Likewise in humans, the quadriceps and hamstring muscles of the thigh are both so crucial to bipedal activities that each alone is much larger than the well-developed biceps of the arms. In addition to the leg muscles, the increased size of the gluteus maximus in humans is an important adaptation as it provides support and stability to the trunk and lessens the amount of stress on the joints when running.
The size of the quadriceps and hamstring muscles in humans' legs is crucial for bipedalism, as is the gluteus maximus muscle, which provides support to the trunk and reduces joint stress when running.