's law, defined mathematically as follows (18): Fe = −kx (1.0), where Fe is elastic force, k is spring stiffness, and x is the amount of stretch or length. Hooke's law refers to deformable bodies affected by external forces. When external forces are not present, these bodies maintain a constant shape (46). However, in the presence of an external force, these bodies generate elastic force to oppose the external force and can store and return elastic energy (46).
How Stiffness Is Measured
Stiffness is the relationship between the deformation of an object and a given force (8). In biomechanics, stiffness can be used to model the force/deformation relationship of many structures from a single muscle fiber to entire limbs as a mass and spring. Most commonly, biomechanical investigations have observed joint stiffness or leg-spring stiffness.
Stiffness can be calculated by dividing the change in force by the change in length (ΔF/ΔL) of the system of interest (46). The stiffness of a specific joint is typically derived by analyzing kinetic and kinematic data and calculated as a change in joint moment (ΔM) divided by the change in joint angle (Δθ) during the braking phase of a gait such as running (46).
Leg-spring stiffness represents an integration of the stiffness of all lower limb musculoskeletal structures (including muscles, tendons, and ligaments acting across joints) during locomotion (16) and describes those structures' ability to interact in unison in a springlike fashion. The leg-spring stiffness model is highly representative of the mechanics of running gait and provides an appropriate means to analyze lower limb performance.
A spring-mass model is used to analyze leg-spring stiffness (Figure 4). This model consists of a single linear leg-spring and a point mass equivalent to body mass (8). The stiffness of this spring-mass model is composed of 2 components: vertical stiffness (Kvert) and overall leg-spring stiffness (Kleg). Kvert describes the mechanism by which the downward velocity of the body is reversed during limb-ground contact. It describes the vertical motions of the center of mass during the ground contact phase of locomotion. It is calculated as the ratio of the peak vertical force in the spring (Fypeak) to the vertical displacement of the center of mass of the spring (ΔL) at the instant the leg spring is maximally compressed. Fypeak and ΔL both occur simultaneously in the mass-spring model. Importantly, in movement tasks where the leg-spring unit acts only vertically, such as jumping and hopping tasks (19), Kvert = Kleg.
When examining the scientific literature, those involved in strength and conditioning research need to be aware that leg-spring stiffness is modulated depending on the specific demands of the criterion task. Movements, which differ only slightly, can induce significant changes in leg-spring stiffness. Strength and conditioning practitioners must exercise caution when interpreting research results or when comparing laboratory-based results to their own data collected in field testing (22).
Stiffness Relative to the Series Elastic and Parallel Elastic Components
The contribution of stored elastic energy between SEC and PEC is reliant on the activity level of the musculature. During passive stretch, the stiffness of the PEC is less than 100 times a tendon at rest (46). Consequently, during relaxed movements, such as occurs during passive stretch, the tendons do not undergo large deformations; rather, the majority of deformation occurs in the PEC. In contrast, during active movement, the stiffness of the muscle tissue and its surrounding PEC far exceeds that of the tendon, reversing the target site for the storage of elastic energy (67).
An analysis of tendinous behavior is described by the tendon force-deformation curve (Figure 5). The curve graphically demonstrates that a tendon behaves as a nonlinear spring. Early on, very little force application is required to produce length changes in tendinous tissue (toe region). This earlier change in length is attributed to a geometric reorientation of tendon fibers (46). As the fibers achieve alignment and slack is taken up, the change in length requires more force and reaches a linear region thought to be related nearly completely by the elastic properties of collagenous fibers. Finally, if force continues to increase failure occurs.
Stiffness and Its Relationship to Human Performance
There appears to be a strong relationship between the amount of stiffness in a human system and various indexes of performance such as force output (64), velocity (43), and even running economy (16). Wilson et al. (64) found that individuals with greater musculotendinous stiffness had a greater rate of force development in concentric and isometric bench press tasks (Figure 6), as well as greater overall force in isometric bench press, compared to individuals with lower musculotendinous stiffness, whereas Kuitunen et al. (43) found that knee joint stiffness increased as running velocity increased.
Leg stiffness also appears to be related to running economy, which refers to the amount of energy used to perform a certain amount of work. Dalleau et al. (16) found a negative linear relationship between energy cost and leg stiffness, whereas Heise and Martin (30) found that as the aerobic demand of running increased, vertical leg-spring stiffness decreased and demonstrated that less economical runners use a more compliant leg spring in their running style during ground contact phases.
