Automated testing of the controlled resistance exercise device (C-red) for spaceflight

Resistance exercise is an effective countermeasure to the muscle and bone atrophy associated with the unloading experienced during spaceflight. Accordingly, the Controlled Resistance Exercise Device (C-RED) developed for NASA uses an electric motor for resistance and capitalizes upon a two-to-one mechanical advantage by loading only one side of the barbell. While this allows for a reduction in the required motor size and weight, the ground reaction force from previous human subject studies has shown higher loads on the motor side, which could be accentuated by the person’s unilateral strength bias or lifting technique. Therefore, an automated testing system was developed to test the bilateral symmetry of only the device without human bias. In addition, the effect of the winching material (cord or strap) on the bilateral symmetry of loading was explored. Findings show that cord leads to a more symmetric loading but is prone to fraying and subsequent breaking below the specified strength. Straps have higher strength and wear resistance properties but lead to slightly less symmetric loading outcomes. In general, the C-RED retains some asymmetric loading regardless of winching material; however the marginally higher bilateral asymmetry of the strap is offset by the gains in strength and wear resistance.