Spaceflight effects on single skeletal muscle fiber function in the rhesusmonkey

The purpose of this investigation was to understand how 14 days of weightle ssness alters the cellular properties of individual slow- and fast-twitch m uscle fibers in the rhesus monkey. The diameter of the soleus (Sol) type I, medial gastrocnemius (MG) type I, and MG type II fibers from the vivarium controls averaged 60 +/- 1, 46 +/- 2, and 59 +/- 2 mm, respectively. Both a control 1-G capsule sit (CS) and spaceflight (SF) significantly reduced th e Sol type I fiber diameter (20 and 13%, respectively) and peak force, with the latter declining from 0.48 +/- 0.01 to 0.31 +/- 0.02 (CS group) and 0. 32 +/- 0.01 mN (SF group). When the peak force was expressed as kiloNewtons per square meter (kN/m(2)), only the SF group showed a significant decline . This group also showed a significant 15% drop in peak fiber stiffness tha t suggests that fewer cross bridges were contracting in parallel. In the MG , SF but not CS depressed the type I fiber diameter and force. Additionally , SF significantly depressed absolute (mN) and relative (kN/m(2)) force in the fast-twitch MG fibers by 30% and 28%, respectively. The Ca2+ sensitivit y of the type I fiber (Sol and MG) was significantly reduced by growth but unaltered by SF. Flight had no significant effect on the mean maximal fiber shortening velocity in any fiber type or muscle. The post-SF Sol type I fi bers showed a reduced peak power and, at peak power, an elevated velocity a nd decreased force. In conclusion, CS and SF caused atrophy and a reduced f orce and power in the Sol type I fiber. However, only SF elicited atrophy a nd reduced force (mN) in the MG type I fiber and a decline in relative forc e (kN/m(2)) in the Sol type I and MG type II fibers.