Locomotion in simulated zero gravity: Ground reaction forces

Background: Exercise is likely to be an important countermeasure to bone demineralization, which remains a concern for astronauts during long-duration spaceflight. However, loads on the feet during exercise with 1 G equivalent gravity replacement are not known. The purpose of this study was to compare ground reaction forces (GRFs) during overground and simulated zero gravity (0 G) locomotion. Hypothesis: It was hypothesized that sufficient gravity replacement loading could be applied to the subjects such that GRF profiles similar to those seen in 1 G would occur during locomotion in a zero-gravity locomotion simulator (ZLS). Methods: GRFs were measured during overground walking and running, and during locomotion in two restraint harness designs in the ZLS with an initial loading of 1 body weight. Load cells measured the gravity replacement load (GRL) in the ZLS. Joint angles at the hip and knee were also measured by goniometers. Results: Peak forces were greater in overground locomotion than in the ZLS; however, loading rates were greater in the ZLS running conditions than in overground running. The knee joint was more flexed at key times in the support phase during running in the ZLS compared with overground. Conclusions: Large loads and loading rates can be generated at the feet during simulated 0 G exercise although peak forces during running in the ZLS are less than overground running at the same speed. The refinement of the gravity replacement system to provide a constant 1 G load should be considered.