Muscle injury can occur by a variety of mechanisms ranging from ischemia to direct in- jury by crush, laceration, or excessive force. Injured muscle undergoes processes of de- generation and regeneration.

When muscle fibers are damaged, an inflammatory pro- cess ensues, followed by removal of debris by macrophages.9 New fibers appear within the connective tissue framework and are believed to be generated from a population of satellite cells that exist in a quiescent state within the original muscle syncytium. Synchronous processes that affect the functional recovery of muscle are connective tissue formation (fibrosis) and revascularization. Fibrosis can be extensive enough to interfere with muscle regeneration, preventing fibers from shortening or lengthening fully.
Forms of muscle injury include lacerations, contusions, and strains. Lacerations typically result from direct trauma from a sharp object. Recovery of function requires reorganization of devitalized tissue, muscle regeneration across the injury site, and rein- nervation of denervated myofibers; it is usually partial rather than total.
Contusions usually result from blunt in- jury, occurring frequently in motor vehicle accidents and sports injuries.

A contusion leads to an inflammatory process and hematoma, with subsequent muscle regeneration and scar formation. Severe blunt injury to muscle may result in heterotopic bone formation (synthesis of bone where it is not normally present) within the muscle, referred to as myositis ossificans.10 Muscle strains are indirect injuries caused by excessive tension on a muscle rather than by direct trauma. A strain may result in either a complete or an incomplete muscle tear. The muscle fibers can avulse from the tendon at the myotendinous junction. How- ever, failure usually occurs in the muscle fiber within several millimeters of the junc- tion. The terminal sarcomeres near the myotendinous junction are stiffer than the middle sarcomeres of a muscle fiber. The injury to muscle occurs within this region of relative stiffness.11

Among the most frequently injured muscles are the hamstrings, the rectus femoris, and the gastrocnemius; these muscles cross at least two joints and as such may be subject to more stretch. In addition, the architecture in these muscles demonstrates an extensive length of the myotendinous junction.