Strain is an injury to the muscle or musculotendinous junction, whereas sprain is an injury to the ligament. Muscle contusion occurs when a muscle is subjected to a sudden, heavy compressive force, such as a direct blow to the muscle.
In strains, an excessive tensile force subjected onto the muscle leads to the overstraining of the myofibres and consequently to a rupture near the musculotendinous junction. Muscle strains typically concern the superficial muscles working across 2 joints, such as the rectus femoris, semitendinosus, and gastrocnemius muscles. Muscle strain usually occurs in eccentric exercise. In eccentric exercise the contracting muscle is forcibly lengthened; in concentric exercise it shortens. While concentric contractions initiate movements, eccentric contractions slow or stop them (e.g., when lowering a weight).