Some of the muscles that support the knee attach at the hip. Like other muscles that are closer to the center or "core" of the body, these muscles give stability to joints. The muscles help put your joints in the right position, and they steady the joints while you squat, walk, or jump. Core muscles form a stable platform, letting you move your leg and foot with precision. If these muscles are weak, your knee will lose some of its control, keeping it from working at its best.