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Lower Extremity Stiffness

Effects on Performance and Injury and Implications for Training

Brazier, Jon MSc1; Bishop, Chris MSc2; Simons, Chris MSc1; Antrobus, Mark MSc1; Read, Paul J. MSc, CSCS3; Turner, Anthony N. MSc, CSCS*D2

Author Information
Strength and Conditioning Journal: October 2014 - Volume 36 - Issue 5 - p 103-112
doi: 10.1519/SSC.0000000000000094
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Abstract

INTRODUCTION

Lower extremity stiffness is considered to be a key attribute in the enhancement of running, jumping, and hopping activities, which are prevalent in most sports (1,4,8,10,19,26,27,32,65). An athlete who can appropriately use greater stiffness characteristics will potentially store more elastic energy at landing and generate more concentric force output at push-off, possibly reducing the onset of fatigue and increasing running speed. Consequently, if a strength and conditioning (S&C) coach is able to advance their athletes' ability to act like a “stiff spring” across an array of sporting movement patterns, performance enhancement may occur.

There are several different classifications and calculations of lower extremity stiffness consisting of joint (Kjoint), vertical (Kvert), and leg (Kleg) stiffness (10), as well as muscle and tendon stiffness. The key terms defining lower extremity stiffness are described in Table 1. Kvert is generally regarded as the “reference” stiffness gauge; thus, models of Kleg and Kjoint have been established based on this (8,10,53,65). Kvert is commonly used to measure jumping and hopping tasks, whereas Kleg would be more appropriate when measuring walking and running tasks as the change in leg length can be measured for each stride. Furthermore, Kjoint is a fundamental measure for all lower extremity tasks, as the stiffness response at the applied joints will have an overall impact on Kvert and Kleg.

T1-12
Table 1:
Classifications and definitions of lower extremity stiffness

Practical limitations existed in the original techniques used to determine leg stiffness, as the use of a force plate was required. However, subsequent research from Dalleau et al. (15) has reported strong validity with the criterion measure using a jump mat by the calculation of flight time and ground contact time, whereby both submaximal and maximal hopping were significantly correlated (r = 0.94 and 0.98). Thus, measurement and monitoring of leg stiffness is now practically viable for the S&C coach. The aim of this article is to review the literature on lower extremity stiffness (Kvert, Kjoint, Kleg) and its effect on both performance and injury. This will provide S&C coaches with practical and purposeful methods for enhancing and measuring appropriate levels of lower extremity stiffness within a field environment.

UNDERPINNING SCIENCE

The concept of stiffness is founded on Hooke's law, which asserts that the force required to deform an object is correlated to a proportionality constant (spring) and the extent that object is deformed (8,65). Simply put, stiffness is the relationship between the deformation of an object in response to an applied force. From a practical perspective, for optimal running, jumping, and hopping performance, an appropriate level of lower extremity stiffness is required to absorb ground reaction forces (GRFs), as well as to store and reuse elastic energy (42). Stiffness is measured as the quotient of force to length and in the human body can be quantified from the level of a single muscle fiber to the modeling of the entire body as a mass-spring (8,10). Therefore, stiffness in the human body, or body parts, portrays its capability to withstand displacement once GRFs are applied and can be defined as the ratio between peak GRFs and peak displacement of the center of mass (15,48). This will require the interaction of muscles, tendons, ligaments, cartilage, and bone to oppose deformation to GRF or joint moments (8,10,63,65).

To create a model for lower extremity stiffness that accounts for all the anatomical structures, muscle reflex time delays, and central nervous system control is very complex and becomes impractical (10). Therefore, a much simpler spring-mass model has been developed to provide an estimate of lower extremity stiffness (2,3,21,31–37,53), comprising a mass and a weightless Hookean spring (Figure). In the human body, the mass represents body mass, and the spring represents the lower extremity, with stiffness recorded in the vertical direction (8). If no pressure is applied to the spring, it will accumulate no potential energy and therefore develop zero force. Conversely, if a force is applied to the spring causing a deformation, it will store elastic energy that can then be returned and reused as the spring shortens and returns to its normal resting length (52). The spring-mass model has been used to assess a variety of whole-body movements that involve the stretch-shortening cycle (SSC) including hopping, jumping, and running (8,52,65).

F7-12
Figure:
Spring-mass model behavior in relation to the human body. k = spring stiffness; m = mass; x = downward displacement.

LOWER EXTREMITY STIFFNESS AND PERFORMANCE

The relationship between stiffness (Kvert, Kjoint, Kleg) and performance is multifaceted and frequently misinterpreted. Numerous studies report that lower extremity stiffness increases with hopping height (19) and running velocity (26,27). However, it is argued that leg compliance (increased joint range of motion and elongation of muscles, tendons, and soft tissues) should be enhanced to improve running and jumping performance, as Kleg has been shown to stay constant (27,53) or even decrease with increasing running speeds (5). Brughelli and Cronin (8) suggest that more compliance will increase the storage and utilization of elastic energy during the SSC, therefore providing more concentric force output at push-off and potentially reducing the onset of fatigue. Consequently, it seems that there is a compromise between stiffness and compliance during athletic tasks.

