PEAK FORCE AND MAXIMAL SHORTENING VELOCITY OF SOLEUS FIBERS AFTER NON-WEIGHT-BEARING AND RESISTANCE EXERCISE

This study examined the effectiveness of resistance exercise as a coun termeasure to non-weightbearing-induced alterations in the absolute pe ak force, normalized peak force (force/fiber cross-sectional area), pe ak stiffness, and maximal shortening velocity (V-o) of single permeabi lized type I soleus muscle fibers. Adult rats were subjected to one of the following treatments: normal weight bearing (WE), non-weight bear ing (NWB), or NWB with exercise treatments (NWB+Ex). The hindlimbs of the NWB and NWB+Ex rats were suspended for 14 days via tail harnesses. Four times each day, the NWB+Ex rats were removed from suspension and performed 10 climbs (similar to 15 cm each) up a steep grid with a 50 0-g mass (similar to 1.5 times body mass) attached to their tail harne ss. NWB was associated with significant reductions in type I fiber dia meter, absolute force, normalized force, and stiffness. Exercise treat ments during NWB attenuated the decline in fiber diameter and absolute force by almost 60% while maintaining normalized force and stiffness at WE levels. Type I fiber V-o increased by 33% with NWB and remained at this elevated level despite the exercise treatments. We conclude th at in comparison to intermittent weight bearing only (J. J. Widrick, J . J. Bangart, M. Karhanek, and R. H. Fitts. J. Appl. Physiol. 80: 981- 987, 1996), resistance exercise was more effective in attenuating alte rations in type I soleus fiber absolute force, normalized force, and s tiffness but was less effective in restoring type I fiber V-o to WE le vels.