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, then 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.

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