Mechanisms Through Which Stiffness Enhances Performance
Stiffness is strongly correlated to decreased ground contact time. For example, Arampatzis et al. (1) had 15 participants perform depth jumps at 3 different heights. After each jump, participants were told to jump as high as possible, but to do so faster than the previous jump. It was found that as ground contact time decreased, stiffness increased. Similarly Kuitunen et al. (43) found that ankle joint stiffness showed a negative correlation with ground contact time at all running speeds.
The decreased contact times are also related to a decrease in flexion in joints such as the knee and hip (43). The rationale is that during the stretching phase, work is done on the muscle, causing the tissue to absorb mechanical energy. The use of this energy, however, depends on how a muscle is being used. If a slow transition occurs, the energy stored will be dissipated as heat. However, decreased ground contact time will lead to a greater return of the elastic energy stored during the contact phase. In this context, Wilson et al. (63) investigated the effect of delays in countermovement during a 95% of maximum bench press in 12 male weightlifters. In condition 1, there was no delay between the downward and upward components, whereas a delay was imposed in condition 2 between the downward and upward components of the lift. Results indicated that the benefits derived from the stretch-shorten cycle had a half-life of 0.85 second and that by 1 second, the benefits dissipated by 55%.
Muscular stiffness can also enhance force transmission from muscular contraction through the tendinous system. According to the Hill (33) model (Figure 1), force produced by a muscle is first transmitted to the SEC. The slack must be stretched out of the SEC before it will transmit any force to the skeletal system. This delay in reactivity is evidenced in a concept known as the electromechanical delay (EMD). The EMD can be defined as the “time lag between an increase in electrical activity and the mechanical response of the muscle” (50, p. 540). The EMD is affected by the amount of slack present in the tendinous tissue. In this context, Muraoka et al. (50) investigated the effects of joint angle and tendon slack on the EMD in 7 healthy participants. Participants were told to lay prone with their leg muscles relaxed and foot tightly secured to a footplate. During the experiment on the dynamometer, participants were administered percutaneous electrical stimulation at 5 different ankle joint angles (range, −30 to 5°). It was found that tendon slack was continually taken up from −30 to −10° of dorsiflexion. There was no tendon slack from −10 to 5° of dorsiflexion. Results indicated that the EMD decreased from −30 to −10° (20%), with no significant differences between −10 to 5°. The rationale is that once tendon slack is taken up, no further decreases in EMD should be found. Interestingly enough, both Granata et al. (36) and Vos et al. (61) found that an increase in the joint angle of the knee did not decrease the EMD. However, Muraoka et al. (50) suggest that the discrepancies between the results of their study and the results of Granata et al. (27) and Vos et al. (61) can be attributed to tendon slack being taken up by the examined musculature at all experimental joint angles analyzed. Tendon slack in both studies was assumed to be taken up as passive knee torque was noted, whereas passive ankle torque was not found until −10° in the Muraoka et al. (50) study.
Mechanisms for the Control of Stiffness
Control of stiffness during various gaits is a function of both feedforward and feedback mechanisms (sensory) (48). Feedforward control would occur through direct input from the central nervous system to the working alpha motor neuron pool. Feedforward control was thought to be a major influence in the Arampatzis et al. (1) study, in which participants increased in vertical stiffness when told to decrease their ground contact times. During feedforward control, both alpha and gamma motor neurons are activated (55). Gamma motor neurons innervate the contractile component of spindle fibers. Therefore, an increase in gamma motor neuron activity will lead to increased tension and stretch of the equatorial region of the spindle fibers (55). This has 2 main effects: first, when spindle fibers are stretched, they depolarize and communicate directly with alpha motor neurons, thereby increasing the intrinsic activity of the musculature; second, a stretch of the spindle fibers makes them more excitable to any further displacement caused by movement. In this manner, it is thought that the gamma motor system heightens the sensitivity of various autogenic reflexes to rapid SSCs (55). Both feedforward and feedback mechanisms were illustrated by Kuitunen et al. (43), who found that as running speed increased, the preactivation of both the plantar flexors and knee extensors increased.
It was suggested that the preactivation of these muscles would increase the stiffness of the muscle tendon units to tolerate and absorb high-impact loads at the beginning of the contact phase. It also appeared that stretch reflex activity, particularly in the soleus muscle, increased, which is reflective of the role of feedback in motor control.
