Muscle and Tendon Basics: Joint motion occurs by muscle contraction. Muscles attach to bone by tendons at each end. Muscle contraction can be concentric, meaning the muscle is shortening, or eccentric, meaning contraction is occurring while the muscle is lengthening. Eccentric contraction occurs because the joint is moving in the opposite direction, even while the muscle is firing. Muscles that contract together on the same side of a joint are called agonistic meaning they are working together. Muscles on the opposite side of the joint causing it to move in the opposite direction are called antagonistic. Joint motion requires muscles working simultaneously on both sides for balanced movement. If the muscle were firing only on one side of the joint, the joint would just collapse in that direction. Muscles firing on the opposite side of the joint, even though they are lengthening, control the speed and amount of joint motion that occurs. Muscle and tendon injuries occur from an eccentric force where the muscle is attempting to contract while it is being stretched. Muscle injuries (strain) occur where the muscle joins with the tendon (myotendinous junction). Tendon injuries occur where the tendon attaches to bone.
Hamstrings: The hamstrings are a group of three muscles (biceps femoris, semimembranosus, and semitendinosus) that originate from the ischium of the pelvis bone in the back of the hip (“sit bone”) and then extend down the back of the thigh, inserting on the top of the lower leg (tibia and fibula) behind the knee. They are “two-joint muscles” because they work across two joints: They extend the hip and flex the knee. (Fig 1) Hamstring injuries are common because they are exposed to more eccentric forces that simultaneously work across the hip and the knee. (Fig 2) These three muscles have various myotendinous junctions at both ends, and thus, a hamstring muscle injury can occur virtually anywhere up and down the back of the thigh.