To stretch or not to stretch?
– Written by Kieran O’Sullivan and Sean McAulliffe, Ireland and Gregory Lehman, Canada
Static stretching (SS) is commonly practiced by athletes, both as a part of a pre-exercise routine and part of an ongoing exercise programme. While different types of stretching can be used for slightly different reasons, the aims of SS are typically to:
· improve flexibility,
· enhance physical performance,
· prevent injury to the musculotendinous unit (MTU) and/or
· improve the speed of return to sport after injury.
Research suggests that other forms of training and rehabilitation (e.g. strength training) can achieve many of these aims as effectively, or even more effectively, than SS. However, SS has remained an integral part of athletic preparation. We recently published a systematic review1 which showed that eccentric strength training programmes consistently increase flexibility, which might have been considered the primary benefit of SS and called into question whether SS has an additional benefit among athletic populations. In this article we wish to discuss whether there is evidence that SS is worth including in athlete management for each of these proposed aims. For each aim, the supporting evidence will be contrasted with the evidence for a graded strength training programme.
STATIC STRETCHING AND FLEXIBILITY
There is consistent evidence that SS increases flexibility in the short-term, although the gains in flexibility decrease relatively quickly, such that they are lost within 30 minutes2-6. There is also consistent evidence that SS performed regularly for several weeks results in meaningful improvements in range of motion (ROM)7-12.
Mechanism of increasing flexibility with SS
The mechanism of how this increased ROM occurs is subject to debate, with changes in neurophysiological (e.g. changes in stretch tolerance) and mechanical (e.g. viscoelastic changes, sarcomerogenesis) factors proposed. There is little doubt from animal studies13,14 that sustained SS, or immobilisation, can induce mechanical tissue changes, with tissues adapting and remodelling in response to the stress they are placed under. Interpreting this animal research must acknowledge that some studies examine SS being applied continuously for days or even weeks13,14. This may not reflect the effect of SS being performed for just a few minutes 3 to 4 times per week. For example, even among animals, intermittent SS every few days (3 times/week for 40 minutes)15 may not prevent reductions in length caused by immobilisation.
Determining whether ROM changes are due to mechanical changes in the MTU among humans typically requires examining whether SS reduces the passive resistance to torque (PRT) when producing a specified ROM or whether SS reduces tendon stiffness (e.g. measured using elastography) during contraction. Several studies have shown that sustained SS does not change MTU mechanical properties11,16-22, with no change in the PRT following SS despite increases in ROM. This would suggest that the increased ROM is due to increased stretch tolerance, rather than any mechanical alteration in the tissues. However, several other studies23-27 have reported changes in PRT after SS, with the degree of increase in ROM being correlated with the decrease in PRT25,26. While the disagreement on changes in PRT is confusing, there is agreement on the lack of change in tendon stiffness following sustained SS, irrespective of whether studies have23,24,26 or have not21 reported a change in PRT. This discrepancy suggests that if structural changes occur following SS, they may be more likely to occur in the parallel elastic component of the musculature or even the joint capsule rather than within the tendon itself. While tendon stiffness does not appear to change following sustained SS, there does appear to be changes in the viscoelastic properties of tendons in the long-term, as evidenced in changes in the hysteresis loop in tendons24.
Strength training has more consistently been shown to influence the mechanical and neurophysiological properties of the MTU. Both animal28,29 and human30-32 studies consistently demonstrate that eccentric strength training results in increased flexibility, measured using indirect measures such as joint ROM30,33 and direct measurement of fascicle length31,32,34. This is thought to reflect the addition of sarcomeres in series (sarcomerogenesis), as documented in different muscle groups30-32. This hypothesis is further supported by the fact that increases in ROM35 and fascicle length34 closely parallel changes in the muscle length-tension curve36. Numerous authors have shown that strength training increases the PRT of muscle17,23, increases tendon stiffness37-39 and increases stiffness of the MTU40,41. This increased stiffness is associated with increased flexibility, where eccentric strength training for as little as 6 weeks significant increases flexibility30,33. The magnitude of increase in flexibility reported after eccentric training1,30,33,42 appears to be similar to that achieved after SS8,12,43 and sufficient to address deficits observed among those with recurrent pain/injury6,44,45. However, it is important to highlight that the precise type of eccentric training performed varies a lot between studies. For example, while all studies in our recent review1 showed increased flexibility after eccentric training, the largest increase was seen in a study30 which incorporated a static hold in the elongated position. Therefore, the improvements in flexibility after some eccentric training may not be as large as those obtained by SS. There is some inconsistency in the literature regarding the effect of other strength training programmes on flexibility. Most studies suggest that eccentric-only programmes result in greater flexibility gains than concentric-only32,46 or isometric-only47 training programmes. However, some research has reported findings suggesting that there may be no difference in flexibility gains dependent on the mode of strength training, with the ROM through which the muscle is exercised possibly being more critical than the mode of exercise34. Further research is therefore needed to clarify whether flexibility can be effectively increased with non-eccentric strength training.
