Fifteen persons with complete SCI at levels T4 and T11 volunteered to participate in the study. None of the participants had a history of cardiopulmonary disease or shoulder dysfunction and none were taking any medications that may affect the results of the study. The participants had completed at least 5 months of FES ambulation training prior to participation. During peak AC testing, participants cranked a Monark 881 arm ergometer at increasing power output until they reached volitional exhaustion (measured with a Borg 6- to 20-point Rating of Perceived Exertion [RPE] scale) or until they could no longer maintain appropriate power output. Open-circuit spirometry was used to measure metabolic activity. Heart rate was monitored by a 12-lead electrocardiogram (ECG). The incremental peak ambulation tests used a Parastep-1 commercial FES system, in which an adapted walker frame is attached via a cable to a battery-powered stimulator, which delivers transcutaneous electrical stimulation to the quadriceps and gluteal/spinal erector muscles. Standby assistance was provided at all times by one or more researchers. Open-circuit spirometry was again used to measure metabolic activity, but use of an ECG device was not possible during FES testing because of electrical interference. Heart rate was monitored manually by a registered physical therapist who had demonstrated a level of precision compared with electrocardiography during AC testing (r2 > 0.91). Participants stepped along a 10-meter walkway beginning at a "very slow, controlled pace" and stopping at the end of the walkway to point to a 6- to 20- point RPE scale. Subsequent trips down the walkway were performed at a "slightly faster pace" until the participants reached volitional exhaustion or could no longer increase the pace of controlled FES ambulation.