The ground reaction force occurring between the foot and ground is equivalent to the resultant force applied to the body's center of mass. Since force is the product of mass and acceleration, an increase in body mass with a given acceleration should result in an increase in ground reaction force. The loading mechanisms used on the ISS to date, appear to be limited in allowing astronauts to achieve normal-gravity-like levels of peak ground reaction force. Thus, other approaches may be necessary to effectively create normal-gravity-like ground reaction force using the current treadmill onboard the ISS. Adding mass to the astronaut could result in an increased ground reaction force. The addition of mass to the subject, however, may also affect the kinematics, kinetics, and adaptations in normal locomotion.
Increasing the mass of the astronaut through a weighted vest offers a potentially economical and easily modifiable enhancement to current exercise countermeasures. The use of a harness capable of providing different levels of mass could save resources by reducing the need to develop new exercise devices. More importantly, the health and well-being of astronauts could be improved efficiently and inexpensively. Furthermore, exercise with body weight support and a weighted vest to increase overall mass may be useful during the rehabilitation process for individuals who are at risk for osteoporosis.
The goal of this study is to determine whether or not increasing subject mass can create more effective exercise countermeasures during long-term space flight and to better understand the motor accommodations made to gait. The primary purpose of this investigation is to determine how the manipulation of mass affects the ground reaction force during treadmill locomotion. The secondary purpose is to examine the kinetic adaptations to increased body mass during locomotion.
Vertical ground reaction force data were collected during the testing trials with a force-measuring treadmill (Kistler Gaitway, Amherst, NY) at 480 Hz. The treadmill was equipped with two force plates beneath the running tread arranged so one plate rested in the front and one in the rear of the locomotion area. Each plate contained four piezoelectric load cells that measure vertical ground reaction force and allow for a determination of the center of pressure during each sample.
Three-dimensional position data from reflective markers placed upon the subject were collected at 60 Hz with eight cameras (Smart Elite motion capture system, BTS Bioengineering Spa, Milanese, IT). Prior to each day of data collection, the motion capture system was calibrated to within 0.44 ± 0.03 mm of marker re-prediction accuracy. All three-dimensional data were expressed relative to an inertial reference frame that was established during calibration. A reference trial was collected after calibration but before the subject arrived at the lab to establish a treadmill reference frame. An electronic pulse was output by the force treadmill upon the initiation of data collection. The signal was recorded by the motion capture system and was used to synchronize the data during post processing.
Mass was added to each subject using a weighted exercise vest (X-Vest, Perform Better, Cranston, RI). The vest was worn over the shoulders and had top and bottom pockets on the front and back of the subject. Each pocket was fitted with slots in which up to twenty individual 1.1 kg masses could be placed. Slots for weight placement were located on the inside and outside inner surface of each pocket (10 on either surface). During AM (morning) trials, masses were added equally to the front and rear of the vest. The masses were always added to the inner-lower slots first, followed by inner-upper, outer-lower, and outer-upper slots. Within each pocket, weights were added to the center of each row of slots first, and then fanned outwards. Body weight was maintained with an overhead un-weighting system (H/P/Cosmos Airwalk, Nussdorf, Germany). The system provided a constant upward force via a pneumatic pump. The subjects wore a harness about their waist and thighs that were provided by the un-weighting system manufacturer.
Data were collected during five mass treatments at two speeds. Subjects walked at 1.34 km/hr (3 mph) and ran at 3.13 km/hr (7 mph). In addition to a control condition with no added mass (0% AM), mass was added while body weight was maintained. Added mass (AM) conditions included, 10% AM, 20% AM, 30% AM, and 40% AM. At each AM condition, subjects had their weight relieved with an unloading system so the net force between the subject and treadmill remained equal to 100% body weight during quiet standing. All trials at each speed were completed during a single data collection session for each subject. Prior to actual data collection, each subject participated in a familiarization session during which they had the opportunity to practice walking and running at each speed and treatment condition.
Subjects completed all added mass treatments at one speed before completing treatments at the other speed. The speed order was randomized for each subject by a coin flip prior to their first testing session. Treatment randomization occurred independently for each speed. In order to assure that there was a balance of increased mass conditions between subjects, a balanced Latin square random assignment was used. The design allowed for a balance of treatment orders so that no two testing sequences were the same for different subjects within each speed. Each subject was randomly assigned a sequence with only one subject completing each specific order. Trial order assignment occurred separately for each speed. The subjects wore the un-weighting harness during all conditions, including the control trial.
Summary of Results During Walking
During walking, stride time increased as mass was added. Adding mass increased impact forces and loading rates. Peak propulsive forces decreased with additional mass, but impulse was not affected. Angular impulse at the hip during the stance phase was affected by the addition of mass during walking. Stance phase hip extension torques increased as mass was added, with the 30% AM condition having the largest angular impulse. Positive work at the hip increased and negative work decreased. Negative work at the ankle joint also decreased as mass was added. During the swing phase, knee extension torques decreased with the addition of mass. There were no differences at any joint in positive or negative work during the swing phase.
Summary of Results During Running
During running, contact time and stride time increased as mass was added. Peak impact forces and loading rates decreased, and impulse increased as mass was added. There were no effects of additional mass upon peak propulsive forces. During the stance phase, hip extension angular impulse and knee flexion impulse increased with additional mass. Positive work at the hip increased and negative work decreased. During the swing phase, hip extension and flexion angular impulse decreased as mass was added. Knee extension and ankle plantar flexion impulses also decreased as mass was added. Negative work at the knee decreased; otherwise, there were no added mass effects on any of the other work dependent variables.
The specific purpose of this investigation was to determine if adding mass to astronauts during locomotive exercise in microgravity would be beneficial to increasing the forces experienced by the musculoskeletal system. In both locomotive modes, hip musculature activity and positive work increased suggesting that the hip is the primary area of adaptation of joint torque. It is possible that increasing mass during locomotive exercise in microgravity may be beneficial to increasing impact forces and hip extensor activity during walking. During running, there may be adaptations in locomotive patterns to minimize increases in ground reaction forces.
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