Whole-Body but Not Muscle-Level Energetic Cost is Reduced During Simulated Hypogravity Running

Maintaining crew health and physical performance during prolonged spaceflight requires effective exercise countermeasures to mitigate the detrimental effects of microgravity on the musculoskeletal and cardiovascular systems. Running exercise represents a critical countermeasure, yet optimal prescription requires understanding how reduced gravity alters the energetic and mechanical demands on working muscles. Running cost of transport (CoT) decreases proportionally, but not always, to the reduced gravity. It remains unclear whether muscle metabolic energy cost is reduced to a similar extent. This study examined whether reductions in whole-body CoT during simulated reduced-gravity running are explained by proportional reductions in medial gastrocnemius (MG) muscle energy cost, or by altered mechanical strategies underlying force production. Sixteen participants (n=11 females, 28±11 years, 176±8 cm, 69±10 kg) ran for six minutes at 80%, 100%, 120% and 140% of preferred running speed under simulated hypogravity conditions of 1.0g and 0.8g using a body-weight unloading system. Speed and gravity conditions were randomized. CoT was calculated from steady-state indirect calorimetery. MG muscle energy cost was estimated from ultrasound-derived fascicle length changes and modeled crossbridge energetics. PS at 1.0g was 2.11±0.21 m/s. Body weight was reduced by 18±3% (equivalent to 0.82±0.03g) due to unloading. CoT was significantly reduced in hypogravity (p< 0.001), and declined faster with PS (p< 0.001). The reduced CoT was not proportional to the magnitude of reduced body weight (r=0.11, p=0.69 at 80%PS to r=0.49, p=0.055 at 140%PS). Speed had a significant effect on the reduction in CoT (p< 0.001); CoT was 17±10% at 80%PS to 26±9% at 140%PS. Reductions in CoT due to unloading were similar from 100%PS to 140% PS (p>0.18). Estimated MG muscle energy cost increased with speed (p=0.02), from 0.52±0.21 J/kg/m at 80%PS to 0.81±0.28 J/kg/m at 140%PS, with no significant main effect of gravity (p=0.17). MG cost represented a higher proportion of total CoT as speed increased (from 12±4% 21±7%, p=0.017), independent of gravity (p=0.71). During stance, the MG shortened to a similar extent across speeds (p=0.15) and gravity (p=0.49), but shortened slower in hypogravity (0.038±0.005 V/Vmax vs 0.050±0.007, p=0.04) which is attributable to a significantly longer stance time in hypogravity (p< 0.001). The muscle-tendon energy ratio was unaffected by speed or gravity (p>0.28), and favored reduced muscle energy cost over amplified tendon strain energy storage, which was significantly lower in hypogravity (p< 0.001). The disproportionate reduction in whole-body CoT relative to MG muscle energy cost during hypogravity running indicates that gravitational unloading primarily reduces external mechanical work rather than intrinsic muscle metabolic demands, highlighting the need for exercise prescriptions that account for these distinct energetic demands. This study was supported by the Canadian Space Health Research Network and NSERC-Canada 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.