Fibrotic myopathy is a mechanical lameness of the hind limb in horses, caused by restriction of the action of the horse’s muscles. It occurs when horses injure their semitendinosus and semimembranosus muscles. The gait change is caused by a mechanical limitation on movement of the hind limb and is not necessarily caused by pain.
Injury and scarring of the hamstring muscles in the upper hindquarter of the horse causes a classic change in gait with shortening of the front phase of the stride, loss of the stretchability of the normal muscle, and slapping of the foot to the ground. The injury can result from a single traumatic injury to these muscles (hyperextension injury) but can also can result from repetitive strain. In some cases, scarring can occur from repeated IM injections into the hamstring muscles.