Hamstring injuries are the most common muscle strain sustained by athletes, and are a frequent occurrence in sports such as Australian Rules football, rugby, soccer, track and field and other sports involving sprinting. Studies have shown that hamstring strains account for up to 29% of injuries in sprinters, and 16–23% of injuries in Australian Rules football. Hamstring injuries are now the most prevalent injury in soccer, accounting for 12% to 16% of all injuries, and have replaced ankle sprains, which were the most commonly reported soccer injury in the 1980’s. In addition to their high incidence, a common problem is the high risk of recurrence. A recurrence rate of 22% within the first 2 months after the initial injury has been reported, and in a Danish professional soccer study, 25% of players with a hamstring injury sustained a recurrent injury during the following soccer season.
Risk factors:
There is evidence showing that previous hamstring strains and older age are independent risk factors for new hamstring strains. Other factors, such as poor hamstring flexibility, muscle fatigue and insufficient warm-up have also been suggested to predispose to hamstring strains, but the evidence for these are less conclusive. Investigators have also suggested that inadequate muscle strength, imbalance in hamstring to quadriceps strength, or side-to-side strength imbalances are risk factors for hamstring strains. Isolated decreased hamstring strength remains a controversial risk factor, and strength imbalances between the quadriceps and hamstrings may play a larger role than isolated hamstring strength alone. A significantly reduced ratio of hamstring strength to quadriceps strength on the injured side has been found in several studies. The ability to exert lower limb swing force may be greater in individuals with increased quadriceps strength relative to hamstring strength. The hamstrings eccentrically (contract while lengthening) slow the lower limb during the swing phase of running before extending the hip to achieve forward motion. Some researchers speculate that professional players, particularly in kicking sports, may be developing a quadriceps-dominant strength profile, which may predispose them to hamstring injury, potentially by placing a greater requirement on the hamstrings to decelerate the lower limb during athletic activity. Overall, the direct influence of risk factors remains inconclusive, as studies do not provide strong evidence to support their individual or collective effect on the development of hamstring strains. However, research does suggest that the most significant predictor of hamstring injury is a history of previous strain to the muscle.
Mechanism of injury:
A muscle strain is defined as an excessive stretch, which leads to muscle fiber damage and disrupts the integrity of related vascular and connective tissue structures. Muscles are commonly strained or torn during rapid acceleration or deceleration movements. A strain can be classified into grades from mild to severe to reflect injury severity. A mild (first degree) strain involves damage to a small number of muscle fibers and localized pain without loss of strength. A clear loss of strength coupled with pain reproduced on resistance testing is indicative of a moderate (second degree) strain. A severe (third degree) strain corresponds with complete rupture of the muscle and loss of strength and function. The hamstring muscle group is at increased risk for strains due to its anatomical configuration. The hamstrings are composed of three muscles (semitendinosus, semimembranosus, and biceps femoris) forming a triad in the posterior compartment of the thigh. The musculotendinous junction of the biceps femoris is the most common site of strain, and a rapid phase change from eccentric to concentric muscle contraction has been suggested as the underlying mechanism for hamstring strains. Eccentric contractions are characterized by active lengthening of muscle fibers, in which the force of contraction increases as the speed of contraction increases. Conversely, concentric contractions involve the shortening of muscle fibers and an inverse relationship between the force and speed of contraction. The functions of the hamstring muscles are hip extension (pulling your leg behind you) and knee flexion (bending the knee). The requirements of the hamstrings in terms of force, velocity, and power are limited during walking and jogging compared with sprinting. The bi-articular (crossing two joints) arrangement of the hamstring muscles across the hip and knee allow the hamstrings to work eccentrically during late swing phase to decelerate the lower leg and control knee extension (straightening of the knee). A concentric contraction follows to initiate hip extension prior to heel strike, and the hamstrings are maximally loaded and lengthened during this rapid phase change. Peak hamstring stretch and force occur during this late swing phase of the running gait cycle, and this force increases significantly with speed. The majority of hamstring injuries seem to occur in this late swing phase, when the hamstring muscles generate tension while lengthening to decelerate knee extension. Increasing the eccentric strength of the hamstring muscles, performed by lengthening the hamstring muscle complex while it is loaded and contracting, has therefore been proposed as a method to prevent hamstring injuries.
