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Training Considerations after Hamstring Injury in Athletes

Comfort, Paul MSc, CSCS1; Green, Carly M BSc, CSCS2; Matthews, Martyn MSc, CSCS1

Strength and Conditioning Journal: February 2009 - Volume 31 - Issue 1 - p 68-74
doi: 10.1519/SSC.0b013e318195d225
ARTICLES
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HAMSTRING STRAINS ARE COMPOUNDED BY A HIGH RECURRENCE RATE OF 12-31% WITHIN THE FIRST YEAR OF RETURN TO SPORT. EXPLANATIONS INCLUDE REDUCED TENSILE STRENGTH, REDUCED STRENGTH OF MUSCLE AND SURROUNDING SITES, AND REDUCED FLEXIBILITY OF THE MUSCLE TENDON UNIT. STRETCHING ALONE IS INSUFFICIENT FOR COMPLETE REHABILITATION, RESULTING IN A RECURRENCE RATE AS HIGH AS 54.5% WITHIN 2 WEEKS OF RETURN TO SPORT. SPORT-SPECIFIC EXERCISE HAS BEEN SHOWN TO BE MORE SUCCESSFUL. FOR COMPLETE REHABILITATION OF HAMSTRING STRAIN, THE INCLUSION OF ECCENTRIC PLYOMETRIC EXERCISE AND SPORT SPECIFIC DRILLS AT THE APPROPRIATE TIME IS ESSENTIAL.

1University of Salford, Greater Manchester; and 2Sports Injury Specialist Clinic, Essex, United Kingdom

Paul Comfortis a lecturer and strength and conditioning coach at University of Salford.

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Figure

Carly Greenis graduate sports rehabilitator, strength and conditioning coach and founder of the Sports Injury Specialist Clinic.

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Figure

Martyn Matthewsis a senior lecturer and strength and conditioning coach at University of Salford.

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INTRODUCTION

Hamstring strains often are difficult to treat, slow to rehabilitate, and are compounded by a high recurrence rate (23,58,60). Normal recurrence rates are between 12% and 31% within 1 year of returning to sport (60) but may be as high as 54.5% (68). Factors responsible for the recurrence in hamstring injury include reduced tensile strength of the scar tissue at the site of injury, reduced strength of surrounding musculature as the result of disuse atrophy, reflex inhibition, reduced flexibility of the muscle tendon unit (MTU), and possible adaptive changes in the biomechanics of sporting movements after the original injury (23,58,60).

Strains to the MTU are among the most prevalent forms of injury for athletes (17,44,48,80) with muscles that span 2 joints, such as the hamstring, being particularly susceptible to strain (22). Hamstrings are the most commonly injured muscles in the lower limb, with strains prevalent in sports such as soccer and track and field athletics (14). Hamstring strains represent 12-16% of injuries among soccer (28,60) and Australian Rules Football players (60), 11% in cricket players (71), and 24% among collegiate sprinters and jumpers (79).

The high recurrence rates reported for hamstring injuries (23,58,60,68,71,79) may be reduced by a progressive reintroduction of activities that will prepare the athlete fully for the demands of the sport. This complete rehabilitation should incorporate and allow sufficient time for several stages of healing and treatment. These stages include mobilization and stretching, to avoid loss of extensibility, improve range of motion (ROM), and help reduce formation of scar tissue (39,40,50,52); avoidance of atrophy and regaining of strength (60,68); and advanced strength and conditioning that is sport specific to appropriately prepare the athlete for their return to sport (15,19,30,32,36,46,54).

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CAUSES OF INJURY

The literature suggests that there are 2 types of hamstring strains, one resulting from explosive high-speed running (75,79) and the other during stretching movements carried out at extreme ROM (5). Reported causes of hamstring strains include poor lumbar posture (33), previous injury (3,21,27,73), lack of flexibility (12,14,22,27,29,33,45,48,74), inadequate warm up (76,77), fatigue (76,77), strength imbalance and inadequate quadriceps to hamstring ratio (16,21,27,48,79), and poor coordination (14,16,27). Hamstring strains also have been associated with eccentric loading (14,16,50), such as during rapid deceleration.

Hamstring strains most commonly occur in the long head of the biceps femoris (7,51,72,75) with most located near the muscle-tendon junction (51,60). Hamstring strains are commonly reported in sprinters when speed is maximal or close to maximal (6) and during powerful eccentric muscle actions (14).

