HYPERTROPHY OF RAT PLANTARIS MUSCLE-FIBERS AFTER VOLUNTARY RUNNING WITH INCREASING LOADS

There have been no systematic comparisons of skeletal muscle adaptatio ns in response to voluntary wheel running under controlled loading con ditions. To accomplish this, a voluntary running wheel for rats and mi ce was developed in which a known load can be controlled and monitored electronically. Five-week-old male Sprague-Dawley rats (10 rats/group ) were assigned randomly to either a 1) sedentary control group (Contr ol); 2) voluntary exercised with no load (Run-No-Load) group; or 3) vo luntary exercised with additional load (Run-Load) group for 8 wk. The load for the Run-Load group was progressively increased to reach simil ar to 60% of body weight during the last week. of training. The propor tions of fast glycolytic (FG), fast oxidative glycolytic (FOG), or slo w oxidative (SO) fibers in the plantaris were similar in all groups. T he absolute and relative plantaris weights were greater in the Run-Loa d group compared with the Control and Run-No-Load groups. The mean fib er cross-sectional areas of FG, FOG, and SO fibers were 20, 25, and 15 % greater in the Run-Load than in Control rats. In addition, these fib er types were 16, 21, and 12% larger in Run-Load than in Run-No-Load r ats. The muscle weights and mean cross-sectional areas of each fiber t ype were highly correlated with the average running distances and tota l work performed in the Run-Load, but not the Run-No-Load, group. The slope of the relationship between fiber size and running distance and total work performed was significant for each fiber type but was highe r for FG and FOG fibers compared with SO fibers. These data show that the load on a rat running voluntarily can determine the magnitude of a hypertrophic response and the population of motor units that are recr uited to perform at a given loading condition.