In the healthy hip joint the femoral head is continually in close and stabile contact with the socket during all movements. The stability of the healthy hip joint is provided by by numerous supporting structures around the hip joint, including a thick joint capsule, a system of joint ligaments built in the joint capsule, and a ligament inside the hip joint itself. These joint structures create a passive resistant force on the hip joint that keeps the femoral head in close contact with the hip joint socket during all movements.
The force necessary to move forward goes beyond the muscle contraction needed to control the hip joints. In most individuals the calf muscles provide the force that pushes them forward into the next step. The hip extensors then pull them forward after the foot hits the floor. Both of these muscles have sacral level innervations and are paralyzed in almost all children who have Spina Bifida. Children with thoracic and upper lumbar levels of paraplegia use their arms to provide the power to move them forward. Unfortunately, the arms are not designed for this activity and are quite inefficient. Consequently, it takes increased effort to walk and the walking is also much slower than normal. Eventually, most children with thoracic and upper lumbar level paraplegia discover that the wheelchair is a much more efficient means of transportation.