There is conflicting evidence within the available literature as to how certain joints affect and regulate lower extremity stiffness. Numerous studies have shown that knee joint stiffness plays a significant role (1,3,4,18,36,42,65), several studies have indicated that ankle stiffness is more essential (20,21,54,63), and some were equivocal (26,34). Reported differences arise from the fact that depending on the task and velocity, the ankle and knee joint will vary in their contribution to lower extremity stiffness (65). However, recent research implies that with increases in hopping frequency there is a greater emphasis on ankle stiffness (35,43).

Sufficient leg stiffness is also a key requirement for effective storage and reutilization of elastic energy in SSC actions (10,46). A review by McMahon et al. (52) found that in vertical jumping and hopping tasks, increased Kvert and Kleg were related to increased vertical GRFs (1,4), increased ground contact frequency (19,25) (i.e., they achieved a higher number of hopping contacts), and to shorter ground contact times (1,4,19). Furthermore, Arampatzis et al. (1) identified a concomitant increase in lower extremity stiffness with reduced ground contact times, as demonstrated by a strong correlation (DJ20 cm: r = −0.82 and DJ40 cm: r = −0.89) during a rebound drop-landing task. Thus, lower extremity stiffness is augmented with increases in drop jump height because of the greater eccentric forces experienced (1,3,4,25,42,66,69).

Conversely, research by Wang (72) found that increased impact loads from drop landings of 40 and 60 cm actually reduced Kleg and when impact loads were increased from drop landings of 60–80 cm, Kleg did not decrease while knee Kjoint increased significantly. Wang (72) highlights that the neuromuscular system has a reaction time of 50 milliseconds, and if the human body cannot initiate a reaction time to absorb impact forces before 50 milliseconds, the chances of injury are high. The study (72) found that the time-to-peak vertical GRF from a drop landing of 80 cm was 40 milliseconds, which suggests that modulation by the human body is insufficient to buffer landing impact. Thus, reduced leg stiffness with accumulating impact load may decrease the risk of future injury, particularly knee joint injury. This notion was reinforced by Laffaye et al. (45) as Kleg decreased with jumping height, advocating that a more compliant leg approach was beneficial to achieving greater jumping heights and faster ground contact times. As the lower limb joints used a larger range of motion (i.e., more compliant), a greater power output could be produced (45).

EFFECTS ON RUNNING PERFORMANCE

Numerous studies have highlighted that as the speed/force demands of physical activities escalate, there is a concomitant increase in stiffness (3,42,66,69). He et al. (27), Morin et al. (53), and Cavagna et al. (11) found that Kvert specifically increased with running speeds while Kleg stayed constant. Furthermore, Stefanyshyn and Nigg (69) established that with increases in running speed there was a substantial increase in ankle Kjoint.

Brughelli and Cronin (8) found comprehensive evidence to suggest that Kvert increases with running speed up to moderate intensities and that Kleg remains constant. However, as speed increased above moderate intensities (5 m/s to maximum), Kleg increased by 60%, potentially suggesting that Kleg is important during running at higher velocities. Further evidence by Bret et al. (7) and Souhaiel and Christian (67) found that Kvert was interrelated with maximal velocity running, and Bret et al. (7) specifically identified that Kvert was significantly correlated with the second and third phases of the 100-m sprint (second phase 30–60 m, third phase 60–100 m) in national level male sprinters (r = 0.66, p < 0.01).

The enhancement of lower extremity stiffness is also a key factor for improving endurance running performance. This was highlighted by Hobara et al. (34), who showed that endurance-trained athletes had greater Kleg and more specifically, greater knee and ankle stiffness than participants from the general population during repeated 5-jump maximal hopping tasks. Furthermore, these results suggest that increases in leg stiffness are likely a result of training-induced adaptations.

LOWER EXTREMITY STIFFNESS AND INJURY

Lower extremity stiffness has been shown to be important for optimal athletic performance (8,10,65); however, there is evidence to suggest that too much stiffness may result in a higher incidence of injury (10,23,51,59,62,72,76). A direct correlation between lower extremity stiffness and lower-body injury has not definitively been established because of a lack of studies. However, extreme levels of lower extremity stiffness have been related to reduced joint motion and increased shock and peak forces in the lower extremity, whereas too low a level of stiffness has been associated with excessive joint motion (10,23).

Williams et al. (74) studied leg stiffness measures between high-arched runners and low-arched runners. High-arched runners were found to have increased leg stiffness and vertical loading rates compared with low-arched runners, potentially leading to the greater incidence of bony injuries compared with low-arched runners (75). Furthermore, earlier research by Williams et al. (75,76) found that runners with low arches and decreased leg stiffness incurred more soft tissue injuries than high-arched runners.

