At the back of each thigh are three hamstring muscles -- the semitendinosus, semimembranosus and biceps femoris, from the inside to the outside. At the top, or proximal end, the three muscles come together, form the hamstring tendon and attach at the base of the pelvis on the ischial tuberosity, or the sitting bone. They run down the back of the thigh, cross the knee joint and attach distally to the tibia and fibula bones in the lower leg. The hamstring muscle group is responsible for the flexing of the lower leg at the knee.
Injuries occur when there is a muscular imbalance, when the hamstrings are not adequately warmed up or are fatigued, or when -- like on the bases -- a sudden burst of speed is required.
Most hamstring strains in baseball occur between the middle of the muscle and the proximal attachment at the pelvis. As with all muscle strains, they are rated by grade. Grade 1 is a mild strain or pull, Grade 2 is a partial tear and Grade 3 is a complete tear in which the hamstring tendon lifts completely away from the bone.
Typical recovery time
Lower-grade injuries can be treated with simple rest, with dry needling to relax angry muscle fibers and reset nerve fibers, or with biologic solutions such as platelet-rich plasma (PRP) or stem-cell injections. Depending on severity of the injury, players may return to play in as little as a few days or as long as a few months. Full tears may need to be repaired surgically and sutured back to the bone, with a return to play in 3-4 months.