Maintaining flexibility is one of the key components of a program for maintaining health and physical fitness according to the American College of Sports Medicine (1,2,23), and evidence of the benefits of regular stretching has been well documented (1,19,28,29). Stretching before exercise or participation in sport has traditionally been accepted as prudent for prevention of injury and optimizing performance (28,32).
However, recent investigations in college-aged subjects have shown that acute static stretching can impair performance. Behm et al. (3,4), Yamaguchi et al. (39), Kokkonen et al. (18), Power et al. (26), and Cramer et al. (11) provided evidence that 18 to 20 minutes of lower-extremity static stretching impaired performance, as measured by reaction and movement time, maximal force and peak torque production, and balance. Winchester et al. (38) and Bradley et al. (7) discovered impaired vertical jump and sprinting ability in 20-year-old athletes after 10 minutes of static stretches. Brandenburg (8) reported that up to 6 minutes of static stretching impaired knee flexor peak torque production. Siatras et al. (31) documented impaired peak torque production by the knee extensors with just 30 to 60 seconds of acute static stretching. The physiologic mechanisms implicated in the negative effect of static stretching on performance are increases in the viscoelastic properties, with concomitant decreases in muscle stiffness of the musculotendonous unit decreasing the force-generating capacity of that muscle (3,4,7,11-13,36,37). Conversely, other studies using college-aged subjects by Unick et al. (36), Egan et al. (13), and Cramer et al. (12) using 6 to 20 minutes of acute static stretches demonstrated no effect on peak torque, mean power output (both concentric and eccentric), and vertical jump height. Little and Williams (20) reported that acute static stretching improved 20-m sprint performance. Shier (29) concluded that static stretching does not impair jumping ability or force production.
Thus, a controversy exists in the current literature on the effects of pre-exercise static stretching on balance and performance. The subjects in these studies were predominantly college aged. To our knowledge, no study has investigated the acute effects of stretching in the middle-aged population. According to Magnusson (22), aging increases muscle stiffness with associated decreases in its viscoelastic properties. Therefore, middle-aged active adults should respond differently than younger adults to static stretches because of differences in the viscoelastic properties and stiffness of their musculotendinous units. It would be poor external validity to extrapolate the results from studies on young adults to middle-aged adults and assume they would be similar because significant physiologic differences exist between these age groups. We hypothesized that jump/hop performance and balance will not be affected by an acute bout of static stretching in active middle-aged adults.
Experimental Approach to the Problem
The purpose of this study was to assess the effects of 10 minutes of acute static stretching on measures of dynamic balance and jump/hop performance in active middle-aged adults. This study used a repeated measures design. A repeated measures design allowed us to control for intersubject differences (25). This age group has not been previously investigated. The research on the effects of stretching on performance has been overwhelmingly performed on college-aged students. The reliability and validity of the dependant variables of this study, the broad jump, hop tests (6,15,27,35), and balance (10,34), as measures of performance have been previously documented.
Subjects reported in the morning hours for the 2 conditions with at least 4 days and not more than 7 days separating them. The 2 independent variables were a stretch and a no-stretching condition that were randomly assigned. Subjects were asked to refrain from exercise, alcohol, or medication that would have an adverse effect on their performance. Paired t-tests were performed on the dependant variables to investigate the effects of stretching on jump/hop performance in this age group.
Ten subjects (6 men and 4 women, aged 40-60 yr) from a Soo Bahk Do martial arts school volunteered to take part in this research study. All the subjects were black belts with at least 4 years of continuous training. Soo Bahk Do is a traditional Korean martial art similar to karate. The Institutional Review Board of New York Institute of Technology approved this investigation. Subjects were informed of the experimental risks and benefits of this study and signed an informed consent document before the investigation.
Subject characteristics are presented in Table 1. The inclusion criteria were subjects to be 40 to 60 years old; subjects to present with medical clearance; subjects to have been martial artists with at least 4 years experience. Exclusion criteria were any medical, musculoskeletal, or vestibular disorder that would impair the ability of the subjects to perform any part of the study's protocol. Each subject was required to attend 2 data collection sessions separated by 4 days and not more than 7 days.
