Regulatory, Integrative and Comparative Physiology

Spaceflight effects on single skeletal muscle fiber function in the rhesus monkey

Robert H. Fitts, Dominique Desplanches, Janell G. Romatowski, Jeffrey J. Widrick


The purpose of this investigation was to understand how 14 days of weightlessness alters the cellular properties of individual slow- and fast-twitch muscle 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 μm, respectively. Both a control 1-G capsule sit (CS) and spaceflight (SF) significantly reduced the 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/m2), only the SF group showed a significant decline. This group also showed a significant 15% drop in peak fiber stiffness that 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/m2) force in the fast-twitch MG fibers by 30% and 28%, respectively. The Ca2+ sensitivity 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 fibers showed a reduced peak power and, at peak power, an elevated velocity and decreased force. In conclusion, CS and SF caused atrophy and a reduced force and power in the Sol type I fiber. However, only SF elicited atrophy and reduced force (mN) in the MG type I fiber and a decline in relative force (kN/m2) in the Sol type I and MG type II fibers.

  • contractile properties
  • morphology
  • slow twitch
  • fast twitch


  • This study was supported by NASA Grant NAG2–636 to R. H. Fitts and CNES Grant 071 N97/CNES 6896 to D. Desplanches.

  • Address for reprint requests and other correspondence: R. H. Fitts, Marquette Univ., Dept. of Biology, Wehr Life Sciences Bldg., P.O. Box 1881, Milwaukee, WI 53201-1881 (E-mail:robert.fitts{at}

  • The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

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