Treadmill exercise is currently employed as a countermeasure to space flight de-conditioning during long duration missions aboard the International Space Station (ISS). The primary treadmill currently in use, a space flight treadmill with vibration isolation and stabilization (TVIS), can be operated in either motorized (TVIS-M) or non-motorized (TVIS-NM) modes. Although nominal treadmill exercise is performed in the motorized mode, non-motorized treadmill exercise has been utilized as a contingency situation in case of treadmill motor failure or low power availability for treadmill operation. Additionally, a Russian non-motorized treadmill (BD-1) has been proposed as a contingency exercise device in the event of a TVIS failure.
Although previous studies have demonstrated differences between motorized and non-motorized treadmill exercise, there has been no direct comparisons made between exercise hardware designed for space flight. Little is known about any differences in metabolic demand, muscle work, gait kinematics, and ground reaction forces between motorized and non-motorized treadmill exercise.
The purpose of this study is to compare the acute metabolic and cardiovascular responses, as well as the kinetic and kinematic aspects of short-duration TVIS-M, TVIS-NM, and BD-1 treadmill exercise in relation to a standard laboratory treadmill (LAB-TM) exercise. The findings from this study provide valuable information regarding ongoing use of treadmill exercise as a countermeasure to musculoskeletal and cardiovascular de-conditioning associated with space flight and assist in the development of future treadmill exercise prescriptions.
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Twenty-four subjects (males and females) were recruited by the Human Test Subject Facility at NASA-Johnson Space Center. Cardiovascular functional capacity of the subjects was screened using a graded exercise test on a motorized treadmill. The investigator team predetermined that the minimal level of aerobic fitness for test subjects, as assessed by peak oxygen consumption (VO2pk
), was 35 ml/kg/min. Previous experience in our laboratory suggests that this is the lower limit of fitness among US astronauts. Of the 24 volunteers, only 21 achieved the desired level of fitness, and one subject later withdrew from the study. Twenty subjects completed the study.
Following the VO2pk test, subjects reported for testing on at least ten different occasions: two familiarization and eight data collection sessions. During the familiarization sessions, subjects practiced walking and running on the motorized and non-motorized treadmills at each of the study speeds (3.2-11.3 km/hr; 2-7 mph). Subjects exercised on the LAB-TM and BD-1 treadmills during one familiarization session and exercised on the TVIS-M and TVIS-NM treadmills during the other session.
During data collection subjects performed two exercise sessions in each of the four treadmill conditions. For each treadmill, the speeds were grouped as 3.2, 6.4, and 9.7 km/hr (2, 4, and 6 mph) for one session and 4.8, 8.0, and 11.3 km/hr (3, 5, and 7 mph) for the other session. Subjects attempted to complete five minutes of exercise at each speed, in order of increasing intensity, to acquire metabolic and cardiovascular measurements.
All but one subject completed five minutes of walking and running at each running speed in the motorized conditions, LAB-TM and TVIS-M. This subject could not complete the entire five minutes at 11.3 km/hr (7 mph). Not all subjects were able to complete walking or running during the non-motorized conditions on the TVIS-NM and BD-1. Subjects were successful at non-motorized treadmill speeds of 3.2 and 4.8 km/hr (2 and 3 mph), but the ability to complete the trials decreased precipitously as the speed increased from 6.4 to 11.3 km/hr (4 to 7 mph).
All subjects completed the five minute walk at 3.2 km/hr (2 mph) in each of the treadmill conditions. Therefore, no comparisons were possible across treadmills for this speed. All subjects completed the five minute walk at 4.8 km/hr (3 mph) in the motorized conditions, and 19 of the 20 subjects completed five minutes of walking during the non-motorized conditions. At 4.8 km/hr (3 mph), there was no significant difference in completion times between the treadmill conditions. At speeds 6.4 km/hr (4 mph) and greater, subjects completed significantly longer exercise periods during the motorized treadmill exercise than during the non-motorized conditions. However, there was no difference in completion times between the LAB-TM and TVIS-M or between the BD-1 and TVIS-NM.
Comprehensive kinematic and kinetic data were collected to determine the differences between treadmill locomotion on motorized and non-motorized treadmills. The results suggest:
- Subjects take shorter and more frequent steps to obtain a given speed on a non-motorized treadmill.
- Subjects will transition to a run at lower speeds on non-motorized treadmills (typically between 4.8 and 6.4 km/hr; 3 and 4 mph) than on motorized treadmills at the same speed.
- Peak ground reaction forces are not different between motorized and non-motorized treadmills. However, distinct impact peaks do not occur during running on non-motorized treadmills, and loading rates were much larger on the motorized treadmills, suggesting that motorized treadmill locomotion may provide a superior osteogenic stimulus.
- The greater forward lean and lack of heel strike during non-motorized locomotion suggest that gait on the non-motorized device should not be considered equivalent to gait on a level motorized treadmill at a similar speed.
- Hip range of motion was greater on the non-motorized treadmills, and knee range of motion was greater on the motorized treadmills, suggesting the emphasis upon different musculature depending on the device.
- Subjects exhibit different joint positions at heel strike, but similar positions at toe-off, suggesting that the physiological effects of locomotive exercise due to impact forces may differ between devices.
- Non-motorized treadmill locomotion on a level treadmill resembles motorized treadmill locomotion on an inclined treadmill.
- Long-term training on a non-motorized treadmill will probably result in different physiological effects than on a motorized treadmill.