Prolonged high force high repetition pulling induces trabecular bone loss and microcracks,while low force high repetition pulling induces bone anabolism

We have an operant rat model of upper extremity reaching and grasping in which we examined the impact of performing a high force high repetition (High-ForceHR) versus a low force low repetition (Low-ForceHR) task for 18 weeks on the radius, compared to age-matched controls. High-ForceHR rats performed at 4 reaches/min and 50% of their maximum voluntary pulling force for 2 hrs/day, 3 days/wk. Low-ForceHR rats performed at 6% maximum voluntary pulling force. High-ForceHR rats showed decreased trabecular bone volume in the distal radius and increased catabolic indices (microcracks and increased osteocyte apoptosis), compared to controls. In contrast, Low-ForceHR rats showed increased trabecular bone volume, and no microcracks or osteocyte apoptosis. Thus, prolonged performance of an upper extremity reaching and grasping task is loading-dependent with high force loading leading to accumulation of bone microdamage. The target grasp force (1.27N) of the High-ForceHR rats was 50% of their mean maximum voluntary force of 2.45N, easily below the predicted maximum acceptable force (MAF) of 1.64N (predicted from published equations). The presence of microcracks and bone loss leads us to suggest that the published equations over-estimated the MAF on these tissues by 1.3 fold.