This was an experimental study with a repeated measures design in which subjects served as their own controls. Subjects took part in a no-stretching session (control) and a stretching session that were randomly assigned. Therefore, subjects who stretched during 1 session became their own controls during the alternate session in which 10 minutes of quiet sitting replaced stretching. The independent variables were the stretching session vs. the no-stretching sessions. The dependent variables were the measured values for single leg dynamic balance, broad jump, single hop, triple hop, 6-m timed hop, and a crossover hop.
All subjects were tested between 9 and 11 am. Upon arrival to the laboratory, the researchers took the medical history of each subject and screened for any of the exclusion criteria. Before either stretching or sitting quietly, each subject was required to perform 1 practice trial of dynamic single-leg standing balance for 20 seconds, on each lower extremity, on the Biodex Balance System SD (Biodex Medical Systems, Inc., Shirley, NY, USA). The practice trial was performed to minimize the effects of learning during the 2 recorded balance trials.
The stretching protocol was adapted from a previously published study in which subjects performed 4 lower-body static stretches (35). The static stretches in this study were held for 30 seconds duration and were repeated 3 times with 30 seconds rest between stretches. The entire stretching protocol required 10 minutes to complete. The “no-stretch” condition consisted of the subjects sitting quietly for 10 minutes before performance testing.
After either the stretching session or the no-stretching control session, subjects were tested for single-leg dynamic balance (Dynamic Stability Index [DSI]) on the Biodex Balance System SD. After the balance testing, the subjects performed a broad jump, single hop, triple hop, 6-m timed hop, and a crossover hop.
The stretching protocol consisted of static stretching to the low back, hip extensors, knee extensors, knee flexors, and ankle plantar flexors for a total of 10 minutes. All stretches were active and were held for 30 seconds at an intensity at which the subject felt minimal discomfort. Each stretch was repeated for 3 successive 30-second repetitions separated by 30 seconds of rest. For the low back and hip extensors, subjects performed a knee-to-chest stretch: subjects were supine and pulled both thighs toward their chest by holding the back of their thighs with both hands. For the knee extensors, subjects performed a quadriceps stretch: subjects were side lying and flexed their superior knee by pulling the foot of that leg toward their buttock with one hand. They then repeated the procedure with the opposite leg. For the knee flexors, subjects performed a hamstring stretch: subjects sat on the floor with both legs extended in front of them and reached forward toward their feet with both hands. For the ankle plantar flexors, subjects performed a standing calf stretch: subjects stood upright facing a wall with their legs in a long stride position, approximately 2 to 3 feet from a wall. The heel of their rear foot was in contact with the floor while that knee was extended to stretch the plantar flexors. Subjects then repeated the procedure with their opposite leg.
Balance testing was conducted using the Balance System SD. Each subject's age and height was entered into the unit so that normative values could be calculated. The subject then stood barefoot with 1 foot centered on the balance platform for single-leg testing. The subject's sway while in single-leg stance would cause the platform to move. The Balance System SD recorded the degrees of motion of the platform as the subject attempted to balance on the moveable surface. The subject received simultaneous visual feedback of the balance platform's position and its movement by a cursor on a target where center was the optimal neutral position. Subjects were instructed to keep the cursor in the middle of the target as they balanced without using their upper extremities for support. Two 20-second dynamic trials were performed and recorded for each leg. Balance ability was measured in units of DSI in which a lower index indicates less platform movement and, therefore, better balance. Reliability of the dynamic balance protocol using the Biodex system has been documented by Testerman and Vander Griend (34).
The performance protocol consisted of a broad jump and 4 hop tests for distance or speed. The reliability and validity of the broad jump and hop tests as measures of performance have been documented (6,15,27,35). Each subject performed 1 practice trial before 3 recorded trials for each lower extremity. To minimize fatigue, each trial was separated by 30 seconds of rest, and different tests were separated by 2 minutes of rest. The best of the 3 recorded trials in each test was used as the value for subsequent data analysis.
The broad jump test was performed with the subject standing with both feet behind the starting line. The subject then jumped horizontally across the floor. The maximal distance jumped from the starting line to the rearmost heel strike was measured and recorded for each of the 3 broad jumps. The use of the broad jump as a performance measure has been described in previous investigations (6,15,35).
