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Hamstring Strain

Hamstring injuries are among the most common muscle strain injuries in all sports and exercise. A prior hamstring injury is the biggest risk factor. The goals of treatment are maximizing comfort and restoring function. Early recognition and optimal management are key to having the best possible outcome following a hamstring injury.

Functional Anatomy

Muscles connect different parts of the body, typically bones on either side of a joint, and when they contract (get shorter), they displace or move a joint. In moving joints, muscle groups have other muscle groups that either support or oppose the specific plane of movement (the agonist group or the antagonist group). Concentric movement occurs when a muscle group contracts and shortens to perform its main function—for example, the hamstring bending the knee. Eccentric movement occurs when a muscle contracts during the antagonistic function—for example, the hamstring restraining knee extension during sprinting; the muscle contracts but does not necessarily shorten in length, and in fact, it may be forced to lengthen by the opposing muscle group.

This concentric-eccentric relationship is important to understand the mechanism of injury involving the hamstring muscle group. The hamstrings include the biceps femoris, the semitendinous, and the semimembranous muscles. The three muscles originate at the ischial tuberosity, and the distal insertions vary based on the muscle. The biceps femoris attaches distally on the tibial lateral condyle and fibula head, the semitendinous attaches on the proximal medial tibia, and the semimembranous attaches in the medial tibial condyle pes anserine region. Like most muscle groups in the human body, the hamstring muscle group works by pulling the insertion toward the origin, so its function is to flex (or bend) the knee (bringing the heel toward the buttocks) and assist in the extension of the hip (moving the leg to the rear).

Mechanism of Injury and Risk Factors

The majority of hamstring injuries in sports occur in velocity athletes (e.g., sprinters, running backs, wide receivers, soccer and basketball players) and result because of an excessive eccentric force during the lengthening of the muscle—especially when running, sprinting, or coming to a sudden stop (deceleration). Hamstring injuries can occur when the muscle is contracting at the same time when other forces are causing the muscle group to lengthen, such as in sprinting and stopping suddenly. Mechanically, hamstring injuries occur frequently at the later part of the swing phase, when the hamstring contracts eccentrically to decelerate knee extension in preparation for foot strike. Another common mechanism of hamstring injury is during the stance phase, when the hamstrings contract in a concentric fashion to assist in hip extension.

An important risk factor for an acute hamstring injury is a past history of the same injury. Other proposed risk factors include a muscle imbalance or weakness of the hamstrings, poor flexibility, inadequate warm-up, muscle fatigue, and overtraining. Many recurrent or chronic hamstring injuries may, in part, be due to a less than optimal rehabilitation of the first initial injury (Figure 1).

Classification

The hamstring injury can occur at any location in the muscle (in the origin, insertion, or muscle belly). The traditional classification is based on the degree of muscle fiber injury. A first-degree strain signifies only minimal structural injury to the muscle fiber and results in minimal strength loss and almost no functional disability. A second-degree strain signifies a partial tear of the muscle, macrotrauma, and the athlete may have loss of strength and some function. A third-degree strain represents a complete rupture of the muscle and is associated with significant functional disability and often significant discomfort. When a third-degree injury to the hamstring occurs at its proximal origin, it may be accompanied by an actual avulsion fracture (bone fragment separation) of the ischial tuberosity.

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