The purpose of this study was to identify overuse of muscles and other alterations in the mechanics of gait in twenty-one patients who had muscular dysfunction as a late consequence of poliomyelitis. All of the patients had good or normal strength (grade 4 or 5) of the vastus lateralis and zero to fair strength (grade 0 to 3) of the calf, as determined by manual testing. Dynamic electromyography was used, while the patients were walking, to quantify the intensity and duration of contraction of the inferior part of the gluteus maximus, the long head of the biceps femoris, the vastus lateralis, and the soleus muscles. Patterns of contact of the foot with the floor, temporal-spatial parameters, and motion of the knee and ankle were recorded. The principal mechanisms of substitution for a weak calf muscle fell into three groups: overuse of the quadriceps (twelve patients) or a hip extensor (the inferior part of the gluteus maximus in eight patients and the long head of the biceps femoris in four), or both; equinus contracture (twelve patients); and avoidance of loading-response flexion of the knee (five patients). Most patients used more than one method of substitution. These observations support the theory that post-poliomyelitis syndrome results from long-term substitutions for muscular weakness that place increased demands on joints, ligaments, and muscles and that treatment--based on the early identification of overuse of muscles and ligamentous strain--should aim at modification of lifestyle and include use of a brace.