The 4 hop tests were a single hop, triple hop, 6-m timed hop, and a crossover hop. The reliability of this standardized hop-test protocol has been demonstrated in previous research by Reid et al. (27). Each subject was required to perform 1 practice trial before the 3 recorded test trials for each leg during the 4 hop tests. The test trial was repeated if the participants were unable to complete it or lost their balance as demonstrated by contacting the ground with the opposite foot. The maximum distance hopped or minimum time required to hop a measured distance during each of the 3 test trials was recorded. The single hop test for distance was performed by the subject standing behind the starting line on the leg to be tested and hopping and landing on the same leg. The subject was instructed to hop as far as possible. The test was then repeated on the opposite leg. The distance hopped was measured from the starting line to the great toe for each of the 3 trials. The triple hop test was performed by the subject standing behind the starting line on the leg to be tested and hopping for 3 consecutive maximum hops on the same leg. The distance from the starting line to the great toe after completing the third hop was measured. The 6-m timed hop test consisted of the subject hopping 6 m as fast as possible. Subjects were instructed to perform large, 1-legged hops, as quickly as possible, from the starting line to the finish line. An electronic stopwatch (Timex Ironman, Waterbury, CT, USA) was used to record the time elapsed. The crossover hop test was performed by the subject standing behind the starting line on the leg to be tested and hopping forward 3 times in succession while crossing a 15 cm-wide marked strip during each hop. The subject was instructed to hop as far as possible. The distance from the starting line to the great toe, after completing the third hop, was measured. The best scores from each leg were added together to yield one compound score, for each of the hop tests, to use for subsequent data analysis.
Statistical analysis was performed with a repeated measures design using SPSS (Windows version 15.0, Chicago, IL, USA). Paired t-tests were used to compare the effects of stretching vs. no-stretching on the values of the following performance variables: balance, broad jump, single hop, triple hop, 6-m timed hop, and crossover hop. A paired t-test is the statistical procedure to be used with this design, and we have met all of the assumptions according to Portney and Watkins (25). A priori sample size calculation based on previously published effect sizes (24) revealed that 10 subjects were needed to detect differences at a power of 80%. Statistical significance was set at an alpha level of p < 0.05.
Group means of measured dynamic balance, broad jump, single hop, triple hop, 6-m timed hop, and crossover hop are presented in Table 2. Measures of dynamic balance were in units of DSI, which indicate improved balance by a lower score. Group means for balance, as presented in Table 2, were significantly different between the stretch and no-stretch conditions (3.5 ± 0.7 vs. 4.3 ± 1.4 DSI, respectively; p = 0.03). There were no significant differences between the 2 conditions for the following dependent variables: broad jump, single hop, triple hop, 6-m timed hop, and crossover hop (Table 2).
We investigated the effects of stretching vs. no stretching on dynamic balance and jump/hop performance. The results of this study suggest that static stretching before exercise improves dynamic balance in middle-aged active adults while not negatively affecting jumping and hopping performance. The results of this study compare favorably with the findings of other investigators using younger subjects on balance (10) and performance (12,13,36).
The ability to balance is dependent on central nervous system integration of afferent feedback from somatosensory, visual, and vestibular receptors resulting in a coordinated efferent response through the musculoskeletal system (5). The increased dynamic balance noted in this study may be a result of enhanced feedback to the central nervous system and more compliant musculotendinous units poststretching (5,26). Stretching of the lower extremity musculature would result in increased afferent information from Golgi tendon organs and muscle spindle receptors to the spinal cord, cerebral cortex, and the cerebellum. There is evidence of improved joint position sense after static stretching of the quadriceps, hamstrings, and adductors (16), and investigations in persons with hemiplegia have shown balance improvements with increased sensory stimulation (21). The increased sensory feedback may be partially responsible for the poststretching balance improvements noted in this study (17). The effect of stretching to counteract the loss of compliance in the stiffer musculoskeletal system of our older subjects may have also contributed to the improvements noted in dynamic balance because more compliant musculotendinous units have been shown to increase reaction times (26).
This study's results of improved balance conflict with those of Behm et al. (3), who showed a nonsignificant decline in balance in the poststretching group (stretching plus warm-up) but a significant improvement in balance in the control group (warm-up only). However, their intervention differed from the current study because it comprised 20 minutes of stretches of 45-second duration with 15-second rest and was preceded by a warm-up activity. The combination of warm-up plus stretching may have been too strenuous to allow optimal performance immediately poststretching. Possibly, an intervention of static stretching alone may have yielded comparable results with this control group's results of improved balance. In addition, the 24-year-old subjects used in Behm et al.'s study (3) are physiologically different from the 50-year-old subjects used in the current study. Morphologic changes associated with aging result in decreased strength, compliance, and flexibility (1,9). Declines in systems independent of the musculoskeletal system such as the visual, vestibular, and somatosensory systems also contribute to the declines in balance that have been associated with aging (24,30). Thus, 20- and 50-year-old adults may respond differently to stretching interventions. Also, the balance measure used by Behm et al. (3) measures balance on both legs simultaneously as opposed to the single-leg dynamic measure used in this study. As such, it may not have been of sufficient difficulty to challenge the balance of their young, athletic subjects.