Pruyn et al. (62) studied the bilateral differences in leg stiffness of a professional Australian rules football league (AFL) team across a whole season to see if this was related to lower-body soft tissue injuries. The injured players demonstrated a significantly higher mean bilateral difference in leg stiffness than the noninjured group across the season (p = 0.05). Similarly, Watsford et al. (73) found that there was a propensity for greater bilateral differences in leg stiffness scores for AFL players who went on to suffer hamstring injury during the season compared with noninjured players. However, hamstring stiffness was significantly higher in the noninvolved limb of the injured players. This study may suggest that reduced hamstring stiffness could be associated with increased injury levels, but as the injured players tended to be significantly older than noninjured players, these findings may be contested.

Contrary evidence by Granata et al. (25) and Williams et al. (75,76) suggests that too little stiffness allows for extreme joint motion therefore leading to soft tissue injury. Granata et al. (25) reported that females exhibited less knee stiffness than males during hopping, advocating that this reduced stiffness may explain the higher incidence of knee ligament injuries sustained by females. Research by Maquirriain (51) found that athletes who had suffered unilateral Achilles tendinopathy presented significantly reduced leg stiffness even after full symptomatic recovery, highlighting that full recovery of muscle-tendon function only occurred in 25% of athletes tested, so the majority had reduced performance. Therefore, it seems that there is an “optimal” level of lower extremity stiffness. Too much can lead to high levels of peak forces and loading rates, which can contribute to greater risk of bony injuries such as stress fractures and knee osteoarthritis (10), whereas too low levels of stiffness have been associated with possible soft tissue injury (23).

EFFECTS OF FATIGUE

Padua et al. (59) investigated the effects of fatigue on Kvert and stiffness control strategies in males and females. Kvert was not affected after fatigue; however, several different control strategies were used to maintain Kvert. Both males and females used an ankle-dominant strategy where greater reliance was placed on the ankle musculature (increased gastrocnemius and soleus peak activation) than the knee musculature (decreased hamstring peak activation). This increased ankle musculature activation may increase the risk of anterior cruciate ligament (ACL) injury, as the gastrocnemius is an antagonist of the ACL and is capable of increasing ACL strain (24). All subjects also used antagonist inhibition strategies by minimizing antagonist coactivation to maintain Kvert, as hamstring and tibialis anterior peak activation was reduced, whereas quadriceps, gastrocnemius, and soleus peak activation was maintained or increased. Thus, a reduction in knee flexor coactivation leads to a quadriceps-dominant strategy and, therefore, increases the load on the ACL due to increased proximal anterior tibial shear forces (6,24,30). This antagonist inhibition strategy was considerably more apparent in females than in males and could go someway to explaining the higher incidence of ACL injuries in females.

Howatson (38) investigated readiness to reperform by measuring electromechanical delay (EMD), which provides information on muscle function changes after exercise interventions. It was reported that after high-volume eccentric training, EMD was significantly greater at 96 hours after exercise and creatine kinase and muscle soreness were significantly elevated, suggesting that optimal recovery had not been achieved and neuromuscular mechanisms could be compromised, leading to altered motor control strategies. This has implications for S&C coaches, particularly in regard to the management of volume load and prescription of training during ballistic and plyometric exercises. Specifically, if the athlete has not fully recovered from periods of high-volume eccentric training, motor control strategies can become altered through a delay in the muscle feedback response, which potentially increases their risk of injury.

TRAINING CONSIDERATIONS FOR LOWER EXTREMITY STIFFNESS

Several papers have stated adaptations in lower extremity stiffness after exercise interventions, such as plyometrics (9,41,68), eccentric strength (61), isometric (9), and general weight (41) training. It has been well established that plyometric training improves sprinting, jumping, and ballistic capabilities (68,70). Plyometric training has also been shown to improve biomechanical technique and neuromuscular control during landing and cutting activities (12,13,29,39,47,55–57), as well as having the potential to reduce lower extremity injuries in team sports (29,49,58,60). There is also evidence to suggest that by performing plyometric training, participants can intentionally alter their stiffness during landing to change the impact forces through the body (4,16,17,78).

