Designing a zero-gravity locomotion simulator

One of the factors that potentially limit the duration humans can stay in a weightless environment concerns spaceflight-induced osteoporosis. At present it is commonly believed that astronauts need to perform exercises during orbital missions, that result in the lower extremity experiencing loads comparable to those normally experienced on earth. In order to investigate whether it is indeed possible to elicit these forces during tethered treadmill locomotion, a simulator was designed to create a weightless environment in a regular gait laboratory. This requirement stemmed from the prohibitive costs and logistical difficulties associated with conducting experiments during an actual space mission. It was felt that a supine suspension device would adequately imitate weightless conditions if certain criteria were met: the method of suspension should not hinder natural locomotion, each body segment's weight should be negated by a constant force, the simulator should allow locomotion at a variety of speeds, the device should be capable of measuring reaction forces during tethered treadmill exercise. In designing a simulator to meet all of these criteria it was found that 10 m long solid latex cords (diameter 13 mm), wrapped around five or less pulleys, would provide relatively constant suspension forces and would not be susceptible to resonance at cadences between 0.5 and 4 Hz.