In summary
· SS increases flexibility in both the short- and long-term.
· The precise mechanism(s) through which SS achieves increased flexibility is still debatable. While enhanced stretch tolerance is likely and changes in tendon stiffness are very unlikely, mechanical changes in PRT have been reported inconsistently.
· Flexibility is also increased by strength training, especially eccentric training. The magnitude of increase achieved appears to be close to that achieved through SS. Interestingly, strength training appears to increase both tendon stiffness and overall MTU stiffness, while simultaneously increasing ROM.
· Neither SS nor strength training appears to consistently decrease the stiffness of the joints.
STATIC STRETCHING AND PERFORMANCE
Acute SS
Effect on strength, power and explosive muscle performance
Several recent reviews and meta-analyses48-51 have agreed that SS maintained for greater than 45 seconds immediately before performance either:
· negatively influences maximal strength, power, muscular explosive performance (e.g. jumping and sprinting), balance and agility or
· has no effect on performance.
In other words, none of the reviews showed a beneficial effect of SS on performance! There is an apparent dose-response relationship with shorter duration SS being less commonly associated with decrements in performance49. Maximal strength appears to be more commonly negatively affected by SS than explosive muscular performance or power48,50.
Effect on endurance performance
The influence of acute SS on activities such as running and cycling is less clear cut. A recent review52 described how several studies53-56 show a negative influence while others52,57-59 show no change in performance. Very few studies60 report improvements in endurance performance following acute SS.
Sustained SS
Effect on strength, power and explosive muscular performance
Overall, fewer studies have assessed the influence of sustained SS on performance. While several studies have shown some improvements in some of these performance measures after sustained SS25,61-63, this is not observed on all measures in these studies61 or at all in many other studies17,25,64. Typically, the studies which have shown improvements in performance after SS have compared to an inactive control group, rather than another exercise group. For the most part SS does not appear to impair performance. In marked contrast, far superior and more consistent improvements in muscular performance are evident after strength training17,25,65. The addition of SS to strength training does not appear to provide additional benefits nor to influence the passive properties of the MTU17,65,66. However a recent study67 showed that SS which was consistently performed immediately before strength training actually reduced the gains in strength achieved compared to strength training alone.
Effect on endurance performance
Sustained SS does not appear to enhance running or walking efficiency25,68,69, even when ROM is increased69. Results are equivocal with SS and endurance performance68,69. In contrast, strength training consistently improves endurance performance66,70-72.
In summary
· Acute SS for greater than 45 seconds should be avoided immediately before participation in activities where strength or power are important, as performance is likely to be reduced without any clear benefits to justify its continued use.
· Shorter durations of SS are also hard to justify immediately before participation in activities where strength or power are important given the potential for decreased performance and lack of clear benefits.
· In endurance activities, acute SS is hard to justify immediately before participation as performance may be reduced with no clear benefits to justify its continued use.
· Sustained SS is not associated with as clear a decline in performance as acute SS and may in fact enhance strength and power compared to not performing any exercise programme. However, since SS is far less effective than strength training in enhancing strength and power and it is unclear whether adding SS might reduce the strength gains achieved, it is hard to justify its continued use.
· Sustained SS does not appear to enhance gait economy, nor consistently enhance endurance performance. In contrast, strength training enhances endurance performance.
STATIC STRETCHING AND INJURY PREVENTION OR TREATMENT
It seems to make sense intuitively that SS should help prevent injury, or help hasten return to activity. For example, athletes with a previous injury often remain less flexible in that limb6,44 and often report ‘tightness’ prior to injury, or intermittently in a previously injured region. In addition, the degree to which flexibility is reduced in the early days post-injury appears to be linked to the speed of return to activity73. Finally, several studies have suggested that decreased flexibility at baseline, or stretching less often, may predispose to injury74-80.