Similarly, Kyrolainen et al. (41) investigated EMG activity during varying running speeds in 17 young runners. The investigators found that coactivity of agonist and antagonist muscles increased in the preactivation and the breaking phases. Also of interest is that the extensor muscles, such as the vastus lateralis, decreased in activity during the toe off. It was suggested that this coordinated response would create a rebound phenomenon in which the active tension developed in the preactivation and breaking phases could be released passively in the toe-off phase. However, the hamstrings maintained activity throughout the toe-off phase, suggesting that they are critical for creating horizontal velocity during the push off and also to directing the stored elastic energy in the proper direction with unopposed antagonist activity from the hip flexors.
Factors Affecting Stiffness
One of the major mechanisms affecting leg and vertical stiffness is fatigue (17). Dutto and Smith (17) found that runners decreased vertical stiffness and stride frequency during a moderate-intensity treadmill run to exhaustion and that these 2 variables were positively correlated. The authors postulated that the runners, either consciously or unconsciously, altered their running kinematics to a longer stride length and hence decreased stride frequency to maintain the given speed. This was due to either local metabolic or central fatigue, resulting in a lowered capacity to maintain the original contraction velocities.
Stretching before exercise also appears to have an effect on muscular stiffness. Fowles et al. (24) found that 13 maximally tolerable passive stretches resulted in a 25% decrease in maximum voluntary contraction (MVC) of the plantar flexors immediately following the protocol and a depression in the ability of participants to activate motor units as inferred through decreased EMG activity. While activation had been restored by 15 minutes, MVC was still depressed by 12-15%. It was calculated that the early deficit (<15 minutes) was caused by 60% neural factors, whereas 99% of the decrement later (>30 minutes) was thought to be caused by factors intrinsic to the muscle. Muscle stiffness decreased by 27% directly after PSmax and was depressed by 14% by 15 minutes. It was suggested that the neural component was explained by inhibitory factors related to Golgi tendon organs and mechanoreceptor activation. A significant relationship (r = 0.62) was found between muscle stiffness and force generation, suggesting that stiffness may assist in torque production and stabilize a muscle to generate force.
A number of other studies have also demonstrated that acute static stretching can degrade performance in movements where success depends on maximal force or power output (2,15). These studies have demonstrated reduced musculotendinous stiffness (15), reduced muscle activation (15), decreases in force production (14), decreases in reaction times (2), and reduced performance in tasks such as countermovement jumps (13), maximal voluntary contractions, and balance-oriented tasks such as stabilizing on a wobble board (2). These studies required subjects to hold static stretches for 30-45 seconds before performing the tasks in question. This research strongly indicates that strength and conditioning coaches should not have athletes warm up in a manner that has them perform long, static stretches immediately before exercises that depend on maximal force and power production. However, whether static stretching followed by dynamic or ballistic warm-up exercises affect negatively force and power output has not been established. Static stretching may still hold a place in the athletic warm-up, but it should not be incorporated just before competition or training tasks that depend on maximal force and power production. More research is required in this area.
The Role of Stiffness in Injury Prevention and Causation
While increases in stiffness have been identified as key to high running velocities and improved running economy, too great a level of stiffness has been associated with a detrimental effect on performance and well-being (38). Increased levels of stiffness in the lower limbs have been identified in patients with cerebral palsy (CP). Fonseca et al. (23) investigated the dynamics of locomotion in children with CP. Kinematic data were collected for children with spastic hemiplegic CP during walking at self-selected speeds ranging from very slow to very fast. Greater levels of stiffness were observed on the affected side of hemiplegic CP children at all walking speeds. Holt et al. (34) observed similar findings in CP and nondisabled children. Granata et al. (27) found that patients with spastic CP demonstrated a significantly shorter EMD than age-matched, normally developing controls.
The high level of stiffness associated with spastic limbs in CP patients is typically characterized by a reduced force-generation capacity, a decreased stance time in affected limbs, and a reduced displacement of the center of mass during gait on the affected limb (23,34). The elevated stiffness is of detriment to performance with low muscle power output and reduced velocities of locomotion observed (23,34).
A number of authors have suggested that the stiffness of the lower limbs is limited in order to preserve the anatomical structures during ground contact phases (9,16). Too high a level of stiffness can increase the stress induced by impact forces on the musculoskeletal system and encourage injuries (9,49).
In healthy, nondisabled individuals, increased leg-spring stiffness can cause reduced flexion in the lower extremity and increased peak ground reaction and joint forces. Butler et al. (9) describe this combination of factors as often leading to increased loading rates, which are associated with an increased impact in the lower extremity. Increased peak forces, loading rates, and shock can contribute to a greater risk of bony injuries including knee osteoarthritis and stress fractures (9).
