Human legs are 171 percent of trunk length; chimps manage 128
Each human leg holds thirty bones, anchored by the femur, tibia and fibula. Evolution reshaped the whole limb for walking upright efficiently, something other primates can only do briefly and at high energy cost. The payoff is legs far longer, relative to the trunk, than any ape's.
Everyday speech uses leg for the whole lower limb, from thigh to foot and sometimes including the hip or buttock. Anatomists are stricter. For them the leg, also called the crus or shank, is only the stretch between knee and ankle, with the calf behind and the shin in front. The shin is built from the tibia and its slimmer partner, the fibula, while the thigh runs from hip to knee around the femur. All three are long bones, and the kneecap in front of the knee is the largest sesamoid bone anywhere in the body.
Two specialisations set the human body apart: hands and arms tuned for manipulation guided by sight, and a lower limb rebuilt for economical two-legged walking. That shift moved the centre of gravity, rearranged internal organs and remodelled the trunk, whose double S-curved spine cushions loads on their way down to the feet. The proportions tell the story. An orangutan's leg equals 111 percent of its trunk length, a chimpanzee's 128 percent, and a human's 171 percent. The gluteal muscles, the knee extensors and the calf muscles show the strongest adaptations to upright gait.
Most of the skeleton can be felt through the skin, and a few bony points act as landmarks for where the limb begins and ends, including the front of the iliac spine, the greater trochanter and the inner ankle knob. The hip joint and the shaft of the femur are the notable parts you cannot palpate.
In a well-aligned limb the hip, knee and ankle sit on one straight mechanical axis, known as the Mikulicz line. Along the tibia that axis matches the bone itself, but along the femur the two diverge by 6 degrees, giving a normal thigh-shin angle of 174 degrees. When the knee drifts outward from the line the result is bow legs, or genu varum; when it drifts inward, knock knees, or genu valgum. Either one loads the joints unevenly. Sex differences exist as well: women tend to have greater hip anteversion, men longer femurs and tibias.
Source: Human leg