The results of this study did compare favorably with Costa et al. (10). In a study of young women, they concluded that a stretching protocol of 45-second hold did not impair balance, whereas a 15-second hold improved balance. They suggested that longer durations may impair balance by decreasing reflex activity resulting from the reduced sensitivity of the muscle spindles to repeated stretches. They also suggested that their moderate stretching protocol of a 15-second hold may have caused changes in the musculotendinous unit that were not detrimental in nature. Our subjects used a 30-second hold, which also improved balance and was not detrimental to performance.
Static stretching can be quite strenuous. Chronic static stretching alone was shown to be a sufficient exercise stimulus for significantly improved performance, endurance, and lower extremity flexibility when compared with a nonexercising control group (19). Ninety minutes of static stretching caused significantly greater delayed onset muscle soreness and greater increases in serum creatine kinase than 90 minutes of ballistic stretching (33). Thus, the specifics of the static stretching protocol may alter its effects on performance. The 20 minutes of static stretching used in previous studies may have been excessive and, therefore, resulted in less than optimal performance (3,18,26,39). Previous investigations that used a 10-minute stretch protocol added 5 minutes of cycling or 30 minutes of dynamic stretching as a warm-up (7,8,31,38). The addition of a warm-up to the static stretch may have prefatigued the musculotendinous units and prevented maximal performance. In addition, a duration/rest ratio of 45-second static stretch with a 15-second rest may also have been too intense for producing optimal performance and could account for the detrimental changes in performance found in these studies (4,26). Feland et al. (14) demonstrated that acute stretches greater or less than 30 seconds in duration (15, 30, 60 s) had different effects on the stiffness of the musculotendinous unit, with longer stretch times resulting in significantly greater flexibility in an older population.
The static stretching protocol of this study did not negatively affect jump/hop performance and, therefore, does not support the findings of a detrimental effect of acute stretching on performance shown by previous investigations with younger subjects (3,4,7,8,11,18,26,31,38,39). The altered biomechanical properties of strength, compliance, stiffness, and resilience of muscles, tendons, ligaments, joint capsules, bones, and articular cartilage may help to explain why the resting values of these components in middle-aged adults may not be optimal for exercise performance (9). Therefore, acute static stretching may not impair the less than optimal status of these tissues. The physiologic mechanism of a decrease in muscle stiffness, involved in the decline of the force-generating capacity and performance of a statically stretched muscle of a younger athlete, may play a diminished role in an older adult. Middle-aged adults have increased muscle stiffness and decreased viscoelastic properties to begin with and, therefore, may be less affected by static stretching than younger adults.
Future areas of investigation should examine the effects of acute static stretching on the force-generating capacity and viscoelastic properties of musculotendinous units in the middle-aged population. The effects of different stretches (i.e., dynamic/ballistic or resistive stretches or the effects of a pure active warm-up on balance and performance) may also yield valuable information for aging athletes. In summary, degenerative changes in connective tissues and neurologic systems are associated with declines in functional performance and independence as we age (1,8). Regular exercise is prescribed as an integral intervention for mitigating the functional declines associated with aging and for improving quality of life (1). The results of this study revealed that acute static stretching, with 30-second hold durations, does improve balance while not adversely affecting jump/hop performance in middle-aged active adults. The finding of this study combined with previous evidence of regular stretching improving performance and decreasing risk of injury emphasize the importance of including static stretching in exercise prescriptions for this population (29).