While high levels of stiffness may be associated with injury to the skeletal structure of the body, elevated muscle and joint stiffness may also play a positive role in soft-tissue injury prevention and rehabilitation (9,53). Houk (35) describes the regulation of stiffness as the first protective mechanism of the neuromuscular system to perturbation. Increases in musculotendinous stiffness would decrease the EMD, allowing muscles to generate tension more rapidly to counteract deleterious forces at joints. Increased stiffness therefore is associated with enhanced functional joint stability, as stiffer structures tend to resist sudden joint displacements more quickly and effectively (9,53).
Bonfim et al. (6) examined the latency of the musculature surrounding the knee in patients following anterior cruciate ligament (ACL) reconstruction. The latency of a muscle group can be described as the time between onset of a stimulus and the onset of the responsive muscular activation. A significantly longer latency of the musculature was detected in subjects' limb with a reconstructed ACL compared to their own uninvolved leg and the legs of an unaffected control group.
While the research of Bonfim et al. (6) does not directly consider stiffness, the longer latency in the knees with reconstructed ACLs demonstrates a delayed muscular response to perturbation. This results in a greater joint motion before muscular forces can equalize the deformation. The joint or the leg-spring system is then more compliant (less stiff) and more susceptible to injury.
Other authors have also associated decreased stiffness with an increased risk of injury to the ACL. Granata et al. (27) examined differences in leg-spring stiffness during hopping tasks between male and female groups. It was observed that at the same hopping frequencies, female subjects exhibited significantly less stiffness than their male counterparts. Similarly, Blackburn et al. (3) observed that males exhibited greater active and passive stiffness at the knee joint compared to females. As control of stiffness contributes to biomechanical stability (9,53), both groups of authors (3,27) suggested that the lower stiffness exhibited by females may contribute to the increased rate of ACL injury observed in females.
This hypothesis is further validated by the examination of stiffness throughout the female menstrual cycle. Higher incidences of ACL injury have been observed in females during ovulation (8). Bryan et al. (8) observed that during the ovulatory phase of menstruation, musculotendinous stiffness was significantly reduced in teenage female netball players. Bryan et al. (8) propose that the decrease in musculotendinous stiffness is a contributing factor to this increased injury incidence.
One mechanism to improve preactivation, stiffness, and muscular latency is neuromuscular training (21,36). In this context, balance and perturbation training have been proposed to be particularly effective as a preventive modality for ACL injury (36). The nature of balance training should progress from static to dynamic balance and become progressively more sport specific (21). Equipment such as rocker boards, stability balls, and inflatable discs can be used to simulate unstable surfaces. Perturbation training is a form of neuromuscular training that creates a sudden change in joint position, destabilizing the knee and inducing reflexive muscle activation that facilitate functional joint stability (21). Perturbation training is highly similar to balance training, with subjects now exposed to unexpected perturbations against which they must attempt to stabilize. A thorough review of balance and perturbation training and their role as a preventive mechanism for ACL injury has been presented in the National Strength and Conditioning Association's Strengthand Conditioning Journal (21).
A high degree of stiffness may assist in the prevention of soft-tissue injuries, but too much stiffness has been associated with skeletal injuries. It seems then that lower extremity stiffness must be optimized within an ideal range that allows for facilitation of successful performance while minimizing the risk of injury. However, Butler et al. (9) correctly assert that the precise relationship between stiffness and injury has not been well established. This is largely due to a dearth of prospective studies observing stiffness levels and injury rates over extended time periods and a lack of studies monitoring stiffness throughout the rehabilitative process.
Numerous studies indicate that elastic energy derived from rapid stretch-shorten cycles can enhance work, power, and efficiency in many activities. In order to maximize the storage and use of elastic energy, it is important to understand that the stretch-shorten cycle is natural in certain movements such as running and walking and learned in others such as the bench press, baseball throw, and slap shot in hockey (66).
