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Objectives: The objective of this study was to compare acute cardiovascular responses to maximal and submaximal arm crank exercise alone and in combination with static and dynamic functional electric stimulation (FES) in individuals with spinal cord injury (SCI).
Clinical Relevance: While a slight increment in metabolic activity might have been expected these results do not indicate a measurable contribution from FES. This should not be taken as a negative indicator of the long term benefits of hybrid exercise. Cardiovascular and pulmonary adaptations that are not seen with acute exercise bouts are nonetheless expected to develop as the lower extremities adapt to regular use of FES. Currently, we are investigating the benefits of static and dynamic hybrid exercise in a 12 week training study. These results will allow us to evaluate the potential for hybrid exercise to modify heart disease risk factors and will be applicable toward informed cost-benefit decisions for implementation of exercise training programs for individuals with SCI.
Methods: Eight males with tetraplegia and six males with paraplegia participated in this study. All subjects had at least minimal spasticity and all subjects' lower extremity musculature responded to FES. All subjects performed submaximal and graded maximal arm crank exercise tests in three different exercise protocols. These were: (1) arm crank exercise alone (ACE); (2) arm cranking combined with FES applied simultaneously to opposing muscle groups of the lower limbs to create isometric co- contractions (Static Hybrid); (3) arm cranking combined with FES applied to leg muscles to produce cycling (Dynamic Hybrid). Cardiorespiratory responses including heart rate, VO2, VE, RER, and submaximal cardiac output were measured. Differences in maximum heart rate, peak VO2, VE, RER, and submaximal cardiac output across exercise protocols were measured using a repeated measures ANOVA.
Results: No differences were found between arm cranking alone, static hybrid and dynamic hybrid in peak V02 (12.8(5.4, 13.1(4.7, and 13.7(5.1 ml/kg/min for ACE, static hybrid, and dynamic hybrid respectively, p=0.79). Likewise, there were no differences found in maximum heart rate between the three exercise protocols (136.4(32.5, 134.6(27.4, 147.7(26.4 bpm, p=0.69). Submaximal steady state cardiac output testing was also similar between protocols (13.3(2.1, 13.2(2.6, 12.5(2.3 L/min, p=0.62). Further, no differences were found in RER and VE between protocols.
Conclusions: While there is evidence from training studies of increased cardiovascular responses to static and dynamic hybrid exercise compared with ACE alone, our results demonstrate that differences do not exist in cardiovascular responses to these three exercise protocols acutely. Our findings indicate that the contribution from untrained lower extremity musculature is insufficient to alter cardiovascular responses during acute hybrid exercise.
Acknowledgments: VA Rehabilitation R&D Merit Review Grant #B2110-RA.