Cardiovascular responses at the onset of passive leg cycle exercise in paraplegics with spinal cord injury
The purpose of this study was to examine the cardiovascular responses at the onset of passive leg cycle exercise (PLCE) in paraplegics with spinal cord injury (PSCI) to investigate the increase in venous return from the paralyzed lower limbs during PLCE. Six male PSCI having lesions at levels ranging from T8 to L1 and five male able-bodied subjects (ABS) participated in this study. The subjects performed PLCE at pedalling frequencies of 40 rpm for 6 min. Cardiac output (Q˙c), stroke volume (SV) and heart rate (fc) were measured before and during PLCE. In the steady state (4th and 5th min) of PLCE, both PSCI and ABS showed a significant increase in Q˙c. At the onset of PLCE, however, clear differences in the cardiovascular response were found between PSCI and ABS. The ABS showed a rapid and marked increase in fc and consequently Q˙c within 20 s of the onset of PLCE. On the other hand, in PSCI, the Q˙c increased more slowly, compared with that in ABS, because of a smaller increase in fc and a delayed increase in SV. The observed delay in the increases of Q˙c and SV at the onset of PLCE in PSCI was presumably due to the absence of afferent reflexes from the lower limbs, and to the additional time needed for venous return to arrive at the heart from the passively moved muscles.