Serres JL, Phillips CA, Reynolds DB, Mohler SR, Rogers DB, Repperger DW, Gerschutz MJ. Pneumatic muscle actuator for resistive exercise in microgravity: test with a leg model. Aviat Space Environ Med 2010; 81:144–8. Introduction: A proof-of-concept demonstration is described in which a DC servomotor (simulating the quadriceps of a human operator) rotated a pulley 90° (simulating knee extension). A pneumatic muscle actuator (PMA) generated an opposing force (antagonist) to the rotating pulley. One application of such a device is for use in microgravity environments because the PMA is compact, simple, and of relatively small mass (283 g). In addition, the operator can set a computer-controlled force-level range in response to individual user changes in exercise conditioning over time. Methods: A PMA was used in this study and interacted with a DC servomotor. For each trial, the PMA contracted in response to internal pressure. An input voltage profile activated the DC servomotor, resulting in the following three phases: an isokinetic counterclockwise pulley rotation of 90° over 5 s (Phase I), the position was held for 5 s (Phase II), and an isokinetic clockwise rotation of 90° over 5 s (Phase III). Root mean square error (RMSE) values were used to evaluate the pulley rotation. Results: For Phase I, when the PMA pressures (in kPa) were 300, 450, and 575, the percent RMSE, respectively, were 5.24, 6.23, and 4.59. For Phase II, the percent RMSE were 2.81, 2.57, and 5.63, respectively. For Phase III, the percent RMSE were 5.69, 2.63, and 3.30, respectively. Discussion: This study presents a demonstration of a PMA device that can enhance exercise by providing a wide range of resistive loads.