Less Work After Spaceflight: Human Performance Biomechanics Following Adaptation to Simulated Hypogravity
Yun, Kristy Sue
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Within the next decade, humans will return to the Moon to establish a permanent presence and prepare for future explorations to Mars. Despite our intuitive knowledge of the influence of gravity, we still do not fully understand how our bodies develop, function, and navigate in hypogravity environments. This study aimed to evaluate the effect of reduced gravity on the biomechanical adaptation of countermovement jumping performance. Fifteen healthy participants performed targeted countermovement jumps in and out of simulated hypogravity using a reduced-gravity simulator that provided a constant upward force near the body’s COM. This constant vertical force effectively reduced bodyweight by 50%, simulating ~0.5g during the vertical jumps. The countermovement jump was divided into two main phases: (i) the Lift phase (from countermovement initiation to take off) and (ii) the Land phase (from touchdown until the stabilization of ground reaction forces). To better understand and investigate which specific parts of the Lift and Land were being affected by hypogravity adaptation, additional partitions were made. In chronological order, the parts of the Lift phase included the Early and Late Unloading phases and Early and Late Propulsive phases, and the Land phase included Early and Late Braking phases and Early and Late Recovery phases. In the first post-adaptation jump upon return to 1.0g, there was a meaningful effect in the normalized work of the Lift phase and a significant decrease in the net normalized work of the Land phase when compared to the baseline pre-adaptation jumps. Further investigation into the different portions of the jump revealed meaningful effects in specifically the last part of the Lift phase, i.e., the Late Propulsive phase, and significant changes in the first part of the Land phase, i.e., the Early Braking phase. These results indicate that humans can adapt to simulated reduced gravity using this jumping adaptation paradigm. More interestingly, observations of normalized work on the COM before and after exposure to hypogravity revealed distinct control strategies for the Lift and Land portions of the countermovement jump. The work generated during the first parts of the Lift phase, i.e., the Unloading phases, appears to be dominantly controlled through a reactive strategy, as it showed no significant after-effects upon return to 1.0g. In contrast, the work generated during the Late Propulsive phase and absorbed during the Early Braking phase of the jump was observed to be predominantly under a predictive control strategy, evidenced by the significantly decreased work upon returning to 1.0g. Thus, upon return to a higher gravity level after exposure to hypogravity, movements requiring the legs to quickly generate and absorb energy will be most affected by sensorimotor control prediction errors. This would increase the likelihood of performance errors or even injury in actions that require rapid acceleration and deceleration of the COM and should be taken into consideration during the post-adaptation re-acclimation process after prolonged exposure to hypogravity.