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

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.

Isometric force and maximal shortening velocity of single muscle fibers from elite master runners.

Single chemically permeabilized gastrocnemius fibers from six elite endurance-trained master runners (RUN group) and five age-matched sedentary controls (SED group) were mounted between a force transducer and a position motor, studied under conditions of maximal and submaximal Ca2+ activation, and subsequently electrophoresed on 5% polyacrylamide gels to determine myosin heavy chain (MHC) composition. For the SED group, peak isometric tension (Pzero) averaged 143 +/- 3, 156 +/- 4, and 170 +/- 4 kN/m2 and maximal shortening velocity (Vzero) averaged 0.43 +/- 0.01, 1.90 +/- 0.08, and 5.59 +/- 0.40 fiber lengths/s for fibers expressing type I, IIa, and IIx MHC, respectively (all comparisons, P < 0.05). Hill plot analysis of relative forces during submaximal Ca2+ activation indicated no SED vs. RUN differences in Ca2+ sensitivity or in the cooperativity of Ca2+ activation. However, at maximal Ca2+ activation, RUN type I and IIa fibers produced 15% less peak absolute force than SED fibers (P < 0.05). This reduction in fiber force was a direct result of the smaller diameter of the RUN fibers (P < 0.05), because Pzero, peak elastic modulus (Ezero), and Pzero/Ezero were not different between SED and RUN groups. RUN type I fibers also displayed a mean Vzero that was 19% higher than the average Vzero of the SED type I fibers (P < 0.05). In separate experiments, quantification of relative myosin light chain (MLC) isoform content revealed a 28% greater ratio of MLC3 to MLC2 in single type I fibers from the RUN group (P < 0.05), suggesting that the elevated Vzero of the RUN type I fibers was related to a greater expression of MLC3. In conclusion, the single fibers from the elite master runners displayed specific morphological and contractile properties that may enhance the performance of these athletes during prolonged muscular activity.

Force-velocity and power characteristics of rat soleus muscle fibers after hindlimb suspension.

The effects of 1, 2, and 3 wk of hindlimb suspension (HS) on force-velocity and power characteristics of single rat soleus fibers were determined. After 1, 2, or 3 wk of HS, small fiber bundles were isolated, placed in skinning solution, and stored at -20 degrees C until studied. Single fibers were isolated and placed between a motor arm and force transducer, functional properties were studied, and fiber protein content was subsequently analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). Additional fibers were isolated from soleus of control and after 1 and 3 wk of HS, and fiber type distribution and myosin light chain stoichiometry were determined from SDS-PAGE analysis. After 1 wk of HS, percent type I fibers declined from 82 to 74%, whereas hybrid fibers increased from 10 to 18%. Percent fast type II fibers increased from 8% in control and 1 wk of HS to 26% by 3 wk of HS. Most fibers showed an increased unloaded maximal shortening velocity (Vo), but myosin heavy chain remained entirely slow type I. The mechanism for increased Vo is unknown. There was a progressive decrease in fiber diameter (14, 30, and 38%) and peak force (38, 56, and 63%) after 1, 2, and 3 wk of HS, respectively. One week of HS resulted in a shift of the force-velocity curve, and between 2 and 3 wk of HS the curve shifted further such that Vo was higher than control at all relative loads < 45% peak isometric force. Peak absolute power output of soleus fibers progressively decreased through 2 wk of HS but showed no further change at 3 wk.(ABSTRACT TRUNCATED AT 250 WORDS)