As a countermeasure to bone and muscle loss, astronauts on the International Space Station exercise using three devices; a treadmill, bicycle and resistance training device. However, as new devices are designed it is important to know the levels of load these devices place on the lower extremities and how the load changes with various settings. Through ground-based simulations, Dr. Peter R. Cavanagh and NASA Glenn Research Center colleagues are working to establish the devices loading characteristics in order to be better able to direct device design and to develop exercise prescriptions. To aid the study, the group will enhance the Zero Gravity Locomotion Simulator at The Cleveland Clinic to provide more flight-like simulation of exercise. Using this device, they will work to define the range of loading possibilities in cycling, treadmill running, resistance and jumping exercise. The results will provide clear guidelines for the prescription of inflight exercise.
Foot Reaction Forces During Simulated ISS Exercise Countermeasures
Peter R. Cavanagh, Ph.D., D.Sc.
The Cleveland Clinic
- To improve the biomechanical validity of the human supine suspension approach to microgravity simulation and to develop hardware and software that will characterize the force magnitude (F), rate of change of force (dF/dt) and vibratory and transmission characteristics of load transferred to the human body.
- To fully characterize lower-extremity loading during high-fidelity simulated 0 gravity exercises on various exercise devices throughout the range of subject load device (SLD) and harness settings.
- To propose a regimen of exercise on the available devices that will be effective in maintaining bone and muscle mass.
- Harness and Subject Load Device.
Locomotor exercise in reduced gravity requires a harness attached to a SLD to return the subject to the treadmill surface. The project team and its collaborators have developed enhanced versions of both these devices. A new pneumatic Subject Load Device (pSLD) has been shown to reduce variability in the applied load to within seven percent at full bodyweight loading. This is superior to all other forms of SLD that we have previously examined. It has also been shown to be reliable in more than 100 hours of subject testing. The Cleveland Clinic (CC) prototype harness for use with exercise countermeasures has been developed and fabricated. Visual analog scale (VAS) comparisons with a replica of the NASA treadmill with vibration isolation stabilization (TVIS) harness during short-duration trials indicated less discomfort in the shoulders using the CC harness. Overall discomfort ratings were less than or equal to the TVIS harness depending on the loading conditions. During long-duration trials the relative overall discomfort was less with the CC harness than the TVIS harness. The success of this development and the harness comfort studies have led to the advancement of this device towards a Detailed Science Objective (DSO) which is currently scheduled for a flight experiment by our colleagues at NASA Glenn Research Center and ZIN Technologies.
- The Enhanced Daily Load Stimulus Model.
The project team has modified existing mathematical theories on daily load stimulus (DLS) to develop a comprehensive algorithm, accounting for new theories on saturation and recovery of the osteogenic potential of bone with distributed mechanical loading and the stimulus provided to the bone through standing. We believe that the new algorithm, the enhanced DLS (eDLS), will provide a better model of the relationship of bone mineral density to daily loading and are currently testing it as a method of prescribing an exercise dose to subjects in an ongoing bed-rest study. We have validated this model against bed-rest data and have submitted it to the journal Aviation, Space and Environmental Medicine.
- Stand-Alone Zero Gravity Locomotion Simulator (sZLS).
Our collaborators at NASA Glenn Research Center have leveraged their experience gained on the development of the enhanced ZLS (eZLS) into a new device, the Stand-Alone Zero Gravity Locomotion Simulator (sZLS) which has been delivered to the NASA Flight Analogs Facility at the University of Texas Medical Branch in Galveston, Texas.
- Ground Reaction Forces with a Compliant Interface.
A computational model of the eZLS environment was created in Simulink to understand system dynamics and their effect on foot reaction forces between the crew member and the exercise countermeasure device. The model simulated rigid body dynamics of the treadmill and its supporting frame or rack as the simulated crew member foot forces excite the dynamics of the isolator components. The model predicted that with a vibration isolation system installed on the treadmill the peak reaction force on the foot could drop by up to 14 percent compared to a grounded system. This finding is significant because astronaut crew exercise prescriptions are formulated based on a grounded system and the on-orbit exercise systems used by astronauts are usually equipped with a vibration isolation system. The implication is that astronauts are not stimulating the musculoskeletal system as much as desired, and we believe this could be a contributing factor in the persistence of bone loss on long-duration space missions despite rigorous exercise. In a subsequent experiment with human subjects, the measured peak ground reaction forces during running were not found to show a major dependence on treadmill compliance within the bounds of the compliance variation used in the study. This finding is unexpected and does not agree with the results of the simulation. The authors are currently refining the model to consider an energy analysis similar to that used in shock analysis.
- New Exercise Devices.
Two new small footprints, low-power exercise devices have been designed, fabricated and tested. Both of these devices (the Impacting Cycle Ergometer ICE and the Impacting Stepper Device ISD) provide a calibrated impact to the foot in order to enhance the loading profile. Based on subject feedback, measured heart rate and foot force measurement, the ISD appears worth further investigation.
These findings provide new opportunities for optimizing in-flight exercise by providing:
- A harness and subject load device that will allow greater, and more consistent, loading than was previously possible;
- The enhanced daily load stimulus model for more accurate matching of individual cumulative loads on Earth with mission exercise loads;
- Experimental testbeds (the eZLS and sZLS) that are available for accurate ground-based simulations of exercise in altered gravity environments;
- Initial insights in the model-predicted reductions that may occur on exercise devices with compliant interfaces; and
- New small footprint, low-power exercise devices for use in long-duration missions.