This investigation evaluated myocellular responses to an integrated resistance and aerobic training program during 70 d of bed rest.Methods
Training was 6 d·wk−1 on a small-footprint gravity-independent flywheel resistance and aerobic device; 3 d of maximal flywheel supine quadriceps and calf exercises with continuous rowing separated by 4 to 6 h, and 3 d of interval rowing. Vastus lateralis (VL) and soleus (SOL) muscle biopsies were obtained from eight healthy males (age, 28 ± 4 yr; BMI, 25 ± 3 kg·m−2; V˙O2max, 42 ± 6 mL·kg−1·min−1) before and after 6° head-down tilt bed rest. Vastus lateralis and SOL myosin heavy chain (MHC) I and IIa single muscle fiber size and functional characteristics, as well as overall fiber type distribution, capillarization, and metabolic enzyme activities were evaluated.Results
In the VL, MHC I size and power (absolute and normalized) were preserved. The MHC IIa fibers hypertrophied (+6%, P < 0.05) without a change in absolute power, so normalized power declined (−7%, P < 0.05). In the SOL, MHC I fibers atrophied (−9%) and absolute power declined (−17%) (P < 0.05), whereas normalized power was maintained. Size, absolute power, and normalized power were protected in the less-abundant MHC IIa fibers. Reduced MHC coexpressing hybrid fibers, generally indicative of an exercise training effect, was apparent in the VL, whereas fiber type was maintained in the SOL. Capillarization and metabolic enzymes were generally preserved or increased in VL and SOL.Conclusions
The integrated resistance and aerobic training protocol on a device maintains several key myocellular characteristics during prolonged unloading, but further refinement of the exercise approach to fully protect the SOL is warranted.