However, the relationship between baseline flexibility and future injury risk is complex, with many studies81,82 demonstrating no relationship between the two. In fact, several systematic reviews have now evaluated the effect of SS on risk of injury83-85. These reviews conclude that: “There is not sufficient evidence to endorse or discontinue routine stretching before or after exercise to prevent injury among competitive or recreational athletes83” and “In light of these findings, routine stretching exercises before initiation of sport activities are not a proven, effective method for reducing injury rates86”. This scientific literature contradicts long-held beliefs about the proposed benefits of SS for injury prevention. It could be argued that many studies have typically included SS as part of a multimodal intervention, making it difficult to pronounce definitive conclusions about the specific contribution of SS. A recent meta-analysis87 addressed this concern, by investigating the effectiveness of exercise interventions at preventing sports injuries while allowing for multimodal exposures and calculating the relative risk of each intervention. Unsurprisingly, their findings “do not support the use of stretching for injury prevention purposes, neither before or after exercise87.” Therefore, at least in in terms of injury prevention, it appears SS has very little to offer and should not be used.
In contrast, a wide range of other interventions can reduce injury rates, with Laursen et al87 stating that “consistently favourable estimates were obtained for all injury prevention measures except for stretching.” For example, consider the strong evidence that gradually progressive strength training programmes which are maintained after return to sport and/or throughout the season can greatly reduce the occurrence and, in particular, recurrence of a range of musculoskeletal pains and/or injuries88-94. The aforementioned meta-analysis87 showed that strength training reduced incidence of sports injuries to less than one third. Thus while there is substantial evidence to suggest a negligible protective effect of SS on sports injury, strength training – and in particular eccentric strength training – positively influences injury rates.
In terms of hastening return to sport, the relatively weak support for the role of SS in reviews of injury management95,96 is based on a single randomised controlled trial97 which showed that performing SS four times per day instead of just once per day reduces the time until return to sport. However, performing SS less frequently only delayed return to sport by less than 2 days, such that this may not be a clinically significant finding. In contrast, another randomised controlled trial89 showed that an eccentric-based strength training programme reduced the time to return to sport significantly more than a ‘conventional’ training programme which included a mix of SS, concentric and eccentric exercises. In this case, the mean difference between groups for time to return to sport was a huge 23 days! It should be acknowledged that the latter study did not allow return to sport until athletes were able to complete the ‘H-test’ satisfactorily, meaning that athletes in both groups did not return to sport as quickly as in the other study97. Unfortunately, there has been no randomised controlled trial comparing either SS or strength training to natural recovery, which is needed to conclusively demonstrate that either of these approaches is better than no treatment.
In summary
· SS does not appear to reduce injury risk and any effect on earlier return to sport is of marginal clinical significance.
· In contrast, a graduated strength training programme appears to significantly reduce injury risk and significantly reduce the time to return to sport after injury.
CLINICAL IMPLICATIONS
It is difficult to justify the use of SS for any of the aims examined in this article:
· Flexibility: SS is very effective in both the short-term and long-term. However, (i) flexibility is not as important a factor in performance and injury prevention as once thought and (ii) other methods of increasing flexibility such as gradually progressive strength training are available. It remains unclear whether increases in flexibility after strength training are equivalent to those after SS, however they appear greatest after eccentric programmes.
· Short-term performance: SS may diminish performance, especially when performed immediately before explosive activities. While the effect of chronic SS is equivocal, strength training programmes are associated with improvements in performance and adding SS to strength training may actually reduce the strength gains achieved.
· Injury prevention or return to sport after injury: there is very little evidence that SS is effective. In contrast, there is considerable evidence that progressive strength training programmes, which typically include an eccentric component, reduce injury risk, pain and disability in a range of musculotendinous conditions, as well as hastening return to sport.
Therefore, the only area in which SS might seem to offer a specific advantage is in the area of increasing flexibility. There may be times when the most important goal is enhancing flexibility (e.g. ballet) and in these isolated circumstances SS may be justifiable. However, there remains a lack of evidence that gains are superior to those of a strength training programme. Even if strength training is eventually confirmed as being inferior to SS at increasing flexibility, the fact that strength training improves performance, pain, disability, injury and return to sport rates mean strength training must be a mainstay of athletic development and training, in contrast to SS.