Hewett et al. (29) instigated a jump-training program that focused on teaching participants to land “softer” (adopting a more flexed knee position to increase hamstring activation and, thus, reduce ACL load). After the 6-week intervention, participants were able to decrease peak vertical GRFs and therefore potentially reduced the risk of injury. Furthermore, Hewett et al. (28) investigated the effect of a jump-training program on knee injury rates in female football, basketball, and volleyball players compared with a control group. The incidence of injury in female athletes who took part in the jump-training program was significantly lower because of their ability to decrease the stiffness (achieve optimal knee flexion) of their landings. Therefore, an athlete's ability to consciously control their lower extremity stiffness can possibly result in a lower chance of injury. Other examples from the literature include the work of Kryolainen et al. (44) who reported that 4 months of plyometric training increased the preactivity of muscles (vastus medialis, vastus lateralis, gastrocnemius, soleus, and tibialis anterior) leading to increased musculotendon stiffness and improved intermuscular coordination. Also, Chimera et al. (13) established that plyometric training might reduce injury rates by improving functional joint stability (Kjoint) in the lower extremities. Therefore, these results suggest that appropriate jump-landing interventions can elicit favorable changes in neuromuscular control and landing biomechanics, which will subsequently reduce joint loading.

Komi (40) proposed that higher stiffness levels in lower extremity muscles during SSC exercises led to an advantage in terms of the larger amount of stored and reused elastic energy. Kubo et al. (41) added further evidence to this by reporting an increase of 63.4% in ankle Kjoint assessed during drop jumps. Kubo et al. (41) suggest that plyometric training significantly increased maximal Achilles tendon elongation and the amount of stored elastic energy, which led to improved SSC jumping performance as confirmed by Wu et al. (77). This, therefore, implies that plyometric training and potentially improving ankle Kjoint also conceivably improve the compliance of the Achilles tendons allowing more elastic energy to be stored and used during athletic performance.

Markovic and Mikili (50) carried out a comprehensive review on the performance adaptations from lower extremity plyometric training, proposing that adaptive changes in neuromuscular function are likely the result of (a) an increased neural drive to the agonist muscles, (b) changes in the muscle activation strategies (i.e., improved intermuscular coordination), (c) changes in the mechanical characteristics of the muscle-tendon complex of plantar flexors, (d) changes in muscle size and/or architecture, and (e) changes in single-fiber mechanics. Therefore, there are numerous mechanisms that plyometric training can affect, which will impact stiffness and compliance performance. In particular, improved intermuscular coordination strategies leading to improved quadriceps-to-hamstring coactivation ratios (50), which could potentially reduce injury rates. Furthermore, there is reported evidence to suggest that short-term plyometric training on nonrigid surfaces (i.e., sand-based or water-based surfaces) can stimulate similar increases in sprinting and jumping performance as traditional plyometric training on rigid surfaces, but with considerably less muscle soreness (50). This could, therefore, substantially reduce the amount of training stress and potentially aid in the prevention of overtraining.

Strength and power training has also been shown to affect lower extremity stiffness. As Cormie et al. (14) established that a 10-week back squat training protocol performed at 75–90% of 1 repetition maximum (RM) significantly increased Kleg during jump squat performance. Furthermore, it was reported that a comparative training group who performed jump squats using 0–30% 1RM increased Kleg during bodyweight jump squat performance. The increased Kleg after back squat training was credited to increased strength, whereas the increased Kleg after 0–30% 1RM jump squat training was attributed to greater SSC utilization through increased eccentric loading leading to enhanced concentric force output.

The combination of weight training and plyometric training together may have greater potential to advance jumping and sprinting performance through improvements in lower extremity stiffness than plyometric training alone. This was evident in a study by Toumi et al. (70) who identified that when the leg press exercise was combined with a plyometric exercise, Kleg assessed during countermovement jump performance was significantly increased after training. This may in part be because of different mechanistic training responses of the 2 modalities as highlighted by Kubo et al. (41) who reported that plyometric training improved concentric and SSC jump performance mostly through changes in mechanical properties of the muscle-tendon complex, whereas weight training produced changes in concentric-only jump performance as a result of increased muscle hypertrophy and neural activation of plantar flexors.

These findings are further substantiated in a recent meta-analysis by Saez-Saez de Villarreal et al. (64) who suggested that the ideal plyometric strategy is to (a) combine weight training and plyometric training, (b) use a training intervention duration of <10 weeks (with >15 sessions), and (c) use high-intensity exercises with >40 jumps per session. However, a precautionary note should be added to the recommendation for >40 jumps per session as the eccentric loading, for example, between 40 drop jumps and 40 ankle jumps is much higher and therefore would need to be accounted for in programming. Thus, although these general guidelines provide some insight, the research is currently equivocal as to effective plyometric program design. Most studies imply that moderate training frequency (2–3 sessions) and short-term interventionist (6–15 weeks) can change the stiffness of various elastic components of the muscle-tendon complex of plantar flexors and improve lower extremity strength, power, and SSC muscle function (50).

