Axial reloading during body weight unloading: Relationship between g-level and cardiorespiratory responses to running – A case study

Objective: Prolonged microgravity exposure induces physiological de-conditioning that is partially mitigated by aerobic exercise. De-conditioning is also anticipated during partial gravity habitation (e.g., Moon or Mars). However, the relationship between gravity, resultant running speed, associated biomechanics, and the cardiorespiratory response is unknown. Thus, this case study evaluated responses to self-selected running across a continuum of simulated gravity levels generated by bodyweight unloading and axial loading via the Mk III Gravity Loading Countermeasure SkinSuit (GLCS). Methods: A healthy male ran at a self-selected speed for 5 min at six simulated gravity (Gz) levels. Bodyweight suspension unloaded the subject's mass to simulate Martian (0.38Gz) and Lunar (0.16Gz) gravity, whilst the GLCS reloaded the subject by 0.8Gz, thereby simulating 1.8Gz and 0.96Gz – comparable to 1Gz. Gait kinematics, heart rate (HR), respiratory variables, subjective ratings were evaluated in the final minute of each run and evaluated across conditions. Results: Cardiorespiratory, biomechanical and subjective responses were broadly similar in 1Gz and 0.96Gz (GLCS + Lunar). Positive linear associations were observed between Gz and minute ventilation (V_E) oxygen consumption (VO₂) and cost of transport responses to self-selected running, where HR responses correlated with self-selected running speed only. Conclusion: Respiratory and biomechanical responses (but not HR) to self-selected running appear related to simulated Gz. Significance: Mk III GLCS is able to induce broadly G-equivalent biomechanical and cardiorespiratory responses, and thus may be a tool to facilitate g-dependence research and hypogravity exercise.