The strain is most likely to occur during 2 phases of the running cycle; late forward swing and toe off (69), because during this phase the hamstrings decelerate hip flexion and knee extension (39,40), resulting in large eccentric loads. It has also been found that although sprinters sustain their injuries during high-speed running, dancers sustain injuries while performing slow stretching-type exercises (6). In activities such as dancing, most hamstring injuries occur during stretching (hip flexion with knee extension) (6,8), resulting in an eccentric load, with the proximal end of the semimembranosus as the site of injury (8).

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RECOMMENDED REHABILITATION

Consensus of how to effectively treat hamstring injury is limited, although a multidisciplinary approach can be recommended (20). Treatment and rehabilitation should be tailored to the severity of injury, and the constraints of the healing process (44,50). Rehabilitation time varies depending on the severity of the injury, with an average duration of 16 weeks, but a range of 6-50 weeks (7). The use of magnetic resonance imaging has identified that hamstring injuries in dancers take an average of 50 weeks to return to preinjury status (8). Cryotherapy (34,41), nonsteroidal antiinflammatory drugs (18,34,35), electrotherapy modalities (34,65), and pain-free strengthening and stretching exercises (60,68) are all incorporated into the rehabilitation of soft tissue injuries; however, to further reduce the recurrence of injury, these strengthening exercises need to be specific to the demand of the individual's chosen sport/activity (15,19,30,32,36,46,54).

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RESTORING ROM

The healing of soft tissue is characterized by the formation of connective fibrous tissue that is both shorter and less elastic than the original structure, leading to a loss of flexibility and impaired function (24). In muscle, the development of scar tissue leads to restricted contractions and increased risk of rupture (23), which can lead to a decrease in the elasticity of the stretch shortening cycle because lengthening is impaired (43), which further increases the risk of recurrence. To regain the lost flexibility and prevent further injury and inflammation (18,24,76,77), the performance of concurrent pain-free stretching and strengthening exercises, beginning with isometrics and progressing to dynamic exercises, is essential (60). During the remodeling phase of rehabilitation, stretching of the muscle determines the stress lines along which collagen will be oriented. If this procedure fails to take place, tensile strength is not regained properly, leading to prolonged pain, limited function, and increased susceptibility to tissue injury (52).

Stretching the hamstrings has been shown to increase ROM at the hip joint with the most successful technique being static stretching performed 3-5 times per week, for a duration of 30-45 seconds, repeated up to 4 times (9-11,52,55,63,64). However, when restoring range of motion in an injured athlete, research has demonstrated that regular stretching (4 × 30 seconds; 3-4 times per day; from 48 hours after injury) reduces the time to restore normal ROM compared with stretching once per day (5.7 versus 7.3 days) (10,52). It is also worth noting that injured muscles with changed viscoelasticity may require longer stretches (>30 seconds) or more repetitions to obtain the same benefits as healthy muscles (52).

Stretching also has been shown to be a powerful stimulant of muscle protein synthesis and muscle growth that can be associated with an adaptation to increased functional length by adding or removing sarcomeres in series (31) and therefore should be continued in conjunction with strengthening exercises. Sarcomere length is adjusted back to the optimum for force generation, velocity, and power output (31) when stretching is performed after exercise or after injury.

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RESTORING STRENGTH

Stretching combined with other treatment protocols such as strengthening increases success, resulting in a decrease in recurrence (65,68,77). Sherry and Best (68) demonstrated only a 7% recurrence of injury when using progressive agility and trunk stabilization exercises compared with a 70% recurrence (within 1 year) in the stretching and strengthening group. The average time to return to sport was 22 days and 37 days, respectively. The progressive agility and trunk stabilization group were subject to isometric, slow and fast concentric and eccentric training, which is more representative of the demands of sport than the stretching and strengthening protocol. Hamstring-specific exercises, such as the leg curl and stiff-leg dead lift (Figure 1), should be incorporated into the strengthening program because they result in greater hamstring activity compared with the back squat (1,78). It is also worth noting that research suggests the back squat, regardless of technique variation, produces comparatively low activation of the hamstring compared to the quadriceps (1,25,26,42,53,57,59,67). The squat may not therefore be an appropriate exercise to strengthen the hamstrings against hamstring strain, although it will form an essential exercise to appropriately condition athletes prior to the performance of plyometric activities. Strength training may help prevent recurrence because it increases MTU stiffness and strength (49). The incorporation of running and agility drills also has demonstrated a much quicker (10-14 days) return to sport in a series of case studies, when combined with stretching, cryotherapy, and electrotherapy (34), with no recurrence of injury for the rest of the season.