Kieran O’Sullivan Ph.D., M Manip Ther, B Physio, S.M.I.S.C.P.
Lecturer in Clinical Therapies
Sean McAulliffe B.Sc. Physio, M.I.S.C.P.
Ph.D. Student
University of Limerick
Limerick, Ireland
Gregory Lehman M.Sc., D.C., M.Sc.P.T.
Private Practice Physiotherapist
The Urban Athlete
Toronto, Canada
Contact: Kieran.OSullivan@ul.ie
References
1. O'Sullivan K, McAuliffe S, DeBurca N. The effects of eccentric training on lower limb flexibility: a systematic review. Br J Sports Med 2012; 46:838-845.
2. de Weijer VC, Gorniak GC, Shamus E. The effect of static stretch and warm-up exercise on hamstring length over the course of 24 hours. J Orthop Sports Phys Ther 2003; 33:727-733.
3. DePino G, Webright W, Arnold B, Duration of maintained hamstring flexibility after cessation of an acute static stretching protocol. J Athl Train 2 2000; 35:56-59.
4. Spernoga SG, Uhl TL, Arnold BL, Gansneder BM. Duration of maintained hamstring flexibility after a one-time, modified hold-relax stretching protocol. J Athl Train 2001; 36:44-48.
5. Ford P, McChesney J. Duration of maintained hamstring ROM following termination of three stretching protocols. J Sport Rehabil 2007; 16:18-27.
6. O'Sullivan K, Murray E, Sainsbury D. The effect of warm-up, static stretching and dynamic stretching on hamstring flexibility in previously injured subjects. BMC Musculoskelet Disord 2009; 10:37.
7. Harvey L, Herbert R, Crosbie J. Does stretching induce lasting increases in joint ROM? A systematic review. Physiother Res Int2002; 7:1-13.
8. Radford JA, Burns J, Buchbinder R, Landorf KB, Cook C. Does stretching increase ankle dorsiflexion range of motion? A systematic review. Br J Sports Med 2006; 40:870-875.
9. Bandy WD, Irion JM, Briggler M. The effect of static stretch and dynamic range of motion training on the flexibility of the hamstring muscles. J Orthop Sports Phys Ther 1998; 27:295-300.
10. 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 2001; 11:81-86.
11. Reid DA, McNair PJ. Passive force, angle, and stiffness changes after stretching of hamstring muscles. Med Sci Sports Exerc 2004; 36:1944-1948.
12. 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 2005; 35:377-387.
13. Herbert RD, Balnave RJ. The effect of position of immobilisation on resting length, resting stiffness, and weight of the soleus muscle of the rabbit. J Orthop Res 1993; 11:358-366.
14. Spector SA, Simard CP, Fournier M, Sternlicht E, Edgerton VR. Architectural alterations of rat hind-limb skeletal muscles immobilized at different lengths. Exp Neurol 1982; 76:94-110.
15. Coutinho EL, Gomes AR, França CN, Oishi J, Salvini TF. Effect of passive stretching on the immobilized soleus muscle fiber morphology. Braz J Med Biol Res 2004; 37:1853-1861.
16. Halbertsma JP, Goeken LN. Stretching exercises: effect on passive extensibility and stiffness in short hamstrings of healthy subjects. Arch Phys Med Rehabil 1994; 75:976-981.
17. Klinge K, Magnusson SP, Simonsen EB, Aagaard P, Klausen K, Kjaer M. The effect of strength and flexibility training on skeletal muscle electromyographic activity, stiffness, and viscoelastic stress relaxation response. Am J Sports Med 1997; 25:710-716.
18. Magnusson SP, Simonsen EB, Aagaard P, Boesen J, Johannsen F, Kjaer M. Determinants of musculoskeletal flexibility: viscoelastic properties, cross‐sectional area, EMG and stretch tolerance. Scand J Med Sci Sports 1997; 7:195-202.
19. Gajdosik RL, Allred JD, Gabbert HL, Sonsteng BA. A stretching program increases the dynamic passive length and passive resistive properties of the calf muscle-tendon unit of unconditioned younger women. Eur J Appl Physiol 2007; 99:449-454.
20. Weppler CH, Magnusson SP. Increasing muscle extensibility: a matter of increasing length or modifying sensation? Phys Ther 2010; 90:438-449.
