A variety of studies have shown that many systems of the body undergo adaptation after exposure to weightlessness. With regard to the skeletal system, there are striking similarities between spaceflight-induced osteoporosis and the process of disuse osteoporosis that patients experience after being confined to bed for an extended period of time. Based on these similarities, it has been suggested that the process of mineral loss may continue for many months in space, primarily in the lower extremities. It has also been contended that even if bone demineralization reaches a plateau during extended spaceflight, the possibility of irreversible osteoporotic changes still exists. This hypothesis seems to be supported by additional data showing a statistically significant loss of bone mineral as much as 5 years post-flight.

Recent investigations suggest that mechanical influences may be critically important in stimulating new bone formation. This kind of loading pattern would typically be found during active exercise--which is in agreement with researchers who advocate running on a treadmill to elicit high loads on the lower extremity during spaceflight. In the case of normal 1G running, factors such as velocity, cadence and treadmill design affect the magnitude of impact forces experienced by the lower extremity. Whether these variables have a similar effect in weightlessness still needs to be ascertained. Although a biomechanical investigation into these factors should ideally be performed during an actual space mission, cost and logistical considerations make such a venture impractical.

The aims of the present study were first, to conduct a detailed investigation into the design of a zero-gravity locomotion simulator, and second, to investigate which factors affected the loads experienced by the legs during tethered treadmill exercise in simulated hypogravity. The underlying assumption was that if a certain combination of velocity, cadence and harness design could produce high-impact forces, part of the problem of demineralization during space travel may be alleviated.

Twelve subjects were recruited for the study that compared running and walking either overground (i.e., in 1G) or on a treadmill that formed part of a 0G locomotion simulator (ZLS). This device required subjects to be suspended horizontally from multiple latex cords--each cord negating the weight of a different limb segment. Once subjects were "weightless," a set of springs was attached to the waist region to act as a tether to the treadmill. The total force in this so-called "gravity replacement system" (GRS) was varied during the experiment, with the mean tension being equal to 60% bodyweight, and the two other levels being greater or less than this setting by an amount of 80N. Other variables that were manipulated were velocity, cadence, and treadmill mode (either motorized or passive).

The results showed that running on active and passive treadmills in the simulator produced similar magnitudes for the maximum ground reaction force (maxGRF). 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.

With regard to the rate of force application at footstrike, it was found that the maximum rate of increase (maxDFDT) was similar for overground running and exercise in simulated OG, provided the "weightless" subjects ran on a motorized treadmill. Running on a passive treadmill in the simulator, however, produced significantly lower magnitudes for maxDFDT when compared to overground running. This ties in with the findings that tibial accelerations were 50% higher for locomotion on a motorized treadmill, and that only when subjects walked or ran in this so-called "active" mode, did the heel region of the foot come into contact with the running surface.

These results are considered important in light of work showing that low strain magnitudes do not prevent bone deterioration except at high strain rates. Thus, although the exact relationship between bone strain and external loads still needs to be investigated, it is possible that the reasonably high maxDFDT responses for active treadmill running could compensate for lower maxGRF values and produce an adequate osteogenic stimulus during space missions.