Secondary Logo

Journal Logo

The Foam Roll as a Tool to Improve Hamstring Flexibility

Junker, Daniel H.; Stöggl, Thomas L.

The Journal of Strength & Conditioning Research: December 2015 - Volume 29 - Issue 12 - p 3480–3485
doi: 10.1519/JSC.0000000000001007
Original Research
Free

Junker, DH and Stöggl, TL. The foam roll as a tool to improve hamstring flexibility. J Strength Cond Res 29(12): 3480–3485, 2015—Although foam rolling is a common myofascial therapy used to increase range of motion (ROM), research is limited on the effectiveness of foam rolling on soft tissue extensibility. The aim of this study was to determine the effect of a 4-week training period of the foam roll method on hamstring flexibility. Furthermore, the study was designed to compare the effectiveness of the foam roll myofascial release with a conventional contract-relax proprioceptive neuromuscular facilitation (PNF) stretching method and a control group. Forty healthy males (age: 17–47 years) were randomly assigned to a foam roll group (FOAM, n = 13), a contract-relax PNF stretching group (CRPNF, n = 14), or a control group (CG, n = 13). The FOAM group massaged their hamstring muscles with the foam roll 3 times per week for 4 weeks (12 training sessions). The CRPNF group was assigned to 12 sessions of contract-relax PNF stretching. The CG underwent no intervention. Hamstring flexibility (ROM) was measured by a stand-and-reach test before and after the intervention period. Two-way repeated-measures analysis of variance showed a significant global time effect (p < 0.001) and an interaction effect for time × treatment (p = 0.004), demonstrating greater improvements in the FOAM and CRPNF compared with the CG, but no difference between the former. Delta changes from baseline to postintervention in ROM were not related to baseline ROM. The foam roll can be seen as an effective tool to increase hamstring flexibility within 4 weeks. The effects are comparable with the scientifically proven contract-relax PNF stretching method.

Department of Sport Science and Kinesiology, University of Salzburg, Salzburg, Austria

Address correspondence to Daniel Junker, daniel@therapie-telfs.at.

Back to Top | Article Outline

Introduction

Flexibility is an important part of motor abilities with human movement depending on the degree of range of motion (ROM) available in synovial joints (19). Furthermore, flexibility is important in both the prevention and the rehabilitation of musculoskeletal injuries (24). Range of motion is determined by joint structure, congruency, capsuloligamentous structures, and muscles. Muscle tension is composed of active and passive tension, with the former defined by alpha and gamma innervation (neuromuscular properties of muscle) and the latter by viscoelasticity and the fascia (19). Muscle tightness is one of many reasons for reduced joint ROM. It is the result of an increase in active or passive tension. Although active tension shortens the muscle through spasm or contraction, passive tension is caused by postural adaptation or scarring. As a consequence, ROM abnormalities may create a muscle imbalance (19).

Shortness and tightness of hamstring muscles are risk factors for back pain (11,16,2111,16,2111,16,21). In this context, Brodersen et al. (2) demonstrated that short hamstrings were fairly common in Danish students older than 10 years, and with that, the incidence of back pain rose significantly, reaching 15% in students with short hamstring muscles. In addition, people with short hamstring muscles also tend to offset with an increased lumbar flexion during bending forward, sitting down, or reaching the toes (21). It was also shown that people with patellofemoral pain had significantly shorter hamstring muscles than asymptomatic controls (31). Witvrouw et al. (32) reported in a prospective study that soccer players with reduced hamstring flexibility were more likely to develop hamstring injuries.

There are different methods or techniques for improving the length of a musculotendinous unit. The classical stretching methods, more precisely, static (active, passive), dynamic (active, ballistic), and precontraction (proprioceptive neuromuscular facilitation [PNF] stretching, postisometric relaxation) stretches (19) or myofascial techniques such as myofascial release or Rolfing can be applied.

One technique known as self-myofascial release is foam rolling. The foam roll is a solid foam cylinder available in different degrees of hardness and size. The exerted pressure of the foam roll stimulates the Golgi tendon unit and decreases muscle tension (12). Another possible effect is improved hydration of tissues. While working, soft tissue is squeezed like a sponge; consequently, it is soaked through with fluid, which improves motion between the different layers of fascia and increases blood flow and temperature (23). It is hypothesized that foam rolling releases fascial adhesions and reduces scar tissue (12). For this reason, it is possible to prevent chronic myofascial pain syndrome and dysfunctional posture. In addition, the foam roll reduces regeneration time and improves muscle performance (12).