FIELD TESTING

A practical way of monitoring lower extremity stiffness levels in athletes in a field environment would be to use a contact mat and measure ground contact time and flight time. Athletes perform submaximal hopping at 2.5 Hz to ensure that movement patterns are reflective of typical spring-mass model behavior (15,48), and a metronome can be used to keep the rhythm of hopping. The contact and flight time data can then be used to calculate leg stiffness (peak GRF/peak displacement of the center of mass) based on the equation proposed by Dalleau et al. (15), which has been established as a valid and reliable measure. Within the following equation, Kn refers to leg stiffness, M is the total body mass, Tc is equal to ground contact time, and Tf represents flight time:

An easy and practical way of measuring fast SSC contact times (<0.25 seconds) would be to use the reactive strength index. This is calculated by dividing the height jumped by the time in contact with the ground before take off (22). The score can be improved by either increasing height jumped or minimizing time spent in contact with the ground but ideally both. The data collated from leg stiffness and reactive strength index results will help S&C coaches identify the lower extremity capabilities of their athletes and help in planning a constructive program that should enable sufficient levels of stiffness to be met relative to the athlete's physiological performance needs at tendon, muscle, and joint level.

PRACTICAL APPLICATIONS

It is important that S&C coaches take a progressive approach to developing “optimal” levels of lower extremity stiffness in their athletes. This should be performed through well-structured periodized programming that focuses on developing all the physiological adaptations required for lower extremity stiffness. A further consideration is that a fundamental level of strength is required to develop knee/hip extensor strength and increase tendon stiffness before the more demanding power and high eccentric loading SSC activities are introduced. The development of weightlifting exercises and their derivatives will be fundamental when programming, and an emphasis on the power catch position for the snatch/clean may provide development of stiffness capabilities about the knee.

Flanagan and Comyns (22) provide a comprehensive breakdown for optimizing fast SSC training to enhance stiffness responses. They recommend following a 4-step progressive plan: Phase 1: eccentric jumping focusing on landing mechanics, quiet landings, and freezing on contact; phase 2: low-intensity fast plyometrics focusing on ankling/skipping, with short contact times and legs acting like stiff springs; phase 3: hurdle jumping, emphasis on short contact time and some degree of jump height, and contact time is used as a feedback tool; and phase 4: depth jumping, short contact times with maximal jump height, “jump fast, jump high.” For a more comprehensive review of developing plyometric exercises, refer to Flanagan and Comyns (22) and Turner and Jeffreys (71). Some general guidelines for exercises that can develop ideal lower extremity stiffness are given in Table 2; Tables 3–5 are examples of base conditioning, strength, and power training sessions to enhance lower extremity stiffness based on the findings from this review. A precautionary note should be added; as these are generic examples that emphasize lower extremity stiffness and have not taken account of specific sporting parameters. The needs, abilities, and sports performance requirements of each athlete should be accounted for when implementing lower extremity stiffness into program design.

T2-12
Table 2:
Guidelines for exercises to develop lower extremity stiffness
T3-12
Table 3:
Example base conditioning session to enhance lower extremity stiffness
T4-12
Table 4:
Example strength training session to enhance lower extremity stiffness
T5-12
Table 5:
Example power training session to enhance lower extremity stiffness

CONCLUSIONS

Lower extremity stiffness has been shown to enhance athletic performance through improvements in running, jumping, and hopping tasks (1,4,8,10,19,26,27,32,65), as well as reducing the incidence of soft tissue injuries (10,23). However, there is growing evidence that too much stiffness is injury inducing (10,23,59,62,72,73,76). Thus, the compromise between gaining ample levels of lower extremity stiffness and also the ability to have a compliant range of motion when needed is not simple to train and will require a multifaceted approach from S&C coaches. Concurrent training methods to improve stiffness and compliance should be applied. For example, when coaching landing mechanics, “stiff” leg landings with minimal joint motion should be taught. However, “soft” landings with optimal knee flexion to absorb heavy forces should also be implemented into the program, thus training the athlete to be able to adapt lower extremity stiffness when required. This will develop their intermusclar coordination and thus potentially reduce the chances of noncontact injuries. Once these movement patterns have been mastered and adequate levels of strength gained, progressions to power and high eccentric loading SSC plyometric training can gradually be made, allowing for further developments in lower extremity stiffness but also improved compliance through SSC mechanics.

