We have proposed a mechanism for the postural balance of the human torso over the hip joint that does not appear to have been fully recognised. In all the published models we could find, the centre of gravity of the torso is usually considered poised above the hips. If this is true, then it is clear that the body is in a state of unstable equilibrium. Examination of the literature suggest that the ligamentous capsule of the hip joint is a substantial structure and early editions of Gray’s Anatomy proposed that the ilio-femoral ligament, one of the three identified as part of the capsule, was the strongest ligament in the body. Though this claim was unreferenced and has disappeared from modern editions, it is still acknowledged to be “very strong”. Considering where the forces are being applied to the pelvis, it is clear that that body weight acts primarily through the sacrum, posterior to the hip. We propose that this is counter-balanced by tension in the ilio-femoral ligament and that the hips then act 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 two-legged stance is then of the order of body-weight on each hip. An implication is that capsular ligaments are important and that consideration should be given to retaining or restoring them as much as possible in hip arthroplasty.
R. M. Aspden, K. E. Rudman, and J. R. Meakin. A mechanism for balancing the human body on the hips. J.Biomech. In Press, 2005.
Figure 1 shows the location of the ilio-femoral ligament and the approximate position of the force due to body weight applied through the sacrum. The idealised model on the right provides the basis for a simple analysis of this balancing mechanism.
Figure 2. The potential energy contributions of the two moments about the hip due to body weight and ligament tension are shown separately. Their sum shows that stable equilibrium can be achieved in the upright posture. |
