At 40% V˙O2peak, oxygen uptake, HR, cardiac output, and arteriovenous O2 differences were significantly lower during FES-LCE than for all the other exercise modalities. ACE elicited 70% greater V˙O2 than FES-LCE, and ACE + FES-LCE and HYBRID elicited 99% and 122% greater V˙O2 than FES-LCE, respectively. ACE evoked a 42% higher HR than FES-LCE, and ACE + FES-LCE and HYBRID elicited 33% and 55% higher HR, respectively, compared with FES-LCE. Q was higher by 31% during ACE and ACE + FES-LCE and 46% greater during HYBRID compared with legs-only exercise. Comparing arm and leg exercise to arms alone, ACE + FES-LCE elicited 17% higher V˙O2, and HYBRID exercise elicited 30% greater V˙O2.
At 60% V˙O2peak, oxygen uptake, HR, cardiac output and arteriovenous O2 differences were significantly lower during FES-LCE than for all the other exercise modalities. Oxygen uptake was also significantly higher during ACE + FES-LCE and HYBRID than ACE alone. ACE elicited 82% higher V˙O2 than FES-LCE, and ACE + FES-LCE and HYBRID elicited 122% and 148% higher V˙O2 than FES-LCE, respectively. ACE evoked a 19% higher HR than FES-LCE, and ACE + FES-LCE and HYBRID elicited 23% and 26% higher HR than legs-only exercise. Q was higher by 36% during ACE and greater by 40% during ACE + FES-LCE and HYBRID. Comparing arm and leg exercise to arms alone, ACE + FES-LCE elicited 22% higher V˙O2 and HYBRID exercise elicited 36% greater V˙O2.
At 80% V˙O2peak, oxygen uptake, HR, and cardiac output were significantly lower during FES-LCE than for all the other exercise modalities. Oxygen uptake was also significantly higher during ACE + FES-LCE and HYBRID than ACE alone. ACE elicited 94% higher V˙O2 than FES-LCE, and ACE + FES-LCE and HYBRID evoked 135% and 132% higher V˙O2 than FES-LCE, respectively. ACE resulted in 47% higher HR than FES-LCE, and ACE + FES-LCE and HYBRID evoked 56% and 43% higher HR than legs-only exercise. Q was greater by 33% during ACE and 49% during ACE + FES-LCE and 47% during HYBRID exercise. Comparing arm and leg exercise to arms alone, ACE + FES-LCE elicited 21% higher V˙O2, and the HYBRID exercise elicited 19% higher V˙O2. ACE + FES-LCE elicited 16% higher Q, and the HYBRID exercise elicited 10% higher Q. ACE + FES-LCE evoked a 6% higher HR, but the HYBRID exercise did not evoke a higher HR response.
There were no significant differences in SV among any exercise modality from 40% to 80% of mode-specific V˙O2peak. However, we observed an 8.3% increase in SV compared with ACE at 40% exercise intensity during FES-LCE and a 13.3% increase in SV compared with ACE at 80% V˙O2peak.
This study compared the acute cardiorespiratory responses during maximal exercise in people with SCI performing four types of exercise involving arm and legs: ACE, FES-LCE, ACE + FES-LCE (two separate pieces of equipment used concurrently), and a commercially available arm and leg hybrid FES tricycle. On the basis of the peak exercise responses in the maximal exercise testing, we then compared the metabolic and cardiovascular responses during submaximal exercise at 40%, 60%, and 80% of mode-specific V˙O2peak in all four exercises.
Cardiorespiratory responses during maximal exercise
The results from this study demonstrated lower oxygen uptakes and HR during FES-LCE compared with ACE or arm and leg exercise (ACE + FES-LCE and HYBRID). This finding agreed with previous studies that have shown lower peak oxygen uptakes during FES leg cycling than other type of exercise (24,30,34). A very early study conducted in the 1980s suggested that ACE alone might be less effective than lower limb exercise for health and fitness promotion in the SCI population due to the relatively small muscle mass in the upper limbs resulting in lower SV and cardiac outputs (10). The current investigation highlighted that leg exercise alone is not always superior to arm effort, even when the muscle mass of the legs exceeds that at the arms in SCI individual. Indeed, just because the paralyzed leg musculature can be artificially activated by FES is not evidence that the metabolism is markedly elevated sufficiently to promote enhanced cardiorespiratory fitness. The combination of ACE and FES-LCE, termed “FES hybrid” exercise, has shown significantly higher peak oxygen uptake, HR, cardiac output, and SV than arm-only or legs-only exercise (22,24,30). Findings from the current study revealed 14%–18% higher peak oxygen uptake during maximal hybrid exercise compared with arm exercise alone. This was likely due to the recruitment of a larger muscle mass with the addition of lower limb FES-evoked cycling to arm exercise. Our findings agreed with Verellen et al. (34) in confirming a significantly lower V˙O2peak attained during FES cycling, compared with ACE or FES hybrid exercises (arm + leg cycling and rowing), without much apparent difference between the latter two.