Strength and conditioning professionals and coaches who are involved with middle-aged athletes should be aware that results of research performed on college-aged athletes should not be extrapolated to middle-aged athletes. Middle-aged athletes are physiologically and morphologically different than college-aged athletes. Middle-aged adults have increased muscle stiffness and decreased viscoelastic properties at rest, and the effects of static stretching on their neuromuscular performance may be different than on their younger counterparts. The static stretching protocol of this study significantly improved balance and did not negatively affect jump/hop performance. Therefore, these results do not support the findings of a detrimental effect of acute stretching on performance as found in previous investigations using younger subjects. The results of this study demonstrate that including static stretching in exercise prescriptions for middle-aged adults can be beneficial for activities that involve balance while not negatively affecting performance in this population. Static stretching should be included as part of a warm-up, before competition and exercise, in fitness programs of active middle-aged adults.
The results of this current study do not constitute endorsement of any of the products by the authors or the NSCA. We did not receive any funding for this study.
1. American College of Sports Medicine Position Stand. Exercise and physical activity for older adults. Med Sci Sports Exerc
30: 992-1008, 1998.
2. American College of Sports Medicine Position Stand. The recommended quantity and quality of exercise for developing and maintaining cardiorespiratory and muscular fitness, and flexibility in healthy adults. Med Sci Sports Exerc
30: 975-991, 1998.
3. Behm, DG, Bambury, A, Cahill, F, and Power, K. Effect of acute static stretching on force, balance
, reaction time, and movement time. Med Sci Sports Exerc
36: 1397-1402, 2004.
4. Behm, DG, Button, DC, and Butt, JC. Factors affecting force loss with prolonged stretching. Can J Appl Physiol
26: 261-272, 2001.
5. Bennet, S and Karnes, J. Neurological Disabilities: Assessment and Treatment
. Philadelphia: Lippincott-Raven, 1998.
6. Birch, K, Sinnerton, S, Reilly, T, and Lees, A. The relation between isometric lifting strength and muscular fitness measures. Ergonomics
37: 87-93, 1994.
7. Bradley, PS, Olsen, PD, and Portas, MD. The effect of static, ballistic, and proprioceptive neuromuscular facilitation stretching on vertical jump performance. J Strength Cond Res
21: 223-226, 2007.
8. Brandenburg, JP. Duration of stretch does not influence the degree of force loss following static stretching. J Sports Med Phys Fitness
46: 526-534, 2006.
9. Buckwalter, JA, Woo, SL, Goldberg, VM, Hadley, EC, Booth, F, Oegema, TR, and Eyre, DR. Soft-tissue aging and musculoskeletal function. J Bone Joint Surg Am
75: 1533-1548, 1993.
10. Costa, PB, Graves, BS, Whitehurst, M, and Jacobs, PL. The acute effects of different durations of static stretching on dynamic balance
performance. J Strength Cond Res
23: 141-147, 2009.
11. Cramer, JT, Housh, TJ, Johnson, GO, Miller, JM, Coburn, JW, and Beck, TW. Acute effects of static stretching on peak torque in women. J Strength Cond Res
18: 236-241, 2004.
12. Cramer, JT, Housh, TJ, Johnson, GO, Weir, JP, Beck, TW, and Coburn, JW. An acute bout of static stretching does not affect maximal eccentric isokinetic peak torque, the joint angle at peak torque, mean power, electromyography, or mechanomyography. J Orthop Sports Phys Ther
37: 130-139, 2007.
13. Egan, AD, Cramer, JT, Massey, LL, and Marek, SM. Acute effects of static stretching on peak torque and mean power output in National Collegiate Athletic Association Division I women's basketball players. J Strength Cond Res
20: 778-782, 2006.
14. Feland, JB, Myrer, JW, Schulthies, SS, Fellingham, GW, and Measom, GW. The effect of duration of stretching of the hamstring muscle group for increasing range of motion in people aged 65 years or older. Phys Ther
81: 1110-1117, 2001.
15. Gamble, RP, Boreham, CA, and Stevens, AB. Effects of a 10-week exercise intervention programme on exercise and work capacities in Belfast's ambulance-men. Occup Med (Lond)
43: 85-89, 1993.
16. Ghaffarinejad, F, Taghizadeh, S, and Mohammadi, F. Effect of static stretching of muscles surrounding the knee on knee joint position sense. Br J Sports Med
41: 684-687, 2007.
17. Hall, C and Brody, L. Therapeutic Exercise: Moving Toward Function. Philadelphia: Lippincott Williams and Wilkins, 2005.
18. Kokkonen, J, Nelson, AG, and Cornwell, A. Acute muscle stretching inhibits maximal strength performance. Res Q Exerc Sport
69: 411-415, 1998.