REFERENCES

1. Arampatzis A, Bruggemann GP, Klapsing GM. Leg stiffness and mechanical energetic processes during jumping on a sprung surface. Med Sci Sports Exerc 33: 923–931, 2001.
2. Arampatzis A, Bruggemann GP, Klapsing GM. A three dimensional shank-foot model to determine the foot motion during landings. Med Sci Sports Exerc 34: 130–138, 2002.
3. Arampatzis A, Bruggemann GP, Metzler V. The effect of speed on leg stiffness and joint kinetics in human running. J Biomech 32: 1349–1353, 1999.
4. Arampatzis A, Schade F, Walsh M, Bruggemann GP. Influence of leg stiffness and its effect on myodynamic jumping performance. J Electromyogr Kinesiol 11: 355–364, 2001.
5. Avogadro P, Chaux C, Bourdin M, Dalleau G, Belli A. The use of treadmill ergometers for extensive calculation external work and leg stiffness during running. Eur J Appl Physiol 92: 182–185, 2004.
6. Beynnon BD, Johnson RJ, Flemming BC, Stankewich CJ, Renstrom PA, Nichols CE. The strain behaviour of anterior cruciate ligament during squatting and active flexion-extension: A comparison of an open and a closed kinetic chain exercise. Am J Sports Med 25: 823–829, 1997.
7. Bret C, Rahmani A, Dufour AB, Messonnier L, Lacour JR. Leg strength and stiffness as ability factors in 100m sprint running. J Sports Med Phys Fitness 42: 274–281, 2002.
8. Brughelli M, Cronin J. Influence of running velocity on vertical, leg and joint stiffness. Sports Med 38: 647–657, 2008.
9. Burgess KE, Connick MJ, Graham-Smith P, Pearson SJ. Plyometric training vs. isometric training influences on tendon properties and muscle output. J Strength Cond Res 21: 986–989, 2007.
10. Butler RJ, Crowell HP III, Davis IM. Lower extremity stiffness: Implications for performance and injury. Clin Biomech 18: 511–517, 2003.
11. Cavagna G, Heglund N, Williams P. Effect of an increase in gravity on the power output and the rebound of the body in human running. J Exp Biol 208: 2333–2346, 2005.
12. Chappell JD, Limpisvasti O. Effect of neuromuscular training program on the kinetics and kinematics of jumping tasks. Am J Sports Med 36: 1081–1086, 2008.
13. Chimera NJ, Swanik KA, Swanik CB, Straub SJ. Effects of plyometric training on muscle-activation strategies and performance in female athletes. J Athl Train 39: 24–31, 2004.
14. Cormie P, McGuigan MR, Newton RU. Changes in the eccentric phase contribute to improved stretch-shorten cycle performance after training. Med Sci Sports Exerc 42: 1731–1744, 2010.
15. Dalleau G, Belli A, Viale F, Lacour JR, Bourdin M. A simple method for field measurements of leg stiffness in hopping. Int J Sports Med 25: 170–176, 2004.
16. Devita P, Skelly WA. Effects of landing stiffness on joint kinetics and energetics in the lower extremity. Med Sci Sports Exerc 24: 108–115, 1992.
17. Dufek JS, Bates BT. The evaluation and prediction of impact forces during landings. Med Sci Sports Exerc 22: 370–377, 1990.
18. Dutto DJ, Braun WA. DOMS-associated changes in ankle and knee joint dynamics during running. Med Sci Sports Exerc 36: 560–566, 2004.
19. Farley CT, Blickman R, Saito J, Taylor CR. Hopping frequency in humans: A test of how springs set stride frequency in bouncing gaits. J Appl Physiol 71: 2127–2132, 1991.
20. Farley CT, Gonzalez O. Leg stiffness and stride frequency in human running. J Biomech 29: 181–186, 1996.
21. Farley CT, Morgenroth DC. Leg stiffness primarily depends on ankle stiffness during human hopping. J Biomech 32: 267–273, 1999.
22. Flanagan EP, Comyns TM. The use of contact time and the reactive strength index to optimize fast stretch-shortening cycle training. Strength Cond J 30: 32–38, 2008.
23. Flanagan EP, Galvin L, Harrison AJ. Force production and reactive strength capabilities after anterior cruciate ligament reconstruction. J Athl Train 43: 249–257, 2008.
24. Flemming BC, Renstrom PA, Ohlen G, Johnson RJ, Peura GD, Beynnon BD, Badger GJ. The gastrocnemius is an antagonist of the anterior cruciate ligament. J Orthop Res 19: 1178–1184, 2001.
25. Granata KP, Padua DA, Wilson SE. Gender differences in active musculoskeletal stiffness. Part II. Quantification of leg stiffness during functional hopping tasks. J Electromyogr Kinesiol 12: 127–135, 2001.
26. Gunther M, Blickman R. Joint stiffness of the ankle and the knee in running. J Biomech 35: 1459–1474, 2002.
27. He JP, Kram R, McMahon TA. Mechanics of running under simulated low gravity. J Appl Physiol 71: 863–870, 1991.
