NASA SPRINT Efficacy for Skeletal Muscle Metabolic Health During Long Duration Simulated Microgravity

Objectives: The purpose of this study was to assess the effectiveness of the NASA SPRINT exercise protocol in preserving the metabolic enzyme profile in slow and fast myofibers of the vastus lateralis and soleus during 70 days of simulated microgravity. Hypotheses: The SPRINT exercise protocol will preserve metabolic enzyme content (absolute amount) and concentration (relative to cell size) in the slow and fast myofibers of the vastus lateralis and soleus during 70 days of simulated microgravity. Methods: Seven males (33±6 y) performed SPRINT exercise countermeasures during 70 days of simulated microgravity (6° head-down-tilt bedrest). The SPRINT protocol consisted of 3 days of high-intensity resistance exercise and continuous cycling alternated with 3 days of sprint intervals on the treadmill. Muscle biopsies were obtained from the vastus lateralis and soleus before and on day 64 of bedrest. Muscle samples were analyzed immunohistochemically for fiber type, fiber size, and fiber specific enzyme content (staining intensity x myofiber cross sectional area (CSA)) and enzyme concentration (staining intensity/myofiber CSA)]. Glycolytic (phosphofructokinase, PFK); fat oxidation (β-hydroxyacyl-CoA, β-HAD; carnitine palmitoyltransferase I, CPT-I); Krebs cycle (oxoglutarate dehydrogenase, OGDH; succinate dehydrogenase, SDH), and electron transport chain (ETC) (cytochrome-I oxidase, CO-I; cytochrome-IV oxidase, CO-IV) enzymes were assessed. Results: Vastus lateralis: Enzyme content was unchanged in slow and fast fibers across all metabolic pathways (P >0.05), except for one ETC enzyme in the fast fibers (CO-I, -6%, P< 0.05). When corrected for cell size, enzyme concentration was unchanged in slow and fast fibers across all metabolic pathways (P >0.05). Soleus: Enzyme content was reduced across all metabolic pathways for slow (-26 to -47%, P< 0.05) and fast (-29% to -56%, P< 0.05) fibers. When corrected for cell size, enzyme concentration was unchanged in slow and fast fibers across all metabolic pathways (P >0.05), except for one fat oxidation pathway enzyme in the fast fibers (β-HAD, +36%, P< 0.05). Conclusions: The metabolic enzyme profile was maintained in the vastus lateralis during long duration simulated microgravity highlighting the efficacy of the SPRINT exercise program for preserving skeletal muscle metabolic health. In contrast, the soleus slow and fast fiber enzyme profiles were reduced across all metabolic pathways suggesting additional muscle-specific exercise countermeasures will be required during long duration microgravity exposure to maintain the metabolic health of this important postural muscle. Funding Source: NASA grant NNX11AJ62G and the Ball State University Human Bioenergetics Program. This abstract was presented at the American Physiology Summit 2026 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.