Figure 1

Figure 1

The inclusion of eccentric (deceleration) training also appears to have a beneficial effect in preventing and rehabilitating hamstring strains and therefore may reduce recurrence of injury (13,14,19,46,61,62,79). This may be due to the fact that hamstring strains are associated with rapid eccentric loading (14,50). Eccentric training such as performing Nordic hamstring lowers (Figures 2 and 3) has been shown to decrease the risk of hamstring injuries (2,4,15,19,30,54) by creating greater strength gains than concentric training (46,47,54) and improving the hamstring:quadriceps ratio, especially at greater velocities (38,54). Eccentric training alters the angle of peak torque closer to full extension (13,19,47,54), which may also aid prevention of injury.

Figure 2

Figure 2

Figure 3

Figure 3

The incorporation of plyometrics and agility drills into an athlete's training program has also been shown to increase hamstring peak torque and improve hamstring quadriceps ratios (36). The incorporation of plyometric training may decrease the risk of reinjury (32), which can be attributed to the rapid eccentric loading. Progression of sport-specific and plyometric activities should develop from unidirectional (e.g., squat jumps) to bidirectional (e.g., bounding), and then to multidirectional movements (e.g., zigzag bounding) (32). Before the athlete commences high-intensity plyometrics, it is recommended that he or she can squat >150% body mass for >1 repetition (37,56). However, if plyometrics are performed in water, creating an unloading effect through buoyancy, they can be introduced earlier and have been shown to be highly effective (66,70).

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APPLICATION: THE CONDITIONING CONTINUUM

From a review of selected literature, it would appear that, post hamstring strain, restoration of ROM, is inadequate as a measure of an athlete's ability to return to sport. Moreover, because of a high recurrence of injury, stretching alone does not appear to be sufficient to fully prepare an athlete for a return to sport (68). However, stretching, combined with strengthening and sport-specific training, may increase success (32,64,68,77). Training should, therefore, be specific to the demands of the individual athlete's sport and consider a range of factors that include the forces exerted, types of muscle action involved, movement patterns and movement velocity, the mechanism of injury, which is commonly eccentric loading (6-8,39,40,69). These principles can be incorporated into a progressive rehabilitation framework that should fully prepare the athlete for their return to sport (Table 1).

Table 1

Table 1

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CONCLUSION

To reduce the risk of an injury recurring, it is essential to understand the mechanism of injury and address these issues by implementing appropriate, progressive exercises in any subsequent training. In the case of hamstring strain injuries, the mechanism of injury appears to be eccentric loading (6,8) at a high velocity (39,40,69); therefore, training needs to address eccentric loading (6-8,39,40,69) and progressively increase velocity of movement using plyometric activities (32,36).

