EFFECT OF FATIGUING EXERCISE ON LONGITUDINAL BONE STRAIN AS RELATED TO STRESS-FRACTURE IN HUMANS

Muscular fatigue in the training athlete or military recruit has been hypothesized to cause increased bone strain that may contribute to the development of a stress fracture. Under normal circumstances, muscles exert a protective effect by contracting to reduce bending strains on cortical bone surfaces. In vivo strain studies in dogs show that musc le fatigue following strenuous exercise elevates bone strain and chang es strain distribution. However, a similar experiment has yet to be pe rformed in humans. The purpose of this work was to test the hypothesis in humans that strenuous fatiguing exercise causes an elevation in bo ne strain. It was also hypothesized that this elevation is greater in younger people than in older people due to the decline in muscle stren gth and endurance that normally occurs with age. To test these hypothe ses, strain in the tibiae of seven human volunteers was measured durin g walking before and after a period of fatiguing exercise. Neither hyp othesis was sustained. Post-hoc analysis of the strain data suggests t hat strain rate increases after fatigue with a greater increase in you nger as opposed to older persons. Although not conclusive, this sugges ts that it is strain rate, rather than strain magnitude, that may be c ausal for stress fracture. (C) 1998 Biomedical Engineering Society.