Activities of pongid thigh muscles during bipedal behavior

Electromyographic recordings were taken from 12 thigh muscles (or major parts of them) in a gorilla, from 6 thigh muscles in a chimpanzee, and from 2 thigh muscles in an orangutan as they engaged in bipedal positional behavior, including stance, reaching overhead, lunging, leaping and walking. In the African apes, symmetric bipedal stances with hindlimb flexure were accompanied by notable EMG activity in many of the hamstring, quadriceps, and adductor muscles. If weight shifted eccentrically onto one hindlimb, EMG potentials generally increased to or remained at moderate and high levels. Our studies on the gluteal (Tuttle et al., '78) and thigh muscles of African apes partly confirm Kummer's ('75) prediction that considerable gluteal and hamstring activity would be required in order for them to stand bipedally with flexed hip and knee joints. The gorilla's thigh muscles exhibited considerable EMG activity during the stance phase and remarkably little activity during the swing phase of bipedal steps. The activity patterns of most thigh and gluteal muscles (Tuttle et al., '78) in the African apes are much more similar to those of bipedal gibbons than to their counterparts in man. The bipedal locomotor cycles of human subjects are accompanied by many more biphasic and triphasic EMG patterns in the thigh muscles than the locomotor cycles of other anthropoid primates are. The evolutionary anthropological significance of these findings should become clearer when they are complemented by EMG studies on human running, arboreal bipedalism and vertical climbing in apes, and central pattern generation in man and apes.