You may think your legs drop straight down from your waist—they do in apes—but in fact the femur angles inward as it descends from pelvis to knee. This has the effect of moving our lower legs closer together, giving us a much smoother, more graceful gait. No ape can be trained to walk like a human. They are compelled by their bone structure to waddle, and to do so in a most inefficient way. A chimpanzee uses four times as much energy to move around at ground level
