Straining of the hamstring, also known as a pulled hamstring, is defined as an excessive stretch or tear of muscle fibers and related tissues. Hamstring injuries are common in athletes participating in many sports and are very difficult to treat and rehabilitate. Track and field athletes are particularly at risk, as hamstring injuries have been estimated to make up 29% of all injuries in sprinters.
The biceps femoris long head is at the most risk for injury, possibly due to its reduced moment of knee and hip flexion as compared to the medial hamstrings.
The hamstrings undergo a complex dynamic process during gait, making it unsurprising that they are frequently injured. They must first contract concentrically during the end of the stance phase in order to bend the knee and allow the foot (along with dorsiflexion at the ankle) to clear the ground. At the end of the swing phase the hamstrings must eccentrically contract while applying a braking moment to knee extension, and the immediately change functions to again concentrically contract and produce hip extension. Studies have shown that “the hamstring group reaches peak elongation and acts eccentrically at the hip and knee during the late swing phases of running” and that “the hamstrings are most active and develop the greatest torques at the hip and knee during the late swing through midstance phase of running.” Thus, the hamstrings reach their maximum length while attempting to forcefully contract eccentrically and switch functions to immediately produce a concentric contraction, which makes the terminal part of swing phase the most vulnerable for injury.
There have been many other proposed predisposing factors to injury. These include muscle weakness, muscle imbalance, poor flexibility, fatigue, inadequate warm up, poor neuromuscular control, and poor running technique. One of the few predisposing factors that most researchers agree upon however is previous hamstring injury. Brokett et al (2004)  stated that “the athletes most at risk of a hamstring strain are those with a previous history of such injury” and noted that 34% of the hamstring injuries were recurrences.” Cameron et al also found that 34% of injuries recur in the same season. Arnason et al  generalized these numbers, saying that previous injury was in itself an independent risk factor for re-injury.
With a grade two hamstring strain there is immediate pain which is more severe than the pain of a grade one injury. It is confirmed by pain on stretch, swelling and contraction of the muscle.
A grade three hamstring strain is a severe injury. There is an immediate burning or stabbing pain and the individual is unable to walk without pain. The muscle is completely torn and there may be a large lump of muscle tissue above a depression where the tear is.
After a few days with grade two and three injuries a large bruise may appear below the injury site caused by the bleeding within the tissues.
Recommended treatment for this injury consists of the RICE protocol — rest, ice, compression and elevation. The RICE method is primarily used to reduce bleeding and damage within the muscle tissue. Lower grade strains can easily become worse if the hamstring is not rested properly. Complete ruptures require surgical repair and rehabilitation.
Initial treatment of the injury, regardless of the severity of the strain, is the same. Within the first five days, the hamstring is rested in an elevated position with an ice pack applied for twenty minutes every two hours. A compression bandage is applied to limit bleeding and swelling in the tissues. After five days of rest, active rehabilitation begins.
- Arnason A, Andersen TE, Holme I, Engebretsen L, Bahr R (2008). "Prevention of hamstring strains in elite soccer: an intervention study". Scand J Med Sci Sports 18 (1): 40–8. doi:10.1111/j.1600-0838.2006.00634.x. PMID 17355322.
- Carlson C (2008). "The natural history and management of hamstring injuries". Curr Rev Musculoskelet Med 1 (2): 120–3. doi:10.1007/s12178-007-9018-8. PMC 2684206. PMID 19468884.
- Cameron, M.; Adams, R.; Maher, C. (2003). "Motor control and strength as predictors of hamstring injury in elite players of Australian football" (PDF). Physical Therapy in Sport 4 (4): 159–166. doi:10.1016/s1466-853x(03)00053-1.
- Brockett CL, Morgan DL, Proske U (2004). "Predicting hamstring strain injury in elite athletes". Med Sci Sports Exerc 36 (3): 379–87. doi:10.1249/01.mss.0000117165.75832.05. PMID 15076778.
- "Marshall University Orthopaedics". Retrieved 2008-01-08.