Introduction: The second-generation ISS treadmill has a faster maximum operating speed than the current ISS treadmill. In normal gravity (1 G), bone loading benefits and cardiorespiratory stress are directly related to locomotion speed. A kinematic comparison of locomotion between 1 G and microgravity will provide information to evaluate the potential efficacy of fast running as an in-flight exercise countermeasure.
Methods: Subjects exercised on a treadmill at 3.13 m x s(-1) (8.5 min x mi(-1)) (JOG; N = 6) and 5.36 m x s(-1) (5 min x mi(-1)) (RUN; N = 5) in microgravity during parabolic flight and in 1 G. During microgravity trials, subjects performed locomotion using a subject loading system (in a configuration identical to ISS) with approximately 80% bodyweight loading. Kinematic analyses of joint position at heel strike were performed using video software.
Results: During the JOG trials, differences were found in thigh angle (microgravity = 54.09 degrees +/- 4.87; 1 G = 64.04 degrees +/- 3.12, mean +/- SD) and knee angle (microgravity = 33.17 degrees +/- 8.68; 1 G = 21.28 degrees +/- 5.22), indicating a more squatted position at heel strike in microgravity. No kinematic differences were found during the RUN condition.
Discussion: The subject loading system and decreased external load throughout the stride in microgravity may account for the observed kinematic differences during JOG. The kinematic compensations for microgravity during JOG may result in in-flight adaptations that are different from expected based on 1-G studies. However, similar kinematics between gravity conditions during RUN suggest in-flight training may provide benefits similar to 1 G.