Adaptations of rodent skeletal muscle to resistance exercise and simulated microgravity

Skeletal muscle can adapt to the functional demands placed upon it. During spaceflight, when postural muscles don't have to work against gravitational forces, these muscles atrophy and weaken. Slow-twitch, postural muscles like the soleus are most affected by disuse. Fast-twitch, locomotor muscles, like the gastrocnemius are more resistant to atrophy. This dissertation investigates resistance exercise as a countermeasure to disuse atrophy. The first experiments investigated adaptations of the soleus and gastrocnemius to 14 weeks of resistance exercise. Male rats (∼300g) were trained to plantarflex against resistance to receive food pellets. Rats trained twice daily, 5 days/week, for 14 weeks and performed 43 ± 0.3 lifts/day with 593 ± 2 g. Upon completion of the training period, the gastrocnemius-to-body weight ratio was increased. There were no changes in the mass or in vitro contractile properties of the soleus. In situ , the soleus displayed a reduced time to peak tension. Whole muscle contractile properties were not determined for the gastrocnemius. Single muscle fibers from the gastrocnemius expressing slow, type I myosin had a greater specific tension, shortening velocity and power after training. These data suggest that high intensity exercise trains locomotor muscles like the gastrocnemius and not the postural soleus. Resistance exercise was combined with a model of weightlessness, hindlimb unweighting, to determine its ability to prevent atrophy. Rats were divided into four groups: control, unweighted, unweighted plus resistance exercise (HU+RE), and unweighted plus standing (HU+ST). Unweighted, HU+RE and HU+ST were unweighted for 14 days. HU+RE and HU+ST were removed from unweighting for 20 minutes, twice daily, 7 days/week to lift weights and stand, respectively. HU+RE rats performed an average 35 ± 2 lifts/day with 402 ± 5 g. Unweighting alone resulted in a 34% decrease in soleus mass and a 71% drop in peak power. Both weight training and standing attenuated the decline in soleus mass and power. Resistance exercise was uniquely able to prevent the loss of power in fast, type IIa fibers of the gastrocnemius. Resistance exercise during unweighting is beneficial to the slow-twitch soleus but can not restore function to control values. It can, however, completely reverse the unweighting-induced loss of power in gastrocnemius type IIa fibers.