Physiologic responses to prolonged electrically stimulated leg-cycle exercise in the spinal cord injured

This study determined the physiologic responses to prolonged functional neuromuscular stimulation (FNS) leg-cycle exercise in seven quadriplegic and seven paraplegic subjects. Each subject completed 30 minutes of continuous FNS leg cycling during which open-circuit spirometry, impedance cardiography, auscultation, and fingertip capillary blood sampling were used to assess metabolic and hemodynamic responses. Compared with resting values, oxygen uptake, carbon dioxide production, respiratory exchange ratio (RER), pulmonary ventilation, heart rate (HR), left ventricular stroke volume (SV), cardiac output (Q̇t), and blood lactate (La) concentration were significantly (p <.05) elevated, whereas plasma volume, bicarbonate concentration, and pH were significantly decreased in both groups during prolonged FNS leg-cycle exercise. Mean arterial pressure remained unchanged in quadriplegic and paraplegic subjects during the prolonged FNS leg-cycle exercise bout. Persons with quadriplegia elicited significantly lower MAP and tended to have lower SV and Q̇t responses than persons with paraplegia, probably due to a higher degree of sympathetic dysfunction and circulatory hypokinesis during FNS leg-cycle exercise. All other physiologic variables responded similarly between groups. We speculate that the relative increases observed for HR (33% to 60%), SV (45% to 69%), and Q̇t (113% to 142%) during prolonged FNS leg-cycle exercise create a sufficient cardiac-volume load to promote central cardiovascular conditioning in persons with both quadriplegia and paraplegia. The La accumulation (4.7 to 5.2mmol·L^-1) in the spinal cord injured during prolonged FNS leg cycling is unusually high for the power output attained (5.2W and 6.1W for quadriplegia and paraplegia, respectively). However, the significant decline in the RER from minute 5 to minute 30 suggests a gradual preference for lipid substrate by exercising muscle as FNS leg cycling continued. Despite impaired sympathetic nervous system function resulting from spinal cord injury and the nonphysiologic manner of FNS muscle contraction, the absolute levels of metabolic and hemodynamic responses achieved indicate that prolonged FNS leg cycling is a relatively safe mode of exercise training for persons with quadriplegia and paraplegia.