28. Hewett TE, Lindenfield TN, Riccobene JV, Noyes FR. The effect of neuromuscular training on the incidence of knee injury in female athletes: A prospective study. Am J Sports Med 27: 699–705, 1999.
29. Hewett TE, Stroupe AL, Nance TA, Noyes FR. Plyometric training in female athletes: Decreased impact forces and decreased hamstring torques. Am J Sports Med 24: 765–773, 1996.
30. Hirokawa S, Solomonow M, Lu Y, Lou Z, D'Ambrosia R. Anterior-posterior and rotational displacement of the tibia elicited by quadriceps contraction. Am J Sports Med 20: 299–306, 1992.
31. Hobara H, Inoue K, Kanosue K. Effect of hopping frequency on bilateral differences in leg stiffness. J Appl Biomech 29: 55–60, 2013.
32. Hobara H, Inoue K, Muraoka T, Omuro K, Sakamoto M, Kanosue K. Leg stiffness adjustment for a range of hopping frequencies in humans. J Biomech 43: 506–511, 2010.
33. Hobara H, Kanosue K, Suzuki S. Changes in muscle activity with increase in leg stiffness during hopping. Neurosci Lett 418: 55–59, 2007.
34. Hobara H, Kimura K, Omuro K, Gomi K, Muraoka T, Iso S. Determinants of difference in leg stiffness between endurance- and power-trained athletes. J Biomech 41: 506–514, 2008.
35. Hobara H, Kimura K, Omuro K, Gomi K, Muraoka T, Sakamoto M. Differences in lower extremity stiffness between endurance trained athletes and untrained subjects. J Sci Med Sport 13: 106–111, 2010.
36. Hobara H, Muraoka T, Omuro K, Gomi K, Sakamoto M, Inoue K, Kanosue K. Knee stiffness is a major determinant of leg stiffness during maximal hopping. J Biomech 42: 1768–1771, 2009.
37. Hobara H, Tominaga S, Umezawa S, Iwashita K, Okino A, Saito T, Usui F, Ogata T. Leg stiffness and sprint ability in amputee sprinters. Prosthet Orthot Int 36: 312–317, 2012.
38. Howatson G. The impact of damaging exercise on electromechanical delay in bicep brachii. J Electrmyogr Kinesiol 20: 477–481, 2010.
39. Irmischer BS, Harris C, Pfeiffer RP, Debeliso MA, Adams KJ, Shea KG. Effects of knee ligament injury prevention exercise program on impact forces in women. J Strength Cond Res 18: 703–707, 2004.
40. Komi PV. Stretch-Shortening Cycle. London, United Kingdom: Blackwell Science, 1992. pp. 169–179.
41. Kubo K, Morimoto M, Komuro T, Yata H, Tsuunoda N, Kanehisa H. Effects of plyometric and weight training on muscle-tendon complex and jump performance. Med Sci Sports Exerc 39: 1801–1810, 2007.
42. Kuitunen S, Komi PV, Kyrolainen H. Knee and ankle joint stiffness in sprint running. Med Sci Sports Exerc 34: 166–173, 2002.
43. Kuitunen S, Ogiso K, Komi PV. Leg and joint stiffness in human hopping. Scand J Med Sci Sports 21: 159–167, 2011.
44. Kyrolainen H, Komi PV, Kim DH. Effects of power training on neuromuscular performance and mechanical efficiency. Scand J Med Sci Sports 1: 78–87, 1991.
45. Laffaye G, Bardy BG, Durey A. Leg stiffness and expertise in men jumping. Med Sci Sports Exerc 37: 536–543, 2005.
46. Latash ML, Zatsiorsky VM. Joint stiffness: Myth or reality? Hum Mov Sci 12: 653–692, 1993.
47. Lephart SM, Abt JP, Ferris CM, Sell TC, Nagai T, Myers JB, Irrgang JJ. Neuromuscular and biomechanical characteristic changes in high school athletes: A plyometric versus basic resistance program. Br J Sports Med 39: 932–938, 2005.
48. Lloyd RS, Oliver JL, Hughes MG, Williams CA. Age related differences in the neural regulation of stretch-shortening cycle activities in male youths during maximal and sub-maximal hopping. J Electromyogr Kinesiol 22: 37–43, 2012.
49. Mandelbaum BR, Silvers HJ, Watanabe DS, Knarr JF, Thomas SD, Griffin LY, Kirkendall DT, Garrett W. Effectiveness of a neuromuscular and proprioceptive training program in preventing anterior cruciate ligament injuries in female athletes: 2-year follow up. Am J Sports Med 33: 1003–1010, 2005.
50. Markovic G, Mikulic P. Neuro-musculoskeletal and performance adaptations to lower extremity plyometric training. Sports Med 40: 859–895, 2010.
51. Marquirriain J. Leg stiffness changes in athletes with achilles tendinopathy. Int J Sports Med 33: 567–571, 2012.
52. McMahon JJ, Comfort P, Pearson S. Lower limb stiffness: Effect on performance and training considerations. Strength Cond J 34: 94–101, 2012.
53. Morin J, Dalleau G, Kyrolainen H, Jeannin T, Belli A. A simple method for measuring stiffness during running. J Appl Biomech 21: 167–180, 2005.
54. Muller R, Grimmer S, Blickman R. Running on uneven ground: Leg adjustments by muscle pre-activation control. Hum Mov Sci 29: 299–310, 2010.
