GROUND REACTION FORCES DURING LOCOMOTION IN SIMULATED MICROGRAVITY

Background: Significant losses in bone density and mineral, primarily in the lower extremities, have been reported following exposure to wei ghtlessness. Recent investigations suggest that mechanical influences such as bone deformation and strain rate may be critically important i n stimulating new bone formation. Hypothesis: It was hypothesized that velocity, cadence, and harness design would significantly affect lowe r limb impact forces during treadmill exercise in simulated zero-gravi ty (0G). Methods: A ground-based hypogravity simulator was used to inv estigate which factors affect limb loading during tethered treadmill e xercise. A fractional factorial design was used and 12 subjects were s tudied. Results: The results showed that running on active and passive treadmills in the simulator with a tethering force close to the maxim um comfortable level produced similar magnitudes for the peak ground r eaction force. It was also found that these maximum forces were signif icantly lower than those obtained during overground trials, even when the speeds of locomotion in the simulator were 66% greater than those in 1G. Cadence had no effect on any of the response variables. The max imum rate of force application (DFDTmax) was similar for overground ru nning and exercise in simulated 0G, provided the ''weightless'' subjec ts ran on a motorized treadmill. Conclusions: These findings have impl ications for the use of treadmill exercise as a countermeasure for hyp okinetic osteoporosis. As the relationship between mechanical factors and osteogenesis becomes better understood, results from human experim ents in 0G simulators will help to design in-flight exercise programs that are more closely targeted to generate appropriate mechanical stim uli.