From a scientific perspective, it is important to mention that many effects are assumed, although they are not yet proven. All studies, except one, on foam rolling address acute effects. MacDonald et al. (13) demonstrated that an acute bout of foam rolling on the quadriceps muscles increases knee joint ROM. Similarly, a stick roller massage (similar principle to foam roll) resulted in an acute increase of hamstring flexibility (26). The study of Miller and Rockey (15) is the only study that investigated chronic effects of foam rolling. They reported a significant improvement of hamstring flexibility after 8 weeks in the foam roll group, as well as in the control group, possibly based on uncontrolled testing time during the day, exclusive inclusion of participants with tight hamstrings, and improvements in ROM for female participants only, however with no control of gender effects. It is worth noting here that the effectiveness of foam rolling on flexibility had not yet been compared with classical stretching methods.

In the past, many studies concerning the effect of different stretching techniques on hamstring flexibility were performed. The evidence appears to indicate that a variety of stretching techniques, positions, and durations increase ROM (5). The contract-relax PNF (CRPNF) method is a dynamic technique whereby a small amount of motion is tolerated. In comparison with static stretching, precontracting stretching yields greater acute gains in ROM (19,2519,25) and at a faster rate than static stretching (27). Furthermore, it is seen to be more functional because it improves active and passive flexibility (25). The aim of this study was to investigate the training effect of the foam roll on flexibility of hamstring muscles and to compare it with CRPNF stretching. The specific hypotheses were (a) that foam rolling increases flexibility of hamstring muscles and (b) that foam rolling provides greater increases in hamstring muscle flexibility compared with CRPNF stretching.

Back to Top | Article Outline

Methods

Experimental Approach to the Problem

A randomized controlled clinical trial using a pretest/posttest design was used. All participants completed a hamstring flexibility test consisting of a stand-and-reach test, after which they were randomly assigned to 2 intervention groups, the foam roll group (FOAM, n = 13) and the contract-relax PNF stretching group (CRPNF, n = 14), and a control group (CG, n = 13). No differences in anthropometric and age-related parameters were found between the 3 groups at baseline (Table 1). Subsequently, the intervention groups were instructed about the foam roll and the contract-relax PNF stretching exercises. In addition, a training protocol was handed out in which participants were asked to document each training session. After a 4-week intervention period, the stand-and-reach test for each group was executed again.

Table 1

Table 1

Back to Top | Article Outline

Subjects

Forty-seven recreationally active male participants performing 2–3 times per week sport activity (mean ± SD; age: 31.3 ± 9.2 years, age range: 19–47 years, weight: 78.0 ± 9.9 kg, height 181.4 ± 7.0 cm, body mass index: 24.3 ± 2.4 kg·m−2) were recruited and tested. Exclusion criteria were recent injury associated with a more than 1-week pause in performing sport. Participants who attended at least 75% of the training sessions were admitted to posttests. Training documentation revealed that participants trained in both the FOAM and CRPNF 12 ± 1 times (range, 10–15 times). Seven participants did not fulfill the criteria of 75% of the training sessions (4 participants) or did not complete the posttests (3 participants). All participants had no previous experience using a foam roll. The study was approved by the institutional review board, and participants were informed in detail about the testing and training procedures, as well as possible benefits and risks of the investigation before signing an institutionally approved informed consent to participate in the study.

Back to Top | Article Outline

Intervention

The intervention period consisted of 3 training sessions per week for 4 weeks. This duration was based on the findings of Chan et al. (4), demonstrating that both a 4-week and an 8-week static stretching period improved hamstring flexibility but with no difference between groups.

The FOAM group was instructed to train their hamstring flexibility with the foam roll in a supine position. They were briefed to use the foam roll with a pressure on their pain threshold. In each subset, they rolled their hamstrings unilaterally for 30–40 seconds (10 times back and forth). After the first leg was finished, they repeated the exercise with the other leg (1 set). Altogether, 3 sets in 1 session were performed. The protocol of foam rolling was based on the recommendations of Lukas (12), which represent practical recommendations known from clinical experience.

