Force-velocity-power and force-pCa relationships of human soleus fibers after 17 days of bed rest

Abstract

Soleus muscle fibers from the rat display a reduction in peak power and Ca2+ sensitivity after hindlimb suspension. To examine human responses to non-weight bearing, we obtained soleus biopsies from eight adult men before and immediately after 17 days of bed rest (BR). Single chemically skinned fibers were mounted between a force transducer and a servo-controlled position motor and activated with maximal (isotonic properties) and/or submaximal (Ca2+ sensitivity) levels of free Ca2+. Gel electrophoresis indicated that all pre- and post-BR fibers expressed type I myosin heavy chain. Post-BR fibers obtained from one subject displayed increases in peak power and Ca2+ sensitivity. In contrast, post-BR fibers obtained from the seven remaining subjects showed an average 11% reduction in peak power (P < 0.05), with each individual displaying a 7-27% reduction in this variable. Post-BR fibers from these subjects were smaller in diameter and produced 21% less force at the shortening velocity associated with peak power. However, the shortening velocity at peak power output was elevated 13% in the post-BR fibers, which partially compensated for their lower force. Post-BR fibers from these same seven subjects also displayed a reduced sensitivity to free Ca2+ (P < 0.05). These results indicate that the reduced functional capacity of human lower limb extensor muscles after BR may be in part caused by alterations in the cross-bridge mechanisms of contraction.

Effect of long-term muscle paralysis on human single fiber mechanics

“…Widrick and coworkers (28) showed that in one subject out of eight, fiber diameter remained unchanged and P 0 was increased after 17 days of bedrest, whereas average values showed significant decreases in both variables. This individual also had increases in V 0 and fiber power (27,28). In another investigation from the same group, one out of four astronauts did not display a decrease in fiber diameter following a 17-day spaceflight mission, in contrast to the other three astronauts (26).…”

“…Experiments based on single fiber models have demonstrated great sensitivity in detecting also a certain degree of functional variability of fibers expressing the same MHC isoforms, especially when the pattern of muscle activity changes. Human studies involving 17 days of bed rest (27,28) or spaceflight (26) revealed that maximal single fiber force (P 0 ) from the soleus muscle was decreased, mainly as a result of a decline in fiber cross-sectional area (CSA), and that maximal unloaded shortening velocity (V 0 ) was increased. Consequently, single fiber power was either maintained or depressed, depending on the study participant or fiber type (29), with fibers expressing type I MHC being generally more affected.…”

The deleterious effects of bed rest on human skeletal muscle fibers are exacerbated by hypercortisolemia and ameliorated by dietary supplementation

“…The single fiber isolation and test procedures were exactly as described previously (21,22) and briefly reviewed here. On the day of Values are means Ϯ SE.…”

“…Fibers were subjected to slack tests to measure unloaded maximal shortening velocity (V 0), and the force-velocity relation was determined from a series of isotonic releases as described previously (21). P 0 was defined as the peak force obtained during the experiment.…”

“…The first and approximately every fourth subsequent contraction was performed at pCa 4.5. Hill plots were used to determine the Ca 2ϩ concentration associated with Ca 2ϩ activation threshold and with half-maximal activation (21). During these experiments, the fibers were subjected to sinusoidal length changes (frequency, 1.5 kHz; amplitude, 0.05% of FL), first in relaxing solution and then at the peak of the Ca 2ϩ activated contraction.…”