Striding Bipedalism

All apes are capable of bipedal locomotion, and some species walk bipedally on a fairly regular basis (although only in humans is bipedal walking the most common form of locomotion). However, human bipedalism differs from that of other apes in a number of ways, and only humans exhibit what is known as “full striding bipedalism.” Striding bipedalism involves: full extension of the hip and knee joints in the support leg during stance phase (apes maintain a degree of flexion at both of these joints during bipedal walking); movement of the hip joint over and in front of the knee and ankle joints in the support leg; and a longer stride length (both absolutely and relative to body size) than seen in ape bipedalism. In addition, the human femoral bicondylar angle (see Bicondylar Angle of the Femur) minimizes mediolateral shifts in the body’s center of gravity when alternating between support legs, while the arrangement of our gluteal abductors (see Pelvic Height and Iliac Flare) minimizes vertical displacements of the center of gravity as weight is shifted from limb to limb. In contrast, apes must usually shift their body weight over their support leg when walking bipedally (resulting in an exaggerated side-to-side swaying), and must tilt their pelvis upwards to counteract gravity operating on the nonsupport leg (while swinging the leg forward to reposition it for the next step), resulting in relatively great vertical displacements of the body’s center of gravity. Because vertical and mediolateral movement of the center of gravity is minimized in human bipedal walking, humans require less muscular effort in bipedal locomotion than do apes. It is generally held that full striding bipedalism evolved to increase energetic efficiency in the context of increased terrestrial mobility, perhaps in conjunction with an adaptive shift towards consumption of greater amounts of savannah resources (such as scavengable carcasses), in early members of the genus Homo.