Abstract:
Plans for extensive manual labor in future translunar and interplanetary space
missions justify the development of a fully effective countermeasure program to prevent
the rampant skeletal muscle deconditioning seen in crew members during spaceflight.
The current investigation: 1) reviewed all existing literature on skeletal muscle responses
to real and simulated microgravity with and without an exercise countermeasure, and 2)
examined the efficacy of the next generation exercise countermeasures program
designed for the International Space Station (SPRINT) on lower limb skeletal muscle
health during 70 days of simulated microgravity (6° head-down-tilt bedrest). Review of the
literature revealed the need for further optimization of the exercise countermeasure
programs for skeletal muscle in real and simulated microgravity. These data are
presented in 2 supplementary files (Ground Muscle Tables.xlsx and Spaceflight Muscle
Tables.xlsx), each containing 6 supplementary tables. To test the efficacy of the SPRINT
protocol, individuals underwent 6° head-down-tilt bedrest (BR, n=9), bedrest with
resistance and aerobic exercise (BRE, n=9), or bedrest with exercise and low-dose
testosterone (BRE+T, n=8). Quadriceps and calf (triceps surae) muscle volumes were
measured via MRI before and after bed rest, and strength was assessed via maximal
isokinetic contractions of the two muscle groups. Vastus lateralis (VL) and soleus (SOL)
muscle biopsies were performed pre- and post-bedrest for the measurement of myosin
heavy chain (MHC) single muscle fiber type distribution, MHC I and IIa muscle fiber size,
metabolic enzyme activities (glycogen phosphorylase, citrate synthase, β-hydroxyacyl-
CoA dehydrogenase), and capillarization. Exercise decreased the number of hybrid
muscle fibers in the VL and blunted the bed rest-induced increase in the SOL. BR
decreased MHC I fiber size in the VL (-7%) and SOL (-12%) (p < .05), while MHC IIa fiber
size was maintained in both muscles (p > .05). In BRE, MHC I and IIa fiber size was
maintained in VL and SOL (p > .05). In BRE+T, MHC I and IIa fiber size was maintained
in the VL (p > .05), while MHC I fiber size decreased (-12%) (p < .05) and MHC IIa fiber
size was maintained (p > .05) in the SOL. Metabolic enzyme activities and capillarity were
unchanged (p > .05) in all three groups across both muscles, suggesting these muscle
health components were regulated proportionally with muscle volume changes
(Quadriceps: BR: -10%, BRE: +3%, BRE+T: +5%; Calf: BR: -22%, BRE: -7%, BRE+T: -
6%). SPRINT, without testosterone, showed better effectiveness than previous
microgravity exercise countermeasures programs regarding quadriceps and calf skeletal
muscle health. The SPRINT exercise program appears viable for thigh skeletal muscle
health, but refinement is needed to completely protect the calf at the myocellular and
whole muscle levels.
