Automated Methods for the Analysis of Skeletal Muscle Fiber Size and Metabolic Type - International Review of Cell and Molecular Biology - page 274

under normal conditions, limits formation of skeletal muscle. Additional
hormones that have positive effects on skeletal muscle growth include
growth hormone (GH) and insulin-like growth factor-1 ( Bidlingmaier
and Strasburger, 2010; Urban, 2011; Veldhuis et al., 2005 ).
2.2. Exercise
When muscles are used more frequently or at higher-work load, the muscles
increase in size and strength, a phenomenon termed adaptation ( Folland and
Williams, 2007; Yan et al., 2011 ). Increases in overall muscle cross-sectional
area of 5-10% are commonly achieved ( Friedmann-Bette et al., 2010; Holm
et al., 2008; Terzis et al., 2006; Vissing et al., 2008 ). Endurance athletes often
feature a higher proportion of Type I fibers than untrained individuals; how-
ever, no study in humans has directly demonstrated an increase in the pro-
portion of Type I fibers in response to a specific exercise regime. Endurance
exercise does increase the proportion of the relatively oxidative Type IIA
fiber type ( Yan et al., 2011 ). While the occurrence of adaptation is well
known, the mechanisms responsible for adaptation are not completely
understood ( Philp and Baar, 2012 ). Second messenger systems implicated
in adaption include intracellular calcium (which is increased with each con-
traction) and Ca 2 þ -calmodulin-dependent serine/threonine kinases, cAMP
( Berdeaux and Stewart, 2012 ), insulin/insulin-like growth factors, and Akt
( Coffey and Hawley, 2007 ). Glycogen content is also hypothesized to mod-
ulate muscle adaptation ( Philp et al., 2012 ), and exercise may increase ste-
roidogenesis within skeletal muscle itself ( Aizawa et al., 2011 ).
2.3. Spaceflight
A special case in which reduced work load leads to reduction in muscle mass
is spaceflight ( Fitts et al., 2001; Graebe et al., 2004; Narici and de Boer,
2011 ). Low gravity leads to marked atrophy of the weight-bearing muscles
as well as demineralization of the skeleton, and other physiological changes,
and these are limiting factors for human space travel. For rats subjected to a
14-day spaceflight, CSA was reduced by 30% in Type I versus an approx-
imately 15% reduction in Type II fibers; in contrast, for humans, CSA was
reduced approximately 35% and 20% for Type IIX and Type I fibers, respec-
tively ( Fitts et al., 2000 ), illustrating a subtle difference between the
responses of the rodent model systems and humans. In earth-bound labora-
tories, low gravity is often simulated using a “hind-limb unloading” model,
in which rodents are suspended by the tail, so that only the forelimbs touch