Oxygen uptake and heart rate responses during arm vs combined arm/electrically stimulated leg exercise in people with paraplegia.

The purpose of this study was to compare the oxygen uptake and heart rate responses during submaximal arm cranking to combined arm cranking + electrical stimulation (ES)-induced leg cycling in individuals with spinal cord injury (SCI). Seven subjects with paraplegia (T4-T12) performed combined arm and leg cycling exercise for 5 min, followed by arm cranking alone at the same power output for a further 5 min. During both exercise conditions, steady state oxygen consumption (VO2), carbon dioxide output (VCO2), expired ventilation (VE) and heart rate (HR) were determined. The respiratory exchange ratio (RER) and oxygen pulse were calculated from the measured variables. During combined arm + electrical stimulation-induced leg cycling exercise, the VO2 was 25% higher (1.58 l min-1 vs 1.26 l min-1), but the HR was 13% lower (132 b min-1 vs 149 b min-1), than during arm cranking exercise alone. Oxygen pulse and VCO2 were also significantly higher (by 42% and 25%, respectively) during combined arm + ES-induced leg exercise, but there were no differences between the two exercise conditions for VE or RER. These data suggest that the absence of the leg 'muscle pump' and a reduced venous return of blood to the heart elevate exercise heart rates during submaximal arm cranking. Conversely, combined arm cranking + ES-induced leg cycling exercise provides the body with a greater metabolic stress than arm cranking alone, while reducing the cardiac stress. The mechanism explaining the heart rate response, however, remains unclear, but may have been influenced by the blood pressure variations across the range of lesions. The findings from this study may have implications for the relative benefit of combined arm + ES-induced leg cycling training for people with paraplegia.