Cardiovascular and metabolic responses to electrical stimulation-induced leg exercise in spinal cord injury

Electrical stimulation-induced leg muscle contractions provide a useful model for examining the role of leg muscle neural afferents during low-intensity exercise in persons with spinal cord-injury and their able-bodied cohorts. Eight persons with paraplegia (SCI) and 8 non-disabled subjects (CONTROL) performed passive knee flexion/extension (PAS), electrical stimulation-induced knee flexion/extension (ES) and voluntary knee flexion/extension (VOL) on an isokinetic dynamometer. In CONTROLs, exercise heart rate was significantly increased during ES (94 +/- 6 bpm) and VOL (85 +/- 4 bpm) over PAS (69 +/- 4 bpm), but no changes were observed in SCI individuals. Stroke volume was significantly augmented in SCI during ES (59 +/- 5 ml) compared to PAS (46 +/- 4 ml). The results of this study suggest that, in able-bodied humans, Group III and IV leg muscle afferents contribute to increased cardiac output during exercise primarily via augmented heart rate. In contrast, SCI achieve raised cardiac output during ES leg exercise via increased venous return in the absence of any change in heart rate.