A Spring-Loaded Constant Force Exercise Device

Abstract

Bone mineral density loss has been observed in astronauts who have spent a significant amount of time in a micro-gravity environment [1]. The lack of mechanical stress placed on the bones while in this environment is a major factor in the decrease in bone mineral density. To counteract these effects, resistive exercise has been the focus of many studies. Weighted plates have been used to provide a constant force resistance in ground-based bed rest studies. Bed rest has been established as one of the best ways to simulate the long-term effects of micro-gravity on earth [2]. In weightlessness, however, an alternate source of resistive force is required Some exercise devices provide resistance with elastic bands. Unfortunately, these elastic bands tend to lose their mechanical stiffness with use, requiring many spare elastic bands to be available for long duration missions in space. A more robust system that requires less maintenance would be preferable. Metallic helical springs meet this requirement The mechanical properties of helical springs are predictable, and these springs have a long life, making them well suited for long duration missions. However, helical springs exhibit linear behavior. That is, the resistive force provided by the spring is directly proportional to the amount of deflection. Therefore, a mechanism was designed to interact with the linear springs to provide a constant output force over a given length of travel.