The importance of pelvic tilt in reducing compressive stress in the spine during flexion-extension exercises.

An explanation for the importance of pelvic tilt exercises is proposed. This explanation derives from a mathematical model that predicts the existence of a relationship between lordosis (pelvic tilt) and the distribution of stresses within the spine. The model predicts that, for every angle of forward flexion, there is a unique degree of lordosis that will minimize and equalize the compressive stress within the spine. This stress-minimizing posture also is associated with the minimum muscular activity; the balance of the moment is supported by passive stretching of the connective tissues. These predictions were tested by comparing them with empirical values of spinal geometry and muscular activity, as measured simultaneously by an opto-electronic motion analysis and electromyographic (EMG) data collection system.