Evidence for eccentric training:
Several studies suggest that eccentric training is effective in primary and secondary (preventing re-injury) prevention of hamstring strains. One eccentric training method, The Nordic Hamstring Protocol (NHP) has gained popularity in recent years. The NHP involves participants kneeling on the floor with an upright trunk position perpendicular to the floor. The feet are supported under a low bench or held by a partner. Arms are kept folded across chest and the body is slowly lowered forward. Participants lower their body until they are no longer able to hold the position, at which point the participant is allowed to relax and use their arms to catch themselves as they reach the floor. The participant then uses his/her arms to return to the starting position, minimizing the concentric phase of the motion. The most robust evidence in favor of eccentric training for the prevention of hamstring strains is provided by Petersen et al., (2011). The authors performed a well-designed study with 942 Danish male professional and amateur soccer players. Players in the intervention group conducted a 10-week progressive eccentric training program followed by a weekly seasonal program, whereas players in the control group followed their usual training program. The main outcome measures were numbers of overall, new, and recurrent acute hamstring injuries during 1 full soccer season. The authors found that their training program was able to reduce the injury rate of new injuries by more than 60%. Furthermore, it was shown that the intervention was also highly effective in reducing the rate of recurrent injuries, which were reduced by approximately 85%.
Risks & adverse events with eccentric training:
Eccentric exercise has the potential to result in delayed onset muscle soreness (DOMS), which needs to be differentiated from muscle strain. DOMS is clinically characterized by muscle soreness, stiffness, inflammation, and loss of function peaking one to three days after unaccustomed exercise. With DOMS, repeated bouts of exercise result in progressively less tissue damage and soreness. In comparison, muscle strain is characterized by immediate acute pain, and exercise too soon after strain can lead to a more disabling injury. Most studies involving eccentric hamstring exercises report that most participants experienced DOMS, especially during the initial week of implementing the program. DOMS were a significant factor leading to high dropout rates in some studies, but these involved high volume eccentric training. Other studies using a more progressive and conservative eccentric exercise program report that most participants experienced some degree of DOMS, but no subjects dropped out of the study due to post-exercise soreness. Most importantly, these “more conservative” studies showed the same positive results!
The bottom line:
- Hamstring strains are a very common athletic injury, and are seen most commonly in sports that involve sprinting.
- Hamstring strains have a high recurrence rate following the initial injury. The 2-month post-injury period is the time of highest risk, although it seems to remain elevated for years after initial injury.
- Evidence shows that previous hamstring strains and advancing age are independent risk factors for new hamstring strains. Many other risk factors have been proposed, but the evidence for these is less robust.
- The musculotendinous junction of the biceps femoris is the most common site of strain, and a rapid phase change from eccentric to concentric muscle contraction has been suggested as the underlying mechanism of injury.
- Overall, evidence shows that the implementation of a progressive eccentric exercise program including the Nordic Hamstring Protocol will result in significant reductions in the incidence of new hamstring injuries, as well as injury recurrence.
- Eccentric exercise programs are associated with delayed-onset muscle soreness (DOMS), which can be a limiting factor for program adherence. A progressive and conservative approach should be employed to limit this effect. Are you struggling with a nagging hamstring injury? Interested in decreasing your/your team’s injury risk through specific preventative exercise?
Contact us!
In good health,
Dr. Steve
References:
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.
Copland ST, Tipton JS & Fields KB. Evidenced-Based Treatment of Hamstring Tears. Current Sports Medicine Reports. (2009). 8(6) 308-314. Gabbe BJ, Branson R & Bennell KL. A pilot randomized controlled trial of eccentric exercise to prevent hamstring injuries in community-level Australian Football. Journal of science and Medicine in Sport. (2006). 9:103-109. Hibbert O, Cheong K, Grant A, Beers A & Moizumi T. A systematic review of the effectiveness of eccentric strength training in the prevention of hamstring muscle strains in otherwise healthy individuals. North American Journal of Sports Physical Therapy. (2008). 3(2) 67-81. Lorenz D & Reiman M. The role and implementation of eccentric training in athletic rehabilitation: tendinopathy, hamstring strains, and ACL reconstruction. The International Journal of Sports Physical Therapy. (2011). 6(1) 27-44. Petersen J, Thorborg K, Nielsen MB, Budtz-Jorgensen E & Holmich P. Preventative Effect of Eccentric Training on Acute Hamstring Injuries in Men’s Soccer: A Cluster-Randomized Controlled Trial. The American Journal of Sports Medicine. (2011). 39(11) 2296-2303.