#Rupture of Muscle or Tendon.#—­A muscle or a tendon may be ruptured in its continuity or torn from its attachment to bone.  The site of rupture in individual muscles is remarkably constant, and is usually at the junction of the muscular and tendinous portions.  When rupture takes place through the belly of a muscle, the ends retract, the amount of retraction depending on the length of the muscle, and the extent of its attachment to adjacent aponeurosis or bone.  The biceps in the arm, and the sartorius in the thigh, furnish examples of muscles in which the separation between the ends may be considerable.

The gap in the muscle becomes filled with blood, and this in time is replaced by connective tissue, which forms a bond of union between the ends.  When the space is considerable the connecting medium consists of fibrous tissue, but when the ends are in contact it contains a number of newly formed muscle fibres.  In the process of repair, one or both ends of the muscle or tendon may become fixed by adhesions to adjacent structures, and if the distal portion of a muscle is deprived of its nerve supply it may undergo degeneration and so have its function impaired.

Rupture of a muscle or tendon is usually the result of a sudden, and often involuntary, movement.  As examples may be cited the rupture of the quadriceps extensor in attempting to regain the balance when falling backwards; of the gastrocnemius, plantaris, or tendo-calcaneus in jumping or dancing; of the adductors of the thigh in gripping a horse when it swerves—­“rider’s sprain”; of the abdominal muscles in vomiting, and of the biceps in sudden movements of the arm.  Sometimes the effort is one that would scarcely be thought likely to rupture a muscle, as in the case recorded by Pagenstecher, where a professional athlete, while sitting at table, ruptured his biceps in a sudden effort to catch a falling glass.  It would appear that the rupture is brought about not so much by the contraction of the muscle concerned, as by the contraction of the antagonistic muscles taking place before that of the muscle which undergoes rupture is completed.  The violent muscular contractions of epilepsy, tetanus, or delirium rarely cause rupture.

The clinical features are usually characteristic.  The patient experiences a sudden pain, with the sensation of being struck with a whip, and of something giving way; sometimes a distant snap is heard.  The limb becomes powerless.  At the seat of rupture there is tenderness and swelling, and there may be ecchymosis.  As the swelling subsides, a gap may be felt between the retracted ends, and this becomes wider when the muscle is thrown into contraction.  If untreated, a hard, fibrous cord remains at the seat of rupture.