21. Konrad A, Gad M, Tilp M. Effect of PNF stretching training on the properties of human muscle and tendon structures. Scand J Med Sci Sports 2014.
22. Ben M, Harvey L. Regular stretch does not increase muscle extensibility: a randomized controlled trial. Scand J Med Sci Sports 2010; 20:136-144.
23. Kubo K, Kanehisa H, Fukunaga T. Effects of resistance and stretching training programmes on the viscoelastic properties of human tendon structures in vivo. J Physiol 2002; 538:219-226.
24. Kubo K, Kanehisa H, Fukunaga T. Effect of stretching training on the viscoelastic properties of human tendon structures in vivo. J Appl Physiol 2002;92:595-601.
25. Guissard N, Duchateau J. Effect of static stretch training on neural and mechanical properties of the human plantar‐flexor muscles. Muscle Nerve 2004; 29:248-255.
26. Mahieu N, McNair P, De Muynck M, Stevens V, Blanckaert I, Smits N, et al. Detection and approach of intrinsic risk factors for achilles tendinopathy: University of Ghent; 2007.
27. Marshall PW, Cashman A, Cheema BS. A randomized controlled trial for the effect of passive stretching on measures of hamstring extensibility, passive stiffness, strength, and stretch tolerance. J Sci Med Sport 2011; 14:535-540.
28. Lynn R, Morgan DL. Decline running produces more sarcomeres in rat vastus intermedius muscle fibres than does incline running. J Appl Physiol 1994; 77:1439-1444.
29. Morgan D. New Insights into the behaviour of muscle during active lengthening. Biophys J 1990; 57:209-221.
30. Nelson RT, Bandy WD. Eccentric training and static stretching improve hamstring flexibility of high school males. J Athl Train 2004; 39:254-258.
31. Duclay J, Martin A, Duclay A, Cometti G, Pousson M. Behavior of fascicles and the myotendinous junction of human medial gastrocnemius following eccentric strength training. Muscle Nerve 2009; 39:819-827.
32. Reeves ND, Maganaris CN, Longo S, Narici MV. Differential adaptations to eccentric versus conventional resistance training in older humans. Exp Physiol 2009; 94:825-833.
33. Mahieu NN, McNair P, Cools A, D'Haen C, Vandermeulen K, Witvrouw E. Effect of eccentric training on the plantar flexor muscle-tendon tissue properties. Med Sci Sports Exerc 2008; 40:117-123.
34. Blazevich AJ, Cannavan D, Coleman DR, Horne S. Influence of concentric and eccentric resistance training on architectural adaptation in human quadriceps muscles. J Appl Physiol 2007; 103:1565-1575
35. Aquino CF, Fonseca ST, Gonçalves GG, Silva PL, Ocarino JM, Mancini MC. Stretching versus strength training in lengthened position in subjects with tight hamstring muscles: a randomized controlled trial. Man Ther 2010; 15:26-31.
36. Kilgallon M, Donnelly AE, Shafat A. Progressive resistance training temporarily alters hamstring torque-angle relationship. Scand J Med Sci Sports 2007; 17:18-24.
37. Kubo K, Ikebukuro T, Maki A, Yata H, Tsunoda N. Time course of changes in the human achilles tendon properties and metabolism during training and detraining in vivo. Eur J Appl Physiol 2012; 112:2679-2691.
38. Waugh CM, Korff T, Fath F, Blazevich AJ. Effects of resistance training on tendon mechanical properties and rapid force production in prepubertal children. J Appl Physiol 2014; 117:257-266.
39. Kubo K, Yata H, Kanehisa H, Fukunaga T. Effects of isometric squat training on the tendon stiffness and jump performance. Eur J Appl Physiol 2006; 96:305-314.
40. Blackburn JT, Norcross MF. The effects of isometric and isotonic training on hamstring stiffness and anterior cruciate ligament loading mechanisms. J Electromyogr Kinesiol 2014; 24:98-103.
41. Tillin NA, Pain MT, Folland JP. Short-term training for explosive strength causes neural and mechanical adaptations. Exp Physiol 2012; 97:630-641.
42. Potier TG, Alexander CM, Seynnes OR. Effects of eccentric strength training on biceps femoris muscle architecture and knee joint range of movement. Eur J Appl Physiol 2009; 105:939-944.
