Effects of early and delayed initiation of exercise training on cardiac and haemodynamic function after spinal cord injury.

NEW FINDINGS: What is the central question of this study? How does early versus delayed initiation of passive hindlimb cycling, as well as detraining, affect cardiac function and blood pressure control in a rodent model of spinal cord injury? What is the main finding and its importance? Early or delayed initiation of hindlimb cycling improves cardiac and haemodynamic function in spinal cord injury, although the benefits of early administration are more pronounced. We also demonstrate the need for exercise to be maintained, because detraining rapidly reverses the cardiac and haemodynamic benefits. Spinal cord injury (SCI) reduces physical activity and alters descending supraspinal cardiovascular control, predisposing this population to early onset of cardiovascular disease. We used a T3 SCI rodent model to investigate the effect of early versus delayed passive hindlimb cycling (PHLC), as well as the effect of detraining on cardiac dysfunction and blood pressure control, including autonomic dysreflexia (AD). Twenty male Wistar rats were divided into an early PHLC initiation group followed by a period of detraining (PHLC starting day 6 post-SCI, for 4 weeks, followed by 4 weeks of detraining; 'early PHLC/detraining' group) and a delayed PHLC intervention group (no PHLC for first 5 weeks post-SCI, followed by PHLC for 4 weeks; 'no PHLC/delayed PHLC' group). At 5 weeks post-SCI, the no PHLC/delayed PHLC group exhibited a decline in almost all cardiac indices (all P < 0.029), which was maintained in the early PHLC/detraining group. Also, the severity of induced AD was reduced in the early PHLC/detraining versus no PHLC/delayed PHLC group (all P < 0.0279). At 9 weeks post-SCI, no PHLC/delayed PHLC animals exhibited a reversal of cardiac dysfunction such that all indices were not different from pre-SCI, whereas early PHLC/detraining rats exhibited a reduction in all cardiac indices relative to pre-SCI (all P < 0.049), except ejection fraction. Between weeks 7 and 9 post-SCI, the no PHLC/delayed PHLC rats exhibited fewer spontaneous AD events than the early PHLC/detraining rats (P < 0.01). We show, for the first time, that delayed exercise promotes similar improvements in cardiac and haemodynamic function to those observed with early initiation. Furthermore, exercise needs to be maintained, because detraining reduces these cardiohaemodynamic benefits.