Because the hamstring muscles extend over both the hip and knee joints, hamstring contraction influences joint movement of both the hip and knee.
Concentric contraction. When the hamstrings contract concentrically, the muscle shortens, causing the leg to move back (hip extension) and the knee to bend (knee flexion). The hamstrings contract concentrically when the foot pushes off the ground in the gait cycle. This motion is called the take-off phase of a person’s gait.
Eccentric contraction. The hamstrings lengthen as they contract eccentrically to slow down the leg in preparation for foot hitting the ground (foot strike).29,30 This motion is called the terminal swing phase of a person’s gait.
Hamstring strains are most likely to occur when the hamstring is lengthened, during eccentric contraction, in the terminal swing phase of the gait cycle.8,16,20,28,31,32
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Where Do Hamstring Injuries Occur?
An acute injury to a hamstring muscle can occur anywhere, but most commonly it occurs in the middle of the muscle, where the tendon and muscle tissues intersect.8,31 This area is called myotendinous junction. Less common are injuries in which the hamstring tendon breaks away from the bone. These injuries are called hamstring avulsions and typically occur at the top of the hamstring (proximal hamstring).
Tears of the myotendinous junction. For runners, hamstring injuries most commonly occur in the mid-thigh, along the myotendinous junction of the biceps femoris muscle.8,16,17,28,31,32 This injury typically occurs when the leg extends forward and the foot prepares to hit the ground (heel strike) during high intensity running and cutting activities.8,29,30,32 This is when the hamstring is contracting eccentrically to try and slow the body down.
Hamstring avulsions. In sports such as dancing and gymnastics, which require extreme stretching of the muscle, hamstring injuries are more common at the proximal hamstring, where it connects with the ischial tuberosity (“sit bone”) near the bottom of the pelvis.4,5,24 These proximal injuries typically require a longer recovery period, sometimes taking 3 to 6 months before the patient is able to return to full activity.24,51,52
When a tendon tears away from the place it inserts into a bone, it is called an avulsion injury. Avulsion literally means to pull or tear away. Sometimes the tendon can pull away a small bone fragment, usually from the ischial tuberosity. Avulsion injuries can happen at the top (proximal origin) or the bottom (distal insertion) of a hamstring muscle, but are more likely proximally at the hamstring origin.
While patients with middle (myotendinous) hamstring injuries normally can be treated without surgery (conservatively), those with high hamstring injuries or proximal hamstring avulsions are more likely to require surgery.