43. Bandy WD, Irion JM, Briggler M. The effect of time and frequency of static stretching on flexibility of the hamstring muscles. Phys Ther 1997; 77:1090-1096.
44. Worrell TW, Perrin DH, Gansneder BM, Gieck JH. Comparison of isokinetic strength and flexibility measures between hamstring injured and noninjured athletes. J Orthop Sports Phys Ther 1991; 13:118-125.
45. McHugh N, McAuliffe S, O'Sullivan K. Eccentric training for hamstring injury, and its relationship to strength and flexibility: a case-series. Physiother Pract Res 2014; 35:111-122.
46. Franchi MV, Atherton PJ, Reeves ND, Flück M, Williams J, Mitchell W, et al. Architectural, functional and molecular responses to concentric and eccentric loading in human skeletal muscle. Acta Physiol 2014; 210:642-654.
47. Albracht K, Arampatzis A. Exercise-induced changes in triceps surae tendon stiffness and muscle strength affect running economy in humans. Eur J Appl Physiol 2013; 113:1605-1615.
48. Behm D, Bambury A, Farrel C, Power K. Effect of acute static stretching on force, balance, reaction time, and movement time. Med Sci Sports Exerc 2004; 36:1397-1402.
49. Kay AD, Blazevich AJ. Effect of acute static stretch on maximal muscle performance: a systematic review. Med Sci Sports Exerc 2012; 44:154-164.
50. Simic L, Sarabon N, Markovic G. Does pre‐exercise static stretching inhibit maximal muscular performance? A meta‐analytical review. Scand J Med Sci Sports 2013; 23:131-148.
51. Winchester JB, Nelson AG, Landin D, Young MA, Schexnayder IC. Static stretching impairs sprint performance in collegiate track and field athletes. J Strength Cond Res 2008; 22:13-18.
52. Peck E, Chomko G, Gaz DV, Farrell AM. The effects of stretching on performance. Curr Sports Med Rep 2014; 13:179-185.
53. Wilson JM, Hornbuckle LM, Kim JS, Ugrinowitsch C, Lee S-R, Zourdos MC, et al. Effects of static stretching on energy cost and running endurance performance. J Strength Cond Res 2010; 24:2274-2279.
54. Lowery RP, Joy JM, Brown LE, Oliveira de Souza E, Wistocki DR, Davis GS, et al. Effects of static stretching on 1-mile uphill run performance. J Strength Cond Res 2014; 28:161-167.
55. Wolfe AE, Brown LE, Coburn JW, Kersey RD, Bottaro M. Time course of the effects of static stretching on cycling economy. J Strength Cond Res 2011; 25:2980-2984.
56. Esposito F, Cè E, Limonta E. Cycling efficiency and time to exhaustion are reduced after acute passive stretching administration. Scand J Med Sci Sports 2012; 22:737-745.
57. Mojock CD, Kim JS, Eccles DW, Panton LB. The effects of static stretching on running economy and endurance performance in female distance runners during treadmill running. J Strength Cond Res 2011; 25:2170-2176.
58. Hayes PR, Walker A. Pre-exercise stretching does not impact upon running economy. J Strength Cond Res 2007; 21:1227-1232.
59. Allison SJ, Bailey DM, Folland JP. Prolonged static stretching does not influence running economy despite changes in neuromuscular function. J Sports Sci 2008; 26:1489-1495.
60. Godges JJ, Macrae H, Longdon C, Tinberg C, Macrae PG. The effects of two stretching procedures on hip range of motion and gait economy. J Orthop Sports Phys Ther. 1989; 10:350-357.
61. Hunter JP, Marshall RN. Effects of power and flexibility training on vertical jump technique. Med Sci Sports Exerc 2002; 34:478-486.
62. Wilson GJ, Elliott BC, Wood GA. Stretch shorten cycle performance enhancement through flexibility training. Med Sci Sports Exerc 1992; 24:116-123.
63. Kokkonen J, Nelson AG, Eldredge C, Winchester JB. Chronic static stretching improves exercise performance. Med Sci Sports Exerc 2007; 39:1825-1831.
64. Bazett-Jones DM, Gibson MH, McBride JM. Sprint and vertical jump performances are not affected by six weeks of static hamstring stretching. J Strength Cond Res 2008; 22:25-31.
