A mechanism for balancing the human body on the hips

A mechanism for the postural balance of the human torso over the hip joints is reported that does not appear to have been fully recognised. The centre of gravity of the torso is usually considered poised above the hips. This is in a state of unstable equilibrium. Here, we propose that body weight acting through the sacrum is counter-balanced by tension in the ilio-femoral ligament; the hips acting as the fulcrum. This places the torso in stable equilibrium and means that balance may be maintained with minimal muscular forces. The joint reaction force due to this mechanism in a two-legged stance is then of the order of the body-weight on each hip. An implication is that capsular ligaments are important and consideration should be given to retaining or restoring them as much as possible in hip arthroplasty.