Rest intervals reduce the number of loading bouts required to enhance bone formation.

Abstract

As our society becomes increasingly sedentary, compliance with exercise regimens that require numerous high-energy activities each week become less likely. Alternatively, given an osteogenic exercise intervention that required minimal effort, it is reasonable to presume that participation would be enhanced. Insertion of brief rest intervals between each cycle of mechanical loading holds potential to achieve this result because substantial osteoblast function is activated by many fewer loading repetitions within each loading bout. Here, we examined the complementary hypothesis that the number of bouts per week of rest-inserted loading could be reduced from three bouts per week without loss of osteogenic efficacy. We conducted a series of 3-wk in vivo experiments that noninvasively exposed the right tibiae of mice to either cyclic (1 Hz) or rest-inserted loading interventions and quantified osteoblast function via dynamic histomorphometry. Although reducing loading bouts from three bouts per week (i.e., nine total bouts) to one bout per week (i.e., three total bouts) effectively mitigated the osteogenic benefit of cyclic loading, the same reduction did not significantly reduce periosteal bone formation parameters induced by rest-inserted loading. The osteogenic response was robust to the timing of the rest-inserted loading bouts (three bouts in the first week vs one bout per week for 3 wk). However, elimination of any single bout of the three one-bout-per-week bouts mitigated the osteogenic response to rest-inserted loading. Finally, periosteal osteoblast function assessed after the 3-wk intervention was not sensitive to the timing or number of rest-inserted loading bouts. We conclude that rest-inserted loading holds potential to retain the osteogenic benefits of mechanical loading with significantly reduced frequency of bouts of activity while also enabling greater flexibility in the timing of the activity.