2007, Piran, Slovenia

Environmental Ergonomics XII
Igor B. Mekjavic, Stelios N. Kounalakis & Nigel A.S. Taylor (Eds.), © BIOMED, Ljubljana 2007
DISCUSSION
Skeletal muscle atrophy is one of the primary problems associated with microgravity
and simulated weightlessness models. Our findings confirm previous observation of a
greater susceptibility to atrophy of the postural muscles, such as the GM, and least of
the non-antigravity muscles, such as the BB.
Contractile changes were described for contraction time (Tc) and the peak amplitude
(Dm) of the twitch muscle contraction response. Tc changes were significantly
reduced in ES and increased in GM muscles. This observation is somewhat surprising
in that both the ES and GM are extensor, i.e. antigravity, muscles. Whereas the
decrease in Tc seems consistent with a greater expression of fast MHC isoforms with
disuse (Trappe et al., 2004), an increase Tc in the GM is difficult to explain,
particularly because previous bed rest studies have found an increase in fast MHC
isoforms expression (Trappe et al., 2004).
Dm changes were more predictable as we observed increased Dm in all leg and lower
back muscles. Statistical significance was confirmed in GM, VM and BF muscles. As
the subjects were allowed to move their arms freely no affect was observed in BB
muscle, neither in Dm nor in Tc. Dm confirmed that subjects with stiffer muscles in
the baseline had bigger Dm increase after bed-rest (P < 0.05 in BB, BF and VM). This
observation may be explained by, 1) an increase in intramuscular connective tissue,
known to occur with atrophy and, 2) an increase in antagonist muscle co-activation,
frequently found in atrophy and sarcopenia (Reeves et al. 2006). It seems plausible
that an increase in muscle stiffness may be needed to compensate for the loss in
tendon stiffness known to occur with prolonged bed rest (Reeves et al. 2005), since
this may preserve the stiffness of the muscle-tendon complex as a whole.
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