|| The degree to which the musculoskeletal system will maintain its integrity during prolonged sojourns in the reduced gravity of the lunar surface is presently unknown. It is, however, likely that without countermeasures there will be adaptive changes in muscle strength, bone mineral density, bone geometry, and sensorimotor status. When the combined effects of these changes are considered in the context of the construction and exploration tasks that will be performed at the lunar base or at other lunar sites, the risk of injury secondary to a fall is likely to be elevated. To address this problem, we constructed a compact platform that integrates a time efficient integrated battery of countermeasures that can be conducted in the confines of the lunar habitat to minimize the risk of musculoskeletal injury. Ultimately, we expect that this battery of countermeasures will be validated using a 10 head-up bedrest simulation of a lunar mission, although it could also be tested in the standard 6 degree head down simulation.
The specific objectives of the countermeasure battery are: to preserve muscle strength and cardiovascular fitness; to minimize decrements in postural stability, dynamic balance, and the ability to make corrective actions prior to a fall; to preserve functional performance on mission relevant tasks; and to minimize bone loss in the proximal femur. To accomplish these objectives, we have constructed a unique multi-functional countermeasure device which integrates cardiovascular, balance control, and resistance training functions. The stepper system provides cardiovascular exercise. When the stepper is locked down, the device may be utilized for lower body strengthening exercises such as squats, leg extensions, and abductor/adductor exercises. For balance training, the stepper/resistive system is mounted on a Stuart Platform allowing 3D translations with a range of +/- 10 cm and pitch/yaw/roll of +/- 10 degrees.
In the second and third years of the study, based on a request from the Human Research Program, we rescoped the project to carry out a training study in which we have evaluated the ability of combined countermeasure device (CCD) exercise to generate improvements in cardiovascular function and lower body resistive strength. 15 subjects underwent a 12 week training study which involved three weekly one hour sessions of cardiovascular and lower body resistive training. The cardiovascular training initially involved stepper exercise (5 subjects, 5% mean 12 week improvement in VO2max, non-significant change), but based on poor results we changed the aerobic protocol to bike exercise (10 subjects, 27% mean 12 week improvement in VO2max, p=0.004), following a design simulation illustrating that a compact exercise bike could be folded into the footprint of the CCD. The 10 subjects exercised on the bike showed improvements ranging from 17%-38%. Leg press strength increased in all 15 subjects over 12 weeks (mean change 68%, range 47-85%, p=0.0001). Isokinetic strength measures showed variable response, with hip abduction, adduction, and ankle plantarflexion strength increasing by 22%, 31%, and 13%, respectively (all p<0.05), but leg extension, leg flexion, and hip flexion strength showed non-significant increases. Weight lifted by all subjects in each exercise increased significantly (all exercises p=0.0001). Thus, we were able to conclude that CCD exercise was well tolerated, and could produce significant improvements in physical fitness, thus achieving the goal of the training study.
Because a project goal is to develop an exercise protocol in which squatting and hip ab/adduction exercise protect against hip bone loss, Dr. Cavanagh's group adapted the Lifemodeler computational tool to simulate the effect of the muscle contractions produced by CCD squatting and ab/adduction exercise. LifeModeler incorporates contractions of 47 muscles in the leg, and fully models all of the CCD exercise. To validate, Drs. Cavanagh and Hanson used the Orthoload Database, which contains results from studies of volunteers who received hip prostheses instrumented with strain sensors, allowing for calculation of hip loading forces associated with different exercises, including abduction and squatting. Simulating the exercise protocols used in the Orthoload Study, the Lifemodeler calculations produced hip loads that were in quantitative agreement with the measured Orthoload results. These calculations showed that 1g CCD abduction exercise produced peak forces of 4 body weights on the hip, compared to 2.5 body weights for squatting. On June 4, we presented our training study and Lifemodeler work to the Human Health Countermeasures (HHC) Control Board. Based on the heavy load on the Bedrest Facility, the CCD was not placed into the bedrest study, but followup on our ab/adduction results were considered highly exciting. Based on this, we carried out a study to evaluate the effects of ab/adduction exercise on hip bone strength and density measured by quantitative computed tomography and finite element modeling. We compared standard Advanced Resistive Exercise Device (aRED) lower body exercise, combined aRED, and ab/adduction and ab/adduction only, maintaining the same number of repetitions per group, in a 16 week study, with three exercise sessions per week. Eight subjects were assigned to each group. aRED exercise resulted in increased spine and femoral neck bone density, as well as increased hip strength by FEM in the stance loading condition. Ab/Add exercise showed increased cortical bone volume at the trochanter and a borderline insignificant increase of hip strength by FEM in the fall loading condition. The combined group showed no changes. Thus, while AbAdd exercise appears to have a modest osteogenic effect, any application of this approach for prevention of hip bone loss would require additional exercise. Given the modest osteogenic effect we observed, the value of Ab/Adductor exercise may rest more in preservation of functional mobility and fall protection rather than bone loss prevention.