The CRPNF group used the contract-relax PNF stretching method. Based on the study of Feland and Marin (7), participants performed 3 separate CRPNF stretches at approximately 25% of their maximal voluntary isometric contraction with each leg. Participants were instructed to lie in a supine position. Next, they stretched their leg using a rope or a towel until an uncomfortable stretching sensation was felt. In this position, a contraction of the hamstring muscles against the rope or towel was performed. After 6 seconds of contraction, they relaxed the muscles while keeping the leg position and then stretched onto the next barrier within 10 seconds. This was repeated 3 times to equal 1 set, and a total of 3 sets were performed. The right and left legs were stretched alternately. The CG performed only the pretest and posttest and were advised to maintain their usual training routine. As mentioned above, all participants were recreationally active and performed in addition to the experimental treatment 2–3 times per week sport activities such as soccer and cycling.

Back to Top | Article Outline

Stand-and-Reach Test

The flexibility of hamstring muscles was measured using the stand-and-reach test. It is a common test for measuring flexibility of the hamstrings and the lower back. Reliability (r = 0.88–0.98) and objectivity of the stand-and-reach test (r = 0.95–0.98) meet the required scientific quality criteria (8).

Before testing, subjects performed 5–10 minutes of light jogging as a general warm-up. After warm-up, the stand-and-reach test was demonstrated by the instructor. Participants stood on a wooden box without shoes, feet together, with legs extended and toes touching the test panel. Participants were then asked to bend forward as far as possible touching the test panel with their fingers, holding the reached position for 2 seconds. The distance from the panel was recorded from a vertical scale in half centimeters. Data above the toe line (0 line) were noted with a minus and data below with a plus. Two measurements for each participant were taken, and the mean was used for further analysis. Both pretest and posttest measurements took place indoors at a standardized room temperature after 5 PM.

Back to Top | Article Outline

Statistical Analyses

Normal distribution was determined by the Shapiro-Wilk test. A 2-way repeated-measures analysis of variance (ANOVA) (time × treatment) was performed to determine treatment, time, and interaction (time × treatment) effects. In the event an interaction effect occurred, a one-way ANOVA over the delta values between pretest and posttest was performed. In case of a main effect for time, paired sample t-tests for post hoc comparisons were applied. The level of significance was set at alpha <0.05. Data were reported as mean ± SD. All data were analyzed using SPSS 23.0 (SPSS, Inc., Chicago, IL, USA).

Back to Top | Article Outline

Results

Baseline and postintervention values for FOAM, CRPNF, and CG are presented in Table 2. Statistical analysis revealed a main effect for time (p < 0.001) with no main effect for group (p = 0.123). An interaction effect for time × treatment (p = 0.004) was found, demonstrating that greater improvements in the CRPNF and FOAM were achieved compared with the CG (p = 0.004 and p = 0.033), whereas no differences were found between the 2 intervention groups (p = 0.60). Within groups, FOAM increased ROM by 3.0 ± 2.1 cm (p = 0.001), CRPNF by 4.0 ± 2.9 cm (p = 0.003), and no change in CG (0.4 ± 1.7 cm, p = 0.46). No significant correlations between baseline ROM and the delta changes within each training group or for the pooled data of both training groups were found. The delta changes from baseline to postintervention measurements are presented in Figure 1.

Table 2

Table 2

Figure 1

Figure 1

Back to Top | Article Outline

Discussion

The aim of this study was to determine the training effect of a foam roll massage on flexibility of hamstring muscles compared with a contract-relax PNF method and a control group. The training period of 4 weeks with 3 training sessions per week improved ROM in the stand-and-reach test, that is, hamstring flexibility, in both the FOAM and CRPNF group, whereas no changes occurred in the CG.

To the best of our knowledge, the study of Miller and Rockey (15) is the only one that analyzed chronic training effects of FOAM rolling. They demonstrated that an 8-week training intervention with 3 sessions per week led to a significant increase in ROM in both the foam roll group and in the control group. These results differ somewhat from those of our study where improvements in hamstring flexibility were found only in the FOAM and CRPNF groups, with no changes in the CG. Ways in which the study of Miller and Rockey (15) differed from this study were (a) the participation of both male and female subjects, (b) different testing setup with active knee extension in supine position for the dominant and nondominant leg was determined using an inclinometer, (c) tight hamstrings with less than 80° of knee extension ROM as an inclusion criteria, and (d) uncontrolled testing time during the day. On closer examination of the results, it is apparent that in the control group, female participants in particular improved their ROM; however, within the statistical analysis, no controls for gender effects were presented. In this study, only male participants with and without tight hamstring muscles were included, and the statistical analysis revealed that baseline ROM was not related to delta changes from baseline to postintervention measurements. Therefore, training-induced changes in hamstring flexibility were not related to tight or nontight hamstrings. Furthermore, it was demonstrated that flexibility is dependent on the time of day testing occurs (9,109,10); therefore, standardization of testing time during the day—for both baseline and postintervention testing—seems to be a relevant detail.

