The muscles of the hip

While the bones and ligaments provide the joint with static stability, muscles and tendons are indispensable for motion. Since the joint can move in any direction, muscles are needed to orchestrate the motion: bending / stretching is performed by the flexors and extensors, spreading apart and pulling together are made possible through the abductors and adductors, while twisting motions come to be by using the inward and outward rotators. These muscular interactions must function perfectly to enable the whole body to remain stable on the ball. Just try to balance 10 kilograms on a ball and you will notice very quickly how difficult this task is!

As far as muscles are concerned, the so-called hip deltoid plays a very special role (marked here in red), since it is responsible for the stability of the pelvis. Some of the most important muscles in this group are abductors, but the tensor fasciae latae is also critical for maintaining balance.

The importance of the interplay of these muscles and maintaining their intactness is still often overlooked by surgeons, especially during hip replacement surgery (see the Micro-Hip chapter).

The whole hip system becomes a bit more complicated when you consider that enormous forces of up to five times a person’s body weight act on the joint.

The torso (40 kg) plus one leg (10 kg) add up to a weight of 50 kg. This weight of 50 kg is concentrated at the center of the pelvis. The distance from the hip joint to the center of the pelvis is four times larger than the distance between hip joint and where the abductor attaches (abductors are marked in red on the left side).

The abductors must pull four times more than the body weight to balance the pelvis. Because of this, training the abductor muscles is of great importance for all those who engage in sports that involve running or jumping. Particularly, runners should not forget to specially train these muscles becuase they are not strengthened enough during regular running.

The geometry of the hip joints is not the same in all persons. Generally, there are 2 categories: we distinguish between steep and flattened hips.

Steep hips are known as “coxa valga” while flattened hips are called “coxa vara.”