Final answer:
When the legs are externally rotated, the pelvis muscles stabilize the body and support its weight. Leg muscles compensate for limited range of motion by providing size and power.
Step-by-step explanation:
When the legs are externally rotated, the best demonstration would be the muscles of the pelvis stabilizing the body. The pelvis, which attaches the lower limbs to the torso, plays a crucial role in supporting the body's weight and maintaining balance. The leg muscles compensate for their limited range of motion by providing size and power, enabling the body's stabilization, posture, and movement.