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 weightlessness. Recent investigations suggest that mechanical influences such as bone deformation and strain rate may be critically important in stimulating new bone formation. HYPOTHESIS It was hypothesized that velocity, cadence, and harness design would significantly affect lower limb impact forces during treadmill exercise in simulated zero-gravity (0G). METHODS A ground-based hypogravity simulator was used to investigate which factors affect limb loading during tethered treadmill exercise. A fractional factorial design was used and 12 subjects were studied. RESULTS The results showed that running on active and passive treadmills in the simulator with a tethering force close to the maximum comfortable level produced similar magnitudes for the peak ground reaction force. It was also found that these maximum forces were significantly 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 maximum rate of force application (DFDTmax) was similar for overground running and exercise in simulated 0G, provided the "weightless" subjects ran on a motorized treadmill. CONCLUSIONS These findings have implications for the use of treadmill exercise as a countermeasure for hypokinetic osteoporosis. As the relationship between mechanical factors and osteogenesis becomes better understood, results from human experiments in 0G simulators will help to design in-flight exercise programs that are more closely targeted to generate appropriate mechanical stimuli.