In-shoe force measurements from locomotion in simulated zero gravity during parabolic flight

INTRODUCTION:: No effective countermeasure for space-induced bone loss has yet been identified. It has been hypothesized that an effective exercise regimen would elicit loads on the lower extremity which resemble those encountered on Earth. Although a treadmill has been used on shuttle flights, the loads to which the lower extremity was exposed have not yet been quantified. It is believed that these loads are much less than the loads experienced in 1G. The purpose of this study was to determine the magnitude of lower extremity loading during tethered treadmill exercise in a 0G environment. METHODS:: Data were collected on five subjects (avg. ht. 177.3+/-10.1 cm, avg. mass 78.3+/-18.0 kg) onboard the KC-135, a NASA airplane used to simulate periods of zero gravity through parabolic flight. Subjects ambulated at 4 speeds: a walk (1.56m/sec), fast walk (2.0m/sec) slow jog (2.75m/sec), and jog (3.35m/sec) on the NASA treadmill operated in either a passive or motorized mode. Each subject wore a harness connected to the Subject Load Device (SLD) to tether them to the treadmill. The tension in the SLD was subjectively adjusted for comfort by each subject. Force data were collected at 60 Hz using Pedar insoles. The number of parabolas per subject was variable due to motion sickness and hardware problems. RESULTS:: Analysis of the insole data showed that the average SLD load was only 35.2% BW, although the values ranged from 20.1% to 56.6%. Maximum ground reaction force values increased with increasing speed and were not affected by treadmill mode. The impulse was higher during walking with the treadmill in the passive mode than in the active mode, but this difference diminished with increasing speed. Subjects tended to run on their forefeet, as shown from the extremely small heel impulse values. At higher speeds, heel contact was absent, while forefoot impulse became more pronounced. DISCUSSION:: All force values were lower than those reported from 1G studies, where typical peak ground reaction forces are 1.2xBW and 2.5xBW for walking and running, respectively. At every speed, the ratio of the rearfoot to forefoot impulse was much lower than reported from 1G studies, and this ratio decreased with increasing speed. CONCLUSIONS:: If the exposure to forces similar to those in 1G is a requirement for countermeasures against space-induced osteoporosis, the loads in the SLD must be greatly increased and should be directly measured before exercise.