MUSCLE-CONTRACTION INCREASES THE IN-VIVO STRUCTURAL STRENGTH TO THE SAME DEGREE IN OSTEOPENIC AND NORMAL RAT TIBIAE

The increase in structural capacity due to muscle contraction in the l ower leg was investigated in osteopenic and normal rats. Osteopenia wa s induced by ovariectomy combined with a low-calcium diet (0.01%). The control rats were sham operated and fed a diet containing 1.1% calciu m. After 7 weeks the right lower leg of all animals were fractured in three-point ventral cantilever bending during muscle contraction induc ed by electrical stimulation of the ischiatic nerve. The left tibiae w ere resected and fractured as each animal's control. During muscle con traction in vivo, the ultimate bending moment, energy absorption, bend ing stiffness, and deflection were significantly lower in the osteopen ic than in the sham-operated animals. However, the increase in mechani cal parameters due to muscle contraction comparing the in vivo and res ected tibiae in each animal were equally high in the osteopenic and sh am-operated animals. Ultimate bending moment in the resected tibiae wa s 10% higher in the sham-operated animals compared with the ovariectom ized, proving mechanically weaker tibiae in the osteopenic rats. In ac cordance with this, the medullary area of the osteopenic rats was 46% larger in the distal tibial diaphysis, and the ultimate stress the tib iae could withstand was 15% lower in the osteopenic compared with the sham-operated rats. The trabecular bone volume in the distal tibial me taphysis of the osteopenic rats was reduced by 70% compared with the s ham operated. This study shows that muscle protection against fracture can be substantial in osteopenic tibia and that it is of the same mag nitude as in rats with normal bone mass.