In this study, FES-LCE did not result in the attainment of “centrally limited” maximal HR because the highest HR observed was at the time when the electrically stimulated muscles had become fatigued. Consistent with previous studies (21,24,30), we did not observe any differences of peak HR responses between ACE and ACE + FES-LCE or HYBRID. These findings contrasted with those of Hooker et al. (14), who observed exercise HR during ACE + FES-LCE to be significantly higher than ACE alone. These differences may be explained by a different subject population because Hooker et al. investigated responses in tetraplegic subjects whereby an increase in HR during exercise was driven by predominantly parasympathetic withdrawal (14). This is in comparison with the current study, where participants were either paraplegics with spinal lesions below T4 or possessed “incomplete” spinal lesions (ASIA B or C). Raymond et al. (31) have proposed that FES-LCE lacks a “central command” component of leg exercise and also lacks complete skeletal muscle afferent feedback due to the spinal cord lesion. Thus, the underlying mechanisms for sympathetically induced exercise cardioacceleration driving such exercise would be blunted or lacking, resulting in the low peak HR observed herein.
The RER values in the current study were all higher than 1.10, indicating maximal effort. However, despite achieving maximal mode-specific effort, the lactate concentration was significantly higher after hybrid exercise compared with ACE alone or FES-LCE. Clearly, the larger muscle mass engendered by arm plus leg exercise and possibly improved circulation at a maximal intensity, resulting in higher lactate production than by arms or legs alone.
Cardiorespiratory responses during submaximal exercise
It is useful to investigate submaximal cardiorespiratory exercise responses because these represent an intensity that can be sustained over prolonged periods of time, and which might represent real-world utility to the SCI individual undertaking fitness training using arms or legs.
During submaximal exercise, the PO was predetermined based on the results from maximal exercise assessments (i.e., the corresponding mode-specific workload at 40%, 60%, and 80% of POpeak). Interestingly, we observed that the V˙O2 achieved at the different submaximal intensities performed at the predetermined PO were higher than the predicted V˙O2 for those intensities. This was attributed to the exercise protocol, whereby the incremental workload (for the given exercise intensity) was ramped up within the first 3 min of exercise before steady state, as compared with the gradual increment for 8–12 min during the maximal effort tests. The sudden increase in dynamic exercise had possibly resulted in the quick rise in oxygen uptake (6) as documented in this study.
In a similar way to maximal exercise, the submaximal V˙O2 during FES-LCE was significantly lower than all other exercise modalities from 40% to 80% V˙O2peak. Further analysis revealed that there were also significant differences in the oxygen uptake between both types of arm and leg exercise compared with arm cranking alone at the highest exercise intensity (i.e., 80% V˙O2peak). During steady-state exercise within the 40%–80% V˙O2peak range, ACE elicited up to 90%, the ACE + FES-LCE up to 135%, and the hybrid bike up to 150% higher V˙O2 than FES-LCE. The ACE + FES-LCE elicited up to 20% and the hybrid bike up to 40% higher V˙O2 than ACE. These findings agreed with earlier studies that examined cardiorespiratory responses during FES hybrid exercise (1,24,34). The addition of arm exercise to FES-LCE clearly elicits a greater whole-body oxygen uptake, supporting the view that hybrid exercise promotes better aerobic fitness potential.
This study also suggested that FES-LCE produced a larger submaximal SV compared with ACE, ACE + FES-LCE, or HYBRID by 3%–13%. This finding, however, did not achieve statistical significance, although it was obvious by visual inspection of the data (Fig. 2). Davis et al. (4) and Raymond et al. (30) demonstrated significant increases of SV when FES leg exercise was superimposed on ACE. Raymond et al. (30) attributed this to an augmented venous return rather than increased sympathetic neural drive augmenting cardiac contractility because there was no simultaneous increase of HR during FES leg cycling.