65. Souza AC, Bentes CM, Salles BF, Reis VM, Alves JV, Miranda H, et al. Influence of inter-set stretching on strength, flexibility and hormonal adaptations. J Hum Kinet 2013; 36:127-135.
66. Saunders PU, Telford RD, Pyne DB, Peltola EM, Cunningham RB, Gore CJ, et al. Short-term plyometric training improves running economy in highly trained middle and long distance runners. J Strength Cond Res 2006; 20:947-954.
67. Borges Bastos CL, Miranda H, Vale RG, Portal Mde N, Gomes MT, Novaes Jda S, et al. Chronic effect of static stretching on strength performance and basal serum Igf-1 levels. J Strength Cond Res 2013; 27:2465-2472.
68. Nelson A, Kokkonen J, Eldredge C, Cornwell A, Glickman‐Weiss E. Chronic stretching and running economy. Scand J Med Sci Sports 2001; 11:260-265.
69. Godges JJ, MacRae PG, Engelke KA. Effects of exercise on hip range of motion, trunk muscle performance, and gait economy. Phys Ther 1993; 73:468-477.
70. Millet GP, Jaouen B, Borrani F, Candau R. Effects of concurrent endurance and strength training on running economy and VO2 kinetics. Med Sci Sports Exerc 2002; 34:1351-1359.
71. Beattie K, Kenny IC, Lyons M, Carson BP. The effect of strength training on performance in endurance athletes. Sports Med 2014; 44:845-865.
72. Rønnestad BR, Mujika I. Optimizing strength training for running and cycling endurance performance: a review. Scand J Med Sci Sports 2013.
73. Malliaropoulos N, Papacostas E, Kiritsi O, Papalada A, Gougoulias N, Maffulli N. Posterior thigh muscle injuries in elite track and field athletes. Am J Sports Med 2010; 38:1813-1819.
74. Hartig DE, Henderson JM. Increasing hamstring flexibility decreases lower extremity overuse injuries in military basic trainees. Am J Sports Med 1999; 27:173-176.
75. Jonhagen S, Németh G, Eriksson E. Hamstring Injuries in sprinters. The role of concentric and eccentric hamstring muscle strength and flexibility. Am J Sports Med 1994; 22:262-266.
76. Cross KM, Worrell TW. Effects of a static stretching program on the incidence of lower extremity musculotendinous strains. J Athl Train 1999; 34:11-14.
77. Liemohn W. Factors related to hamstring strains. J Sports Med Phys Fitness. 1978; 18:71-76.
78. Witvrouw E, Danneels L, Asselman P, D’Have T, Cambier D. Muscle flexibility as a risk ractor for developing muscle injuries in male professional soccer players: a prospective study. Am J Sports Med 2003; 31:41-46.
79. Witvrouw E, Bellemans J, Lysens R, Danneels L, Cambier D. Intrinsic Risk Factors for the Development of Patellar Tendinitis in an Athletic Population a Two-Year Prospective Study. Am J Sports Med 2001; 29:190-195.
80. Dadebo B, White J, George KP. A survey of flexibility training protocols and hamstring strains in professional football clubs in England. Br J Sports Med 2004; 38:388-394.
81. Bennell K, Tully E, Harvey N. Does the toe-touch test predict hamstring injury in Australian Rules footballers? Aust J Physiother 1999; 45:103-109.
82. Wang HK, Chen CH, Shiang TY, Jan MH, Lin KH. Risk-factor analysis of high school basketball–player ankle injuries: a prospective controlled cohort study evaluating postural sway, ankle strength, and flexibility. Arch Phys Med Rehabil 2006; 87:821-825.
83. Thacker SB, Gilchrist J, Stroup DF, Kimsey CJ. The impact of stretching on sports injury risk: a systematic review of the literature. Med Sci Sports Exerc 2004; 36:371-378.
84. Herbert RD, Gabriel M. Effects of stretching before and after exercising on muscle soreness and risk of injury: systematic review. BMJ 2002; 325:468.
85. Shrier I. Does stretching improve performance? A systematic and critical review of the literature. Clin J Sport Med 2004; 14:267-273.
86. Schiff MA, Caine DJ, O'Halloran R. Injury prevention in sports. Am J Lifestyle Med 2010; 4:42-64.
87. Lauersen JB, Bertelsen DM, Andersen LB. The effectiveness of exercise interventions to prevent sports injuries: a systematic review and meta-analysis of randomised controlled trials. Br J Sports Med 2014; 48:871-877
88. Arnason A, Andersen TE, Holme I, Engebretsen L, Bahr R. Prevention of hamstring strains in elite soccer: an intervention study. Scand J Med Sci Sports 2008; 18:40-48.
