Posture and Locomotion
Walking on two legs, or bipedal locomotion, was adopted by our ancestors millions of years ago, and was among the earliest features distinguishing the hominid line from the apes. Bipedal locomotion was an adaptation to life on the African plains, where food sources were widely scattered. It allowed the ability to move from one place to another while holding things at the same time. It also provided a higher vantage point from which to scan the area for predators.
While advantageous in many ways, the upright posture is inherently unstable, like a two-legged stool. Over time, evolution has brought changes to the human musculoskeletal system, especially the pelvis and lower limbs, to help balance and support the mass of the body with a minimum expenditure of energy. These include powerful and bulky leg muscles, and a wide pelvic girdle to support the body's weight and transmit it to the lower limbs.
Humans employ a plantigrade gait, in which the entire sole of the foot is placed on the ground. The heel strikes the ground first, followed by the rest of the sole. The ankle and toes are then flexed, pushing off to provide forward motion. Meanwhile, the other leg is off the ground. It raises, swings forward, and is planted on the ground to continue the cycle.
Because the body's center of gravity generally falls between the two feet, every time we lift a foot to take a step, we create even more instability for our musculoskeletal system to handle. It does this by shifting the body's weight over the supporting leg using the hip abductor muscles.
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