In more natural movements such as running, elastic energy is used most efficiently at relatively higher levels of stiffness, which are optimized at higher stride frequencies (16). A major factor that affects stiffness and stride frequency is the level of fatigue of the athlete. At higher levels of fatigue, athletes' stiffness decreases, resulting in less efficiency of movement. A major implication is that a proper periodized training program should focus heavily on the preparatory phase in which the athlete's concern is to enhance his or her resistance to fatigue. Commonly, it has been advised to perform power-related training activities in the nonfatigued condition. When fatigued, neural adaptations are reduced and the magnitude of positive adaptation to the training stimulus is decreased. In the nonfatigued state, greater power outputs can be achieved so the training stimulus is thought to be optimal. However, in many competitive sporting activities, high-power outputs are required under fatigued conditions. So performing some power-oriented exercise when fatigued could enhance specificity of training. However, the safety of this practice has not been well established. With the neuromuscular system fatigued, there may be a reduction in the body's capacity to deal with impact loadings during activities such as plyometrics and power cleans or power snatches. Excessive, unattenuated impacts can cause chronic injury or performing complex tasks in the fatigued condition could cause increased likelihood of acute injury. Strength coaches should be aware that performing power activities in the fatigued state may offer a useful, specific training stimulus for many athletes, but currently more research in this area is required to determine exactly which power-oriented training activities are safe and effective to perform in the fatigued state.
In skilled movements, one strong implication concerns teaching athletes from an early training period the importance of transitioning from eccentric to concentric movements, as evidence suggests that greater time delays are associated with decreased reuse of stored energy. This may also apply to characteristic sticking points in exercises such as the squat. By decreasing the time in sticking points, athletes may elicit greater performances in their respective lifts. However, the athlete is cautioned to transition in a controlled manner so as to avoid possible injury.
One specific training tool for athletes working to improve their use of the stretch-shorten cycle is plyometric exercises. Plyometric exercises are exercises using rapid powerful movements that are preceded by a preloading countermovement that creates a stretch-shorten cycle of muscle (62). Plyometrics are used to train muscle to optimally use the stretch-shorten cycle to produce maximal force in as short an amount of time as possible (45). Examples of plyometric exercises are depth jumps, hopping activities, and bounding.
Plyometric training can increase musculotendinous stiffness (57) and promote effective use of the SSC. Plyometric exercise is increasingly used in training athletes involved in power-oriented sports. Such training has been demonstrated to increase power output (51,52), improve jumping performance (52), and improve running velocity (54) in athletic and nonathletic populations. Research has also demonstrated that through its effects on musculotendinous stiffness, plyometric training can increase energy efficiency during running and improve aerobic performance (57,58).
One important aspect, of which strength and conditioning practitioners should be aware, is that effective performance of plyometric exercises depends on the eccentric loading and timing with the concentric phase of the muscle contraction (62). Essentially, if the time between the eccentric and concentric phase of movement is kept minimal, the stretch-shorten cycle will be optimized and muscular contraction will be more powerful. During plyometric exercises, coaches should encourage rapid powerful movements that minimize the eccentric/concentric transition times of the activities from their athletes. This can be achieved by emphasizing the importance of minimizing ground contact times. In a practical sense, using training equipment such as contact mats, ground contact times can be measured and monitored. A simple calculation, such as the reactive strength index that divides the height jumped by the ground contact time during activities such as depth jumps, can provide practitioners with a strong indication of athletes' abilities to transition quickly from the eccentric to concentric contraction (65,67).
As discussed previously, elevated muscle and joint stiffness has been hypothesized to play a role in soft-tissue injury prevention and rehabilitation. Considering plyometric training can increase musculotendinous stiffness (57), plyometrics may be an appropriate training modality to incorporate into athletes' preparation as a preventive measure against injury. Research has suggested that a plyometric training intervention over the course of a season in athletes involved in sports with large jumping and cutting components may reduce the incidence of knee injury (32).
Following such research, many scholastic, collegiate, and Olympic level teams have developed plyometric programs as methods to improve performance and decrease injury risk in their athletes (29,32).
Stretching before activities has also been reviewed, and evidence indicates that long static stretches before high-intensity activities lowers performance. As such, strength and conditioning coaches are advised to avoid long static stretches before activities that require maximal power or strength output.
In summary, it is clear that athletes and strength and conditioning coaches alike can benefit from an understanding of the principles of elastic energy and its application to athletic performance. Through an understanding of these principles, athletes can be trained to exploit the benefits of countermovements before concentric portions of a given activity through specific training techniques such as plyometrics or through avoidance of contraindicating warm-up methods such as long static stretches before high-intensity performance events. Research implications provided from this review include the suggestion to further investigate the nature of the relationship between the level of stiffness and optimal performance, as well as protection against injuries.
The authors thank Dr. John Ostarello, California State University East Bay and President of the Western Society of Kinesiology and Wellness, for his inspiration and thoughtful insights into the manuscript.
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