19. Kokkonen, J, Nelson, AG, Eldredge, C, and Winchester, JB. Chronic static stretching improves exercise performance. Med Sci Sports Exerc
39: 1825-1831, 2007.
20. Little, T and Williams, AG. Effects of differential stretching protocols during warm-ups on high-speed motor capacities in professional soccer players. J Strength Cond Res
20: 203-207, 2006.
21. Magnusson, M, Johansson, K, and Johansson, BB. Sensory stimulation promotes normalization of postural control after stroke. Stroke
25: 1176-1180, 1994.
22. Magnusson, SP. Passive properties of human skeletal muscle during stretch maneuvers. A review. Scand J Med Sci Sports
8: 65-77, 1998.
23. Nelson, ME, Rejeski, WJ, Blair, SN, Duncan, PW, Judge, JO, King, AC, Macera, CA, and Castaneda-Sceppa, C. Physical activity and public health in older adults: recommendation from the American College of Sports Medicine and the American Heart Association. Med Sci Sports Exerc
39: 1435-1445, 2007.
24. Onambele, GL, Narici, MV, Rejc, E, and Maganaris, CN. Contribution of calf muscle-tendon properties to single-leg stance ability in the absence of visual feedback in relation to ageing. Gait Posture
26: 343-348, 2007.
25. Portney, GL and Watkins, MP. Foundations of Clinical Research Applications to Practice
. Upper Saddle River, NJ: Pearson Prentice Hall, 2009.
26. Power, K, Behm, D, Cahill, F, Carroll, M, and Young, W. An acute bout of static stretching: effects on force and jumping performance. Med Sci Sports Exerc
36: 1389-1396, 2004.
27. Reid, A, Birmingham, TB, Stratford, PW, Alcock, GK, and Giffin, JR. Hop testing provides a reliable and valid outcome measure during rehabilitation after anterior cruciate ligament reconstruction. Phys Ther
87: 337-349, 2007.
28. Shellock, FG and Prentice, WE. Warming-up and stretching for improved physical performance and prevention of sports-related injuries. Sports Med
2: 267-278, 1985.
29. Shrier, I. Does stretching improve performance? A systematic and critical review of the literature. Clin J Sport Med
14: 267-273, 2004.
30. Shumway-Cook, A, Gruber, W, Baldwin, M, and Liao, S. The effect of multidimensional exercises on balance
, mobility, and fall risk in community-dwelling older adults. Phys Ther
77: 46-57, 1997.
31. Siatras, TA, Mittas, VP, Mameletzi, DN, and Vamvakoudis, EA. The duration of the inhibitory effects with static stretching on quadriceps peak torque production. J Strength Cond Res
22: 40-46, 2008.
32. Smith, CA. The warm-up procedure: to stretch or not to stretch. A brief review. J Orthop Sports Phys Ther
19: 12-17, 1994.
33. Smith, LL, Brunetz, MH, Chenier, TC, McCammon, MR, Houmard, JA, Franklin, ME, and Israel, RG. The effects of static and ballistic stretching on delayed onset muscle soreness and creatine kinase. Res Q Exerc Sport
64: 103-107, 1993.
34. Testerman, C and Vander Griend, R. Evaluation of ankle instability using the Biodex Stability System. Foot Ankle Int
20: 317-321, 1999.
35. Thompsen, AG, Kackley, T, Palumbo, MA, and Faigenbaum, AD. Acute effects of different warm-up protocols with and without a weighted vest on jumping performance in athletic women. J Strength Cond Res
21: 52-56, 2007.
36. Unick, J, Kieffer, HS, Cheesman, W, and Feeney, A. The acute effects of static and ballistic stretching on vertical jump performance in trained women. J Strength Cond Res
19: 206-212, 2005.
37. Wilson, JM and Flanagan, EP. The role of elastic energy in activities with high force and power requirements: a brief review. J Strength Cond Res
22: 1705-1715, 2008.
38. Winchester, JB, Nelson, AG, Landin, D, Young, MA, and Schexnayder, IC. Static stretching impairs sprint performance in collegiate track and field athletes. J Strength Cond Res
22: 13-19, 2008.
39. Yamaguchi, T, Ishii, K, Yamanaka, M, and Yasuda, K. Acute effect of static stretching on power output during concentric dynamic constant external resistance leg extension. J Strength Cond Res
20: 804-810, 2006.