Effect of intermittent weight bearing on soleus fiber force-velocity-power and force-pCa relationships.

Rat permeabilized type I soleus fibers displayed a 33% reduction in peak power output and a 36% increase in the free Ca2+ concentration required for one-half maximal activation after 14 days of hindlimb non-weight bearing (NWB). We examined the effectiveness of intermittent weight bearing (IWB; consisting of four 10-min periods of weight bearing/day) as a countermeasure to these functional changes. At peak power output, type I fibers from NWB animals produced 54% less force and shortened at a 56% greater velocity than did type I fibers from control weight-bearing animals while type I fibers from the IWB rats produced 26% more absolute force than did fibers from the NWB group and shortened at a velocity that was only 80% of the NWB group mean. As a result, no difference was observed in the average peak power of fibers from the IWB and NWB animals. Hill plot analysis of force-pCa relationships indicated that fibers from the IWB group required similar levels of free Ca2+ to reach half-maximal activation in comparison to fibers from the weight-bearing group. However, at forces < 50% of peak force, the force-pCa curve for fibers from the IWB animals clearly fell between the relationships observed for the other two groups. In summary, IWB treatments 1) attenuated the NWB-induced reduction in fiber Ca2+ sensitivity but 2) failed to prevent the decline in peak power that occurs during NWB because of opposing effects on fiber force (an increase vs. NWB) and shortening velocity (a decrease vs. NWB).