Figure 4

Figure 4

Figure 5

Figure 5

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REFERENCES

1. Andersen LL, Magnussun SP, Nielson M, Haleem J, Poulsen K, and Aagaard P. Neuromuscular activation in conventional therapeutic exercises and heavy resistance exercises: Implications for rehabilitation. Phys Ther 86: 683-697, 2006.
2. Arnason A, Anderson TE, Holme I, Engebretsen L, and Bahr R. Prevention of hamstring strains in elite soccer: An intervention study. Scand J Med Sci Sports 18: 40-48, 2008.
3. Arnason A, Sigurdsson SB, Gudmundsson A, Holme I, Engebretsen L, and Bahr R. Risk factors for injuries in football. Am J Sports Med 32: S4-S16, 2004.
4. Askling C, Karlsson J, and Thorstensson A. Hamstring injury occurrence in elite soccer players after preseason strength training with eccentric overload. Scand J Med Sci Sports 13: 244-250, 2003.
5. Askling CM, Lund H, Saartok T, and Thorstensson A. Self reported hamstring injuries in student dancers. Scand J Med Sci Sports 12: 230-235, 2002.
6. Askling CM, Saartok T, and Thorstensson A. Type of acute hamstring strain affects flexibility, strength, and time to return to pre-injury level. Br J Sports Med 40: 40-44, 2006.
7. Askling CM, Tengvar M, Saartok T, and Thorstensson A. Acute first-time hamstring strains during high-speed running: a longitudinal study including clinical and magnetic resonance imaging findings. Am J Sports Med 35: 197-206, 2007.
8. Askling CM, Tengvar M, Saartok T, and Thorstensson A. Acute first-time hamstring strainsduring slow-speed stretching: clinical, magnetic resonance imaging, and recovery characteristics. Am J Sports Med 35: 1716-1724,2007.
9. Bandy WD and Irion JM. The effect of time on static stretch on the flexibility of the hamstring muscles. Phys Ther 74: 845-852, 1994.
10. Bandy WD, Irion JM, and Briggler M. The effect of time and frequency of static stretching on the flexibility of the hamstring muscles. Phys Ther 77: 1090-1096, 1997.
11. Bandy WD, Irion JM, and 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 27: 295-300, 1998.
12. Bennell K, Wajswelner H, Lew P, Schall-Riaucour A, Leslie S, Plant D, and Cirone, J. Isokinetic strength does not predict hamstring injury in Australian Rules footballers. Br J Sports Med 32: 309-314, 1998.
13. Brockett CL, Morgan DL, and Proske U. Human hamstring muscles adapt to eccentric exercise by changing optimum length. Med Sci Sports Exerc 33: 783-790, 2001.
14. Brockett CL, Morgan DL, and Proske U. Predicting hamstring strain injury in elite athletes. Med Sci Sports Exerc 36: 379-387, 2004.
15. Brooks JH, Fuller CW, Kemp SP, and Reddin DB. Incidence, risk and prevention of hamstring muscle injuries in professional rugby union. Am J Sports Med 38: 1297-1306, 2006.
16. Cameron M, Adams R, and Maher C. Motor control and strength as predictors of hamstring injury in elite players of Australian football. Phys Ther Sport 4: 159-166, 2003.
17. Canale ST, Cantler ED, Sisk TD, and Freeman BL. A chronicle of injuries of an American intercollegiate football team. Am J Sports Med 9: 384-389, 1981.
18. Clanton TO and Coupe KJ. Hamstring strains in athletes. J Am Acad Orthop Surg 6: 237-248, 1998.
19. Clark R, Bryant A, Culgan J, and Hartley B. The effects of eccentric hamstring strength training on dynamic jumping performance and isokinetic strength parameters: a pilot study on the implications for the prevention of hamstring injuries. Phys Ther Sport 6: 67-73, 2005.
20. Croisier JL. Factors associated with recurrence hamstring injuries. Sports Med 34: 681-695, 2004.
21. Croisier JL, Forthomme B, Namurois MH, Vanderthommen M, and Crielaard JM. Hamstring muscle strain recurrence and strength performance disorders. Am J Sports Med 30: 199-203, 2003.
22. Cross KM and Worrell TW. Effects of a static stretching program on the incidence of lower extremity musculotendinous strains. J Athl Train 34: 11-14, 1999.
23. Dadebo B, White J, and George KP. A survey of flexibility training protocols and hamstring strains in professional football clubs in England. Br J Sports Med 38: 388-394, 2004.
24. Drezner JA. Practical management: hamstring muscle injuries. Clin J Sport Med 13: 48-52, 2003.
25. Escamilla RF, Fleisig GS, Zheng N, Barrentine SW, Wilke KE, and Andrews JR. Biomechanics of the knee during closed kinetic chain and open kinetic chain exercises. Med Sci Sport Exerc 30: 556-569, 1998.
26. Escamilla RF, Fleisig GS, Zheng N, Lander JE, Barrentine SW, Andrews JR, Bergemann BW, and Moorman CT. Effects of technique variations on knee biomechanics during the squat and leg press. Med Sci Sports Exerc 33: 1552-1566, 2001.
