Reaction forces during exercises on earth and in simulated zero-gravity

Many systems of the body are adversely affected by exposure to hypogravity. In particular, considerable mineral loss can occur in the lower extremity skeleton, amounting to almost 20% in the os calcis after a six month spaceflight. To date, a variety of countermeasures have had limited success in preventing bone mineral loss, though it is now accepted that mechanical deformation is critical for bone homeostasis on earth. To examine the issue of applying loads to the musculoskeletal system, a study was conducted in two phases. In the first stage, three exercises under active consideration for use in the Space Station were compared in terms of the peak loads and rates of change of load elicited under the feet. It was found that peak loading rate in running was over 30 times greater than in cycling, while peak loads were over 5 times greater than those in rowing. These findings suggested that exercise during spaceflight may be more beneficial if some form of tethered locomotion could be performed on a treadmill, rather than on a stationary rowing device or bicycle. In the second stage of the study, experiments were conducted to ascertain whether it is possible to reproduce 1-G-like ground reaction forces during tethered locomotion in a weightless environment. This phase necessitated that a zero-gravity locomotion simulator be built to investigate the nature of loads on the lower extremity while running and walking at various speeds.