The anatomy, architecture, and innervation patterns of the hamstring muscles, biceps femoris, and semitendinosus were examined in adult cats using microdissection and glycogen-depletion techniques. The biceps femoris muscle consists of two heads. The anterior head, which attaches mainly to the femur, is divided into two parts by the extramuscular branches of its nerve. The posterior head is innervated by a single nerve. Semitendinosus is composed of two heads, one proximal and one distal to a tendinosus inscription, each of which is separately innervated. The extramuscular branches of the nerves to these hamstring muscles thus partition them into innervation subvolumes termed parts. The available evidence suggests that each of the parts of these muscles so innervated is not equivalent to the collections of single motor units that have been described for ankle extensors as neuromuscular compartments. It is quite likely that each of the parts of the hamstring muscles may contain more than one neuromuscular compartment. Using chronically implanted EMG electrodes, the activation patterns of different parts of the hamstring muscles were analyzed during locomotion. The anterior and middle parts of biceps femoris are active during the early stance phase, probably producing hip extensor torque. The posterior part of biceps femoris and semi-tendinosus act most consistently as flexors, during the early swing phase, but also may function in synergy with hip, knee, and ankle joint extensors near the time of foot placement. Greater variability is found in the activity patterns of posterior biceps femoris and semitendinosus with respect to the kinematics of the step cycle than is observed for anterior and middle biceps femoris. It is suggested that this variation may reflect a larger role of sensory feedback in shaping the timing of activity in posterior biceps femoris and semitendinosus than in "nonarticular" muscles.