27. Foreman TK, Addy T, Baker S, Burns J, Hill N, and Madden T. Prospective studies into the causation of hamstring injuries in sport: A systematic review. Phys Ther Sport 7: 101-109, 2006.
28. Fuller CW and Walker J. Quantifying the functional rehabilitation of injuries football players. Br J Sport Med 40: 151-157, 2006.
29. Funk D, Swank AM, Adams KJ, and Treolo D. Efficacy of moist heat pack application over static stretching on hamstring flexibility. J Strength Cond Res 15: 123-126, 2001.
30. Gabbe BJ, Branson R, and Bennell KL. A pilot randomised controlled trial of eccentric exercise to prevent hamstring injuries in community-level Australian Football. J Sci Med Sport 9: 103-109, 2006.
31. Goldspink G. Changes in muscle mass and phenotype and the expression of autocrine and systemic growth factors by muscle response to stretch and overload. J Anat 194: 323-334, 1999.
32. Heidt RS, Sweeterman LM, Carlonas RL, Traub JA, and Tekulve FX. Avoidance of soccer injuries with pre-season conditioning. Am J Sports Med 28: 659-662, 2000.
33. Hennessey, L and Watson, AWS. Flexibility and posture assessment in relation to hamstring injury. Br J Sports Med 27: 243-246, 1993.
34. Herrington L. Patients with hamstring muscle strains returning to sport in less than 14 days: A case report of treatment used. Phys Ther Sport 1: 137-138, 2000.
35. Hertal J. The role of non-steroidal anti-inflammatory drugs in the treatment of soft tissue injuries. J Athl Train 32: 350-358, 1997.
36. Hewett TE, Stroupe AL, Nance TA, and Noyes FR. Plyometric training in female athletes. Decreased impact forces and increased hamstring torques. Am J Sports Med 24: 765-773, 1996.
37. Holcomb WR, Kleiner DM, and Chu DA. Plyometrics: Considerations for safe and effective training. Strength Cond 20: 36-41, 1998.
38. Holcomb WR, Rubley MD, Lee HJ, and Guadagnoli MA. Effect of hamstring-emphasized resistance training on hamstring-quadriceps strength ratios. J Strength Cond Res 21: 41-47, 2007.
39. Hoskins WT and Pollard HP. Successful management of hamstring injuries in Australian Rules footballers: Two case reports. Chiropr Osteopat 13: 4, 2005.
40. Hoskins W and Pollard H. The management of hamstring injury - Part 1: Issues in diagnosis. Man Ther 10: 96-107, 2005.
41. Hubbard TJ, Aronson SL, and Denegar CR. Does cryotherapy hasten the return to participation? A systematic review. J Athl Train 39: 88-94, 2004.
42. Isear JA, Erickson JC, and Worrell TW. EMG analysis of lower extremity muscle recruitment patterns during an unloaded squat. Med Sci Sports Exerc 29: 532-539, 1997.
43. Ishikawa M and Komi PV. Effects of different dropping intensities on fascicle and tendinous tissue behavior during stretch-shortening cycle exercise. J Appl Physiol 96: 848-852, 2004.
44. Jarvinen TA, Kaariainen M, Jarvinen M, and Kalimo, H. Muscle strain injuries. Curr Opin Rheumatol 12: 155-161, 2000.
45. Jonhagen S, Nemeth G, and Eriksson, E. Hamstring injuries in sprinters. The role of concentric and eccentric hamstring muscle strength and flexibility. Am J Sports Med 22: 262-266, 1994.
46. Kaminski TW, Webberson CV, and Murphy RM. Concentric versus enhanced eccentric hamstring strength training: Clinical implications. J Athl Train 33: 216-221, 1998.
47. Kilgallon M, Donnelly AE, and Shafat A. Progressive resistance training temporarily alters hamstring torque angle relationship. Sand J Med Sci Sports 17: 18-24, 2007.
48. Knapik JJ, Bauman CL, Jones BH, Harris JM, and Vaughan L. Preseason strength and flexibility imbalances associated with athletic injuries in female collegiate athletes. Am J Sports Med 19: 76-81, 1991.
49. Kubo K, Kanehisa H, and Fukanaga T. Effects of resistance and stretching training programmes on the viscoelastic properties of human tendon structures in vivo. J Physiol 538: 219-226, 2002.
50. Kujala UM, Orava S, and Jarvinen M. Hamstring injuries: current trends in treatment and prevention. Sports Med 23: 397-404, 1997.
51. Lempainen L, Sarimo J, Mattila K, Heikkila J, and Orava S. Distal tears of the hamstring muscle: review of the literature and our results of surgical treatment. Br J Sports Med 41: 80-83, 2007.
52. Malliaropoulos N, Papalexandris S, Papalada A, and Papacostas E. The role of stretching in rehabilitation of hamstring injuries: 80 athlete follow-up. Med Sci Sports Exerc 36: 756-759, 2004.
53. McCaw ST and Melrose DR. Stance width and bar load effects on leg muscle activity during the parallel squat. Med Sci Sports Exerc 31: 428-436, 1999.