89. Askling CM, Tengvar M, Thorstensson A. Acute hamstring injuries in Swedish elite football: a prospective randomised controlled clinical trial comparing two rehabilitation protocols. Br J Sports Med 2013; 47:953-959.
90. Petersen J, Thorborg K, Nielsen MB, Budtz-Jørgensen E, Hölmich P. Preventive effect of eccentric training on acute hamstring injuries in men’s soccer a cluster-randomized controlled trial. Am J Sports Med 2011; 39:2296-2303.
91. Mjolsnes R, Arnason A, Østhagen T, Raastad T, Bahr R. A 10-Week randomized trial comparing eccentric vs. concentric hamstring strength training in well-trained soccer players. Scand J Med Sci Sports 2004 ;14:311-317.
92. Alfredson H, Pietilä T, Jonsson P, Lorentzon R. Heavy-Load eccentric calf muscle training for the treatment of chronic achilles tendinosis. Am Journal of Sports Med 1998; 26:360-366.
93. Waldén M, Atroshi I, Magnusson H, Wagner P, Hägglund M. Prevention of acute knee injuries in adolescent female football players: cluster randomised controlled trial. BMJ 2012; 344:e3042.
94. Wasielewski NJ, Kotsko KM. Does eccentric exercise reduce pain and improve strength in physically active adults with symptomatic lower extremity tendinosis? A systematic review. J Athl Train 2007; 42:409-421.
95. Petersen J, Holmich P. Evidence based prevention of hamstring injuries in sport. Br J Sports Med 2005; 39:319-323.
96. Mason DL, Dickens VA, Vail A. Rehabilitation for hamstring injuries. Cochrane Database Syst Rev 2012.
97. Malliaropoulos N, Papalexandris S, Papalada A, Papacostas E. The role of stretching in rehabilitation of hamstring injuries : 80 athletes follow-up. Med Sci Sports Exerc 2004; 36:756-759.
98. Ryan J, McCreesh K, Deburca N. To identify intrinsic risk factors for groin/hip Injuries among academy level rugby union players: a prospective cohort study. Br J Sports Med 2014; 48:655
99. Wyss TF, Clark JM, Weishaupt D, Nötzli HP. Correlation between internal rotation and bony anatomy in the hip. Clin Orthop Relat Res 2007; 460:152-158.
100. Askling CM, Nilsson J, Thorstensson A. A new hamstring test to complement the common clinical examination before return to sport after injury. Knee Surg Sports Traumatol Arthrosc 2010; 18:1798-1803.
101. Schache AG, Crossley KM, Macindoe IG, Fahrner BB, Pandy MG. Can a clinical test of hamstring strength identify football players at risk of hamstring strain? Knee Surg Sports Traumatol Arthrosc 2011; 19:38-41.
102. Kingma JJ, de Knikker R, Wittink HM, Takken T. Eccentric overload training in patients with chronic achilles tendinopathy: a systematic review. Br J Sports Med 2007; 41:e3.
103. Young MA, Cook JL, Purdam C, Kiss ZS, Alfredson H. Eccentric decline squat protocol offers superior results at 12 months compared with traditional eccentric protocol for patellar tendinopathy in volleyball players. Br J Sports Med 2005; 39:102-105.
104. Gabbett TJ. Reductions in pre-season training loads reduce training Injury rates in rugby league players. Br J Sports Med 2004; 38:743-749.
105. Gabbett TJ, Jenkins DG. Relationship between training load and injury in professional rugby league players. J Sci Med Sport 2011; 14:204-209.
106. Coutts AJ, Reaburn P, Piva TJ, Rowsell GJ. Monitoring for overreaching in rugby league players. Eur J Appl Physiol 2007; 99:313-324.
107. Brink MS, Visscher C, Coutts AJ, Lemmink KA. Changes in perceived stress and recovery in overreached young elite soccer players. Scand J Med Sci Sports 2012; 22:285-292.
108. Fletcher IM, Anness R. The acute effects of combined static and dynamic stretch protocols on fifty-meter sprint performance in track-and-field athletes. J Strength Cond Res 2007; 21:784-787.