55. Myer GD, Ford KR, Brent JL, Hewett TE. The effects of plyometric vs. dynamic stabilization and balance training on power, balance, and landing force in female athletes. J Strength Cond Res 20: 345–353, 2006.
56. Myer GD, Ford KR, McClean SG, Hewett TE. The effects of plyometric versus dynamic stabilization and balance training on lower extremity biomechanics. Am J Sports Med 34: 445–455, 2006.
57. Myer GD, Ford KR, Palumbo JP, Hewett TE. Neuromuscular training improves performance and lower extremity biomechanics in female athletes. J Strength Cond Res 19: 51–60, 2005.
58. Myklebust G, Engebresten L, Braekken IH, Skjolberg A, Olsen OE, Bahr R. Prevention of anterior cruciate ligament injuries in female team handball players: A prospective intervention study over three seasons. Clin J Sport Med 13: 71–78, 2003.
59. Padua DA, Arnold AL, Perrin DH, Gansneder BM, Carcia CR, Granata KP. Fatigue, vertical leg stiffness and stiffness control strategies in males and females. J Athl Train 41: 294–304, 2006.
60. Petersen W, Braum C, Bock W, Schmidt K, Weimann A, Drescher W, Eiling E, Stange R, Fuchs T, Hedderich J, Zantop T. A controlled prospective case control study of a prevention training program in female team handball players: The German experience. Arch Orthop Trauma Surg 125: 614–621, 2005.
61. Pousson M, Van Hoecke J, Goubel F. Changes in elastic characteristics of human muscle induced by eccentric exercise. J Biomech 23: 343–348, 1990.
62. Pruyn EC, Watsford ML, Murphy AJ, Pine MJ, Spurrs RW, Cameron ML, Johnston RJ. Relationship between leg stiffness and lower body injuries in professional Australian football. J Sports Sci 30: 71–78, 2011.
63. Rapoport S, Mizrahi J, Kimmil E, Verbitsky O, Isakov E. Constant and variable stiffness and damping of the leg joints in human hopping. J Biomech Eng 125: 507–514, 2003.
64. Saez-Saez de Villarreal E, Requena B, Newton RU. Does plyometric training improve strength performance? A meta-analysis. J Sci Med Sport 13: 513–522, 2009.
65. Serpell BG, Ball NB, Scarvell JM, Smith PN. A review of models of vertical, leg and knee stiffness in adults for running, jumping or hopping tasks. J Sports Sci 30: 1347–1363, 2012.
66. Seyfarth A, Geyer H, Gunther M, Blickman RA. A movement criterion for running. J Biomech 35: 649–655, 2002.
67. Souhaiel CM, Christian D. Leg power and hopping stiffness: Relationship with sprint running performance. Med Sci Sports Exerc 33: 326–333, 2000.
68. Spurrs RW, Murphym AJ, Watsford ML. The effect of plyometric training on distance running performance. Eur J Appl Physiol 89: 1–7, 2003.
69. Stefanyshyn DJ, Nigg BM. Dynamic angular stiffness of the ankle joint during running and sprinting. J Appl Biomech 14: 292–299, 1998.
70. Toumi H, Best TM, Martin A, Poumarat G. Muscle plasticity after weight and combined (weight + jump) training. Med Sci Sports Exerc 36: 1580–1588, 2004.
71. Turner A, Jeffreys I. The stretch-shortening cycle: Proposed mechanisms and methods for enhancement. Strength Cond J 32: 87–99, 2010.
72. Wang L. Lower extremity stiffness modulation: Effect of impact load of a landing task from different drop heights. Int J Sports Med 10: 186–193, 2009.
73. Watsford ML, Murphy AJ, Mclachlan KA, Bryant AL, Cameron ML, Crossley KM. A prospective study of the relationship between lower body stiffness and hamstring injury in professional Australian rules footballers. Am J Sports Med 19: 2058–2064, 2010.
74. Williams DS, Davis IM, Scholz JP, Hamill J, Buchanan TS. High-arched runners exhibit increased leg stiffness compared to low arched runners. Gait Posture 19: 263–269, 2004.
75. Williams DS, McClay IS, Hamill J. Arch structure and injury patterns in runners. Clin Biomech 16: 341–347, 2001.
76. Williams DS, McClay IS, Scholz JP, Hamill J, Buchanan TS. Lower extremity stiffness in runners with different foot types. American Society of Biomechanics Conference: Gait Posture. Chicago, IL, September 16, 2000.
77. Wu YK, Lien YH, Lin KH, Shih TT, Wang TG, Wang HK. Relationship between three potentiation effects of plyometric training and performance. Scand J Med Sci Sports 20: 80–86, 2009.
78. Zhang S, Bates BT, Dufek JS. Contributions of lower extremity joints to energy dissipation during landings. Med Sci Sports Exerc 32: 812–819, 2000.
Keywords:

lower extremity stiffness; vertical stiffness; leg stiffness; joint stiffness; compliance; stretch-shortening cycle; plyometrics; intermuscular coordination

© 2014 by the National Strength & Conditioning Association