The chronicle improvements in hamstring flexibility in this study are comparable with studies about acute effects of the foam roll or comparable tools. In a study by MacDonald et al. (13), the acute effect of 2 one-minute bouts of self-myofascial release with the foam roll (range of hip extension with knee flexed) was found to significantly increase quadriceps flexibility 2 minutes (10°) and 10 minutes (8°) after foam rolling. Comparable findings were demonstrated by Sullivan et al. (26) where instead of a foam roll, a stick roller massager was used. They observed an acute increase of 4.3% in the sit-and-reach test after using the roller massager for either 10 or 5 seconds. However, in this study, the last training session was, at the latest, 1 day before postintervention; therefore, these changes can be regarded as training induced and not acute effects.

Several mechanisms might lie behind the improvement in hamstring flexibility by foam rolling in this study. The fascia mainly consists of collagen fibers (as well as, to a lesser degree, elastic and reticular fibers), fibroblasts, and water-binding ground substance (22). As a natural consequence of trauma, inflammation, or immobility, the fascia loses flexibility and becomes restricted. According to Pischinger's ground regulation system, the phase state of the connective tissue solidifies and develops adhesions (1). The aim of myofascial release methods is to rehydrate the fascia and in this way create a fluid gel-like extracellular environment to provide a greater increase in ROM (1,231,23), called the thixotropic property of fascia (22). Okamoto et al. (18) reported that self-myofascial release with the foam roll led to an acute reduced arterial stiffness and an improved endothelial vascular function. Therefore, the encouragement of blood flow is seen as another purpose of myofascial release with the foam roll because arterial distensibility is associated with flexibility. These mechanisms are likely to explain the effects of foam rolling in this study; however, long-term training effects were not analyzed.

As mentioned earlier, 2 key aspects of the loss of flexibility are fascial restriction and adhesion. As a consequence, stiffness accrues, which in turn leads to not only local but also overall problems in the body with acute and chronic dysbalance (myofascial imbalance, joint dysfunction, pain, and dysfunction in venous and lymphatic systems). It is assumed that self-myofascial release with the foam roll remedies these consequences (12). In this context, Pohl (20) explored a significant difference of the collagen matrix before and after a skin rolling treatment. In his opinion, this is caused by changes in the mechanical forces of fibroblasts and increased microcirculation. Carano and Siciliani (3) found out that cyclical forces stimulate the production of collagenase—an enzyme responsible for remodeling the extracellular matrix—by the fibroblast. Therefore, based on the results of this study, it might be speculated that there is a positive long-term effect of foam rolling on fascial restriction and adhesion.

The results revealed a significant difference in improvement of ROM between the FOAM group and the CG, as well as for the CRPNF group and the CG. No differences occurred between the 2 intervention groups. Precontraction stretching is a common and very effective method to increase ROM (7,19,24,257,19,24,257,19,24,257,19,24,25). The increase in ROM due to precontraction stretching is also attributed to a possible neurologic phenomenon (14), although the specific mechanism of action still remains unclear. Most signs point to an increased tolerance to stretching and not to increased muscle length. The perception of sensation is changed and allows a greater ROM (17,25,3017,25,3017,25,30). The improvement of ROM in the FOAM group was similar to the CRPNF group. There are many mechanoreceptors in fascia; these are sensory endings that are responsive to compressive and tensile loading. It is claimed that the stimulation of Golgi receptors are essential in myofascial release with the foam roll. The stimulation of Golgi receptors inhibits the muscle spindle activity and decreases muscular tension. This phenomenon is known as autogenic inhibition (12,2812,28). Fama and Bueti (6) suggested that it is likely that the pressure of the foam roll causes stimulation of the Golgi receptors by ischemic compression. They demonstrated that there was a negative effect of a warm-up with foam rolling on jump performance, especially for the countermovement jump, when compared with a dynamic warm-up. Nevertheless, the stimulation of Golgi receptors only explains the immediate effects of foam rolling and not the observed effects as in this study. Ruffini's receptors and free nerve endings react on sustaining and alternating pressure. On the contrary, Pacini's receptors are only responsive to varying pressure (22) and are essential for proprioception, a requirement for proper movement. In fascial training, a proprioceptive refinement is encouraged (23). In this study, no precise declaration for pressure was given. Participants were advised to train with a melting pressure on their pain threshold. Therefore, it is likely that the pressure varied during training, and as a consequence, the application of the foam roll might have stimulated these receptors.

