Aerobic training effects of electrically induced lower extremity exercises in spinal cord injured people

Eleven people with spinal cord injury (SCI) (C4-T6) participated in a program of functional electric stimulation (FES) of their paralyzed leg muscles using the REGYS I system. Individualized protocols consisted of an initial phase of weight lifting, an intermediate phase of ergometer pedalling against 0 Kilopond (kp) alternated with 1/8kp for six two-minute runs separated by two-minute rest periods, and a final phase of 36 sessions of continuous ergometer pedalling against variable resistance. A metabolic analyzer measured exercise stress test parameters before and after each phase while subjects pedalled against incremented resistance. Peak oxygen consumption and total stress test time increased markedly. The respiratory exchange ratio (R=VCO2/VO2) at termination, however, did not differ from unity at any phase, indicating that fatigue (defined as a failure to maintain a pedalling frequency of 35 rpm) occurred when the anaerobic threshold was reached, and that FES exercise can increase the aerobic capacity of persons with SCI. The initial velocity of quadriceps shortening (derived from patellar tendon displacements) also decreased in five of eight subjects tested, suggesting corresponding increases in quadriceps twitch time. Since muscle inactivity converts slow-twitch to fast-twitch fibers, our subjects' increased muscle endurance accompanied by decreased muscle-shortening velocity were compatible with a disproportionate increase in the function of slow-twitch fibers relative to fast-twitch fibers. Although these findings demonstrate that lower extremity FES exercises can safely achieve significant aerobic training effects in patients with SCI, the peak levels of cardiorespiratory performance were similar to those reported for quadriplegic people performing maximal voluntary upper extremity exercises.