Effects of Erect Bipedal Standing on the Cross-sectional Geometry of the Rat Femur

Abstract

Effects of erect bipedal standing exercise on the skeletal morphology were investigated in seventeen growing male rats divided into control and exercise group. Using the newly devised 'bipedal training box', in which rats achieved a fully upright stance through positively reinforced operant conditioning, the exercise group was burdened with the bipedal standing exercise from 64 days to 140 days of age, totally in 136-138 sessions. At the age of 140 days, the left femur was dissected out, ten serial cross sections of the femoral diaphysis were cut from proximal to distal and cross-sectional properties were calculated from the photographs of the sections. The bipedal standing exercise had the following effects on the femoral diaphysis; an increase in the cross-sectional area, area moments of inertia in the proximal half of the shaft, i.e. the strength of the femoral diaphysis increased against axial compressive, antero-posterior bending and medio-lateral bending, respectively; an increase in the polar moment of inertia and an external rotation of the principal axis in the vicinity of mid-shaft, i.e. the strength against the torsional load increased and the direction to resist the maximum bending load more or less approached the antero-posterior direction. These observations were discussed in comparison with the effects of quadrupedal running exercise on the femoral cross section previously observed by us.