Both hamstring flexibility and lower back flexibility influence the stand-and-reach test. However, in this study, only hamstring muscles were considered. During myofascial release therapy, not only isolated muscles but also muscle chains should be treated. It is also known that there is only one connected fascia and not different fasciae (1). Therefore, if the whole posterior chain was treated by foam rolling, the changes might have been even more pronounced. Additionally, the treatment protocols did not include trigger points, although the existence of trigger points has a negative influence on myofascial function. A passive technique to release trigger points is to use compression. It is likely that the compression exerted by the foam roll is suitable to release trigger points. However, this treatment of trigger points with the foam roll would have required a subjective and varying time treatment protocol (6,296,29).

Back to Top | Article Outline

Practical Applications

This study demonstrates that foam rolling can be applied as an effective technique for increasing hamstring flexibility within a 4-week training period. The improvements were similar to the CRPNF method, which is known to be one of the most effective stretching methods to increase ROM. For both techniques, 3 training sessions per week consisting of 3 repetitions of 30–40 seconds (FOAM) or 50 seconds (CRPNF) were sufficient to improve ROM. In addition, with foam rolling, there is a massage effect that does not occur with CRPNF stretching. However, the exact mechanisms of foam rolling still remain unclear, and future studies are needed to investigate this issue further.

Back to Top | Article Outline

Acknowledgments

The authors would like to thank the participants for their participation, enthusiasm, and cooperation. The authors also would like to express appreciation for the support of Donna Kennedy. The results of this study do not constitute endorsement of the product by the authors or the National Strength and Conditioning Association. Disclosure of funding received for this work from any of the following organizations: National Institutes of Health (NIH); Wellcome Trust; Howard Hughes Medical Institute (HHMI); and other(s).