54. Mjolsnes R, Arnason A, Osthagen T, Raastad T, and Bahr R. A10-week randomized trial comparing eccentric vs. concentric hamstring strength training in well trained soccer players. Scand J Med Sci Sports 14: 311-317, 2004.
55. Nelson RT and Bandy WD. Eccentric training and static stretching improve hamstring flexibility of high school males. J Athl Train 39: 254-258, 2004.
56. Newton H. Explosive Lifting for Sports. Champaign, IL: Human Kinetics, 2002. pp. 33.
57. Ninos JC, Irrgang JJ, Burdett R, and Weiss JR. Electromyographic analysis of the squat performed in self-selected lower extremity neutral rotation and 30° of lower extremity turn-out from the self-selected neutral position. J Orthop Sports Phys Ther 25: 307-315, 1997.
58. Orchard J and Best TM. The management of muscle strain injuries: An early return verses the risk of recurrence. Clin J Sports Med 12: 3-5, 2002.
59. Panerillo RA, Backus SI, and Parker JW. The effect of the squat exercise on anterior-posterior knee translation in professional football players. Am J Sports Med 22: 768-773, 1994.
60. Petersen J and Holmich P. Evidence based prevention of hamstring injuries in sport. Br J Sports Med 39: 319-323, 2005.
61. Proske U and Morgan DL. Muscle damage from eccentric exercise: mechanism, mechanical signs, adaptation and clinical applications. J Physiol 537: 333-345, 2001.
62. Proske U, Morgan DL, Brockett DL, and Percival P. Identifying athletes at risk of hamstring strains and how to protect them. Clin Exp Pharmacol Physiol 31: 546-550, 2004.
63. Reid DA and McNair PJ. Passive force, angle, and stiffness changes after stretching of hamstring muscles. Med Sci Sports Exerc 36: 1944-1948,2004.
64. Roberts JM and Wilson K. Effects of stretching duration on active and passive range of motion in the lower extremity. Br J Sports Med 33: 259-263, 1999.
65. Robertson VJ and Baker KG. A review of therapeutic ultrasound: Effectiveness studies. J Phys Ther 81: 1339-1350, 2001.
66. Robinson LE, Devor ST, Merrick ME, and Buckworth J. The effect of land vs aquatic plyometrics on power, torque, velocity, and muscle soreness in women. J Strength Cond Res 18: 84-91, 2004.
67. Schaub PA and Worrell TW. EMG activity of six muscles and VMO: VL ratio determination during a maximal squat exercise. J Sports Rehabil 4: 195-202, 1995.
68. Sherry MA and Best TM. A comparison of 2 rehabilitation programs in the treatment of acute hamstring strains. J Orthop Sports Phys Ther 34: 116-125, 2004.
69. Stanton PE. Hamstring injuries in sprinting-the role of eccentric exercise. J Orthop Sports Phys Ther 10:343-349, 1989.
70. Stemm JD and Jacobson BH. Comparison of land and aquatic based plyometric training on vertical jump performance. J Strength Cond Res 21: 568-571, 2007.
71. Stretch RA. Cricket Injuries: a longitudinal study of the nature of injuries to South African Cricketers. Br J Sports Med 37: 250-253, 2003.
72. Thelen DG, Chumanov ES, Sherry MA, and Heiderscheit BC. Neuromusculoskeletal models provide insights into the mechanisms and rehabilitation of hamstring strains. Exerc Sport Sci Rev 34: 135-141, 2006.
73. Verall GM, Slavotinek JP, Barnes PG, Fon GT, and Spriggins AJ. Clinical risk factors for hamstring muscle strain injury: A prospective study with correlation of injury by magnetic resonance imaging. Br J Sports Med 35: 435-440, 2001.
74. Witvrouw E, Bellemans J, Lysens R, Danneels L, and Cambier, D. Intrinsic Risk Factors for the development of patellar tendonitis in the athletic population. A two year prospective study. Am J Sports Med 29: 190-195, 2001.
75. Woods C, Hawkins RD, Maltby S, Hulse M, Thomas A, and Hodson A; Football Association Medical Research Programme. The Football Association Medical Research Programme: an audit of injuries in professional football-analysis of hamstring injuries. Br J Sports Med 38: 36-41, 2004.
76. Worrell TW. Factors associated with hamstring injuries: An approach to treatment and preventative measures. J Sports Med 17: 338-345, 1994.
77. Worrell TW, Smith TL, and Winegardner J. Effect of hamstring stretching on hamstring muscle performance. J Orthop Sports Phys Ther 20:154-159, 1994.
78. Wright GA, Delong TH, and Gehlsen G. Electromyographic activity of the hamstrings during performance of the leg curl, stiff-leg deadlift and back squat movements. J Strength Cond Res 13: 168-174, 1999.
79. Yamaoto T. Relationship between hamstring strains and leg muscle strength. A follow up study of collegiate track and field athletes. J Sports Med Phys Fitness 33: 194-199, 1993.
80. Zemper ED and Pieter W. Injury rates during the 1988 US Olympic Team Trials. Br J Sports Med 23: 161-164, 1999.
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