A) Recumbent stepping machine with real-time computer-controlled resistance and force and position sensors (modified TRS 4000, NuStep Inc, Ann Arbor, MI). The handles and seat are adjustable. Velcro gloves, foot straps, and a torso belt help minimize unwanted movement. B) The machine provides smooth consistent stepping regardless of subject effort through a custom designed position control of a prescribed sine wave stepping profile. The group mean profile for the actual stepper position (black dashed line) closely follows the target sine wave (thick dark grey line). The thin grey line is the maximum error which occurred during the Active Upper & Active Lower condition. Dotted line is zero.