Back to Top | Article Outline

References

1. Barnes MF. The basic science of myofascial release: Morphologic change in connective tissue. J Bodyw Mov Ther 1: 231–238, 1997.
2. Brodersen A, Pedersen B, Reimers J. Incidence of complaints about heel-, knee- and back-related discomfort among Danish children, possible relation to short muscles [in Danish]. Ugeskr Laeger 156: 2243–2245, 1994.
3. Carano A, Siciliani G. Effects of continuous and intermittent forces on human fibroblasts in vitro. Eur J Orthod 18: 19–26, 1996.
4. Chan SP, Hong Y, Robinson PD. Flexibility and passive resistance of the hamstrings of young adults using two different static stretching protocols. Scand J Med Sci Sports 11: 81–86, 2001.
5. Decoster LC, Cleland J, Altieri C, Russell P. The effects of hamstring stretching on range of motion: A systematic literature review. J Orthop Sports Phys Ther 35: 377–387, 2005.
6. Fama BJ, Bueti DR. The Acute Effect of Self-Myofascial Release on Lower Extremity Plyometric Performance: Theses and Dissertations, Paper 2. Fairfield, CT: Sacred Heart University, 2011.
7. Feland JB, Marin HN. Effect of submaximal contraction intensity in contract-relax proprioceptive neuromuscular facilitation stretching. Br J Sports Med 38: E18, 2004.
8. Fetz F, Kornexl E. Sportmotorische Tests. Praktische Anleitung zu sportmotorischen Tests in Schule und Verein [sportmotoric tests. Practical instructions for sportmotoric tests for school and clubs]. Wien, Austria: ÖBV, 1993.
9. Grosser M, Starischka S, Zimmermann E. Das Neue Konditionstraining [The New Conditioning Training]. München, Germany: BLV-Sportwissenschaft, 2001.
10. Guariglia DA, Pereira LM, Dias JM, Pereira HM, Menacho MO, Silva DA, Cyrino ES, Cardoso JR. Time-of-day effect on hip flexibility associated with the modified sit-and- reach test in males. Int J Sports Med 32: 947–952, 2011.
11. Harreby M, Nygaard B, Jessen T, Larsen E, Storr-Paulsen A, Lindahl A, Fisker I, Laegaard E. Risk factors for low back pain in a cohort of 1389 Danish school children: An epidemiologic study. Eur Spine J 8: 444–450, 1999.
12. Lukas C. Faszienbehandlung mit der Blackroll [Treatment of fascia with the blackroll]. Norderstedt: BoD, Books on Demand, 2012.
13. MacDonald GZ, Penney MD, Mullaley ME, Cuconato AL, Drake CD, Behm DG, Button DC. An acute bout of self-myofascial release increases range of motion without a subsequent decrease in muscle activation or force. J Strength Cond Res 27: 812–821, 2013.
14. Markos PD. Ipsilateral and contralateral effects of proprioceptive neuromuscular facilitation techniques on hip motion and electromyographic activity. Phys Ther 59: 1366–1373, 1979.
15. Miller JK, Rockey AM. Foam rollers show no increase in the flexibility of the hamstring muscle group. UW-L Journal of Undergraduate Research IX: 1–4, 2006.
16. Mistry GS, Vyas NJ, Sheth MS. Comparison of hamstrings flexibility in subjects with chronic low back pain versus normal individuals. J Clin Exp Res 2: 85, 2014.
17. Mitchell UH, Myrer JW, Hopkins JT, Hunter I, Feland JB, Hilton SC. Acute stretch perception alteration contributes to the success of the PNF “contract-relax” stretch. J Sport Rehabil 16: 85–92, 2007.
18. Okamoto T, Masuhara M, Ikuta K. Acute effects of self-myofascial release using a foam roller on arterial function. J Strength Cond Res 28: 69–73, 2014.
19. Page P. Current concepts in muscle stretching for exercise and rehabilitation. Int J Sports Phys Ther 7: 109–119, 2012.
20. Pohl H. Changes in the structure of collagen distribution in the skin caused by a manual technique. J Bodyw Mov Ther 14: 27–34, 2010.
21. Sahrmann SA. Diagnosis and Treatment of Movement Impairment Syncdromes. St. Louis, MO: Mosby, 2002.
22. Schleip R. The Importance of Fasciae in Manual Therapy. Deutsche Z für Osteopathie 1: 10–16, 2004.
23. Schleip R, Müller DG. Training principles for fascial connective tissues: Scientific foundation and suggested practical applications. J Bodyw Mov Ther 17: 103–115, 2013.
24. Shadmehr A, Hadian MR, Naiemi SS, Jalaie S. Hamstring flexibility in young women following passive stretch and muscle energy technique. J Back Musculoskelet Rehabil 22: 143–148, 2009.
25. Sharman MJ, Cresswell AG, Riek S. Proprioceptive neuromuscular facilitation stretching: Mechanisms and clinical implications. Sports Med 36: 929–939, 2006.
26. Sullivan KM, Silvey DB, Button DC, Behm DG. Roller-massager application to the hamstrings increases sit-and-reach range of motion within five to ten seconds without performance impairments. Int J Sports Phys Ther 8: 228–236, 2013.
27. Tanigawa MC. Comparison of the hold-relax procedure and passive mobilization on increasing muscle length. Phys Ther 52: 725–735, 1972.
28. Thacker SB, Gilchrist J, Stroup DF, Kimsey CD Jr. The impact of stretching on sports injury risk: A systematic review of the literature. Med Sci Sports Exerc 36: 371–378, 2004.
29. Vernon H, Schneider M. Chiropractic management of myofascial trigger points and myofascial pain syndrome: A systematic review of the literature. J Manipulative Physiol Ther 32: 14–24, 2009.
30. Weppler CH, Magnusson SP. Increasing muscle extensibility: A matter of increasing length or modifying sensation? Phys Ther 90: 438–449, 2010.
31. White LC, Dolphin P, Dixon J. Hamstring length in patellofemoral pain syndrome. Physiotherapy 95: 24–28, 2009.
32. Witvrouw E, Danneels L, Asselman P, D'Have T, Cambier D. Muscle flexibility as a risk factor for developing muscle injuries in male professional soccer players. A prospective study. Am J Sports Med 31: 41–46, 2003.
Keywords:

contract-relax PNF; myofascial release; range of motion; stretching

Copyright © 2015 by the National Strength & Conditioning Association.