In the current study, the HR responses during steady state were significantly lower during FES-LCE across all exercise intensities compared with the other modes of exercise. In addition, there was no significant difference of steady-state HR between ACE and the combined arm and leg exercise modes. Only two previous studies that investigated HR response during arms exercise, FES leg exercise, and hybrid exercise have suggested a lack of difference in steady-state HR between ACE and hybrid exercise (4,29). Interestingly, in one of these, Raymond et al. (29) noted significantly lower HR responses during combined arm and leg exercise compared with arm cranking exercise alone, and they concluded that combined arm and leg exercise reduced cardiac stress for a given oxygen uptake. In contrast, Hooker et al. (14) observed that hybrid exercise elicited significantly higher HR (up to 33%) compared with ACE or FES-LCE. In that early study, the authors investigated tetraplegic subjects and attributed their findings to a diminished vagal tone in the absence of sympathetic-evoked cardioacceleration.
Cardiac output (Q) during FES-LCE was significantly lower than all other exercise modalities, across the range of effort intensities. There was no significant difference, however, in the Q between ACE and ACE + FES-LCE at all exercise intensities. During steady-state exercise within the 40%–80% V˙O2peak range, ACE elicited up to 36% and ACE + FES-LCE and HYBRID up to 50% higher Q than FES-LCE. The ACE + FES-LCE and HYBRID elicited 10% higher Q than ACE.
Some studies have noted a lower cardiac output during maximal or submaximal arm exercise in paraplegic individuals compared with able-bodied subjects. This has been due to a greater increase in HR in the paraplegic individuals, which was largely responsible for their increase in cardiac output while the SV was not significantly altered (13,20,27). Arm exercise alone may not be capable of stressing the cardiovascular system for a sustained period to enable a beneficial training effect to occur. Active lower limb exercises in SCI paralyzed limbs via electrical stimulation enable improvement of central and peripheral circulation by the activation of venous muscle pumps in the lower limbs. However, electrical stimulation of the lower limbs alone does not result in substantial elevation of oxygen uptake or cardiac output (15,30). As demonstrated in this study, combined arm and leg exercises result in a higher cardiac output with no significant difference in HR responses compared with arm exercise alone. Davis et al. (4) suggested that elevated central hemodynamic responses during submaximal hybrid exercise may make blood more available to the working upper body musculature for improved exercise performance.
There is still sparse literature on the acute cardiovascular responses during hybrid FES cycling in individuals with traumatic SCI, and the findings of HR changes corresponding to increases in oxygen consumption and cardiac output have been conflicting. This perhaps can be attributed to the difference in exercise testing protocols, electrical stimulation procedures, and different subject profiles, whether high paraplegics or low paraplegics or tetraplegics, which can all influence the outcomes of cardiovascular and cardiorespiratory responses in the maximal and submaximal exercise testing.
The current study has provided insights into the cardiovascular and metabolic responses during different exercise modalities by measuring cardiac output, SV (Fig. 2) and arteriovenous oxygen extractions during arm, and FES leg or arm plus leg exercise in an SCI cohort (Table 2). Taken together, these variables clearly showed that lower submaximal exercise PO during FES leg cycling exercise could be seen as the end point in a chain of ablated underlying physiological variables. During steady-state FES-LCE from 40% to 80% of mode-specific V˙O2peak, lower HR resulted in reduced cardiac outputs, and this played a role in lower submaximal oxygen consumptions. Even a slightly greater SV during FES-LCE could not compensate for a “lower HR on cardiac output” effect. However, in addition, lower whole-body arteriovenous oxygen extractions also contributed to lower V˙O2 during legs-only exercise. In contrast, when voluntary exercise using musculature above the spinal cord lesion was added (e.g., ACE + FES-LCE, HYBRID), these differences of physiological responses were eliminated. The real-world utility of these findings to the SCI individual undertaking fitness training using arms or legs is that legs-only training may not always provide sufficient intensity for promotion of whole-body aerobic fitness. Conversely, some component of upper body exercise may be needed to achieve sufficient intensity to increase aerobic fitness for cardiovascular health in this population.
This study demonstrated that the cardiorespiratory demands during submaximal ACE + FES-LCE were higher than that in FES-LCE in all exercise intensities. These findings suggest that hybrid-FES training within the submaximal exercise intensities may lead to greater gains in cardiovascular fitness than arm exercise training alone.
The authors wish to express their sincere appreciation to Mr. Raymond Patton for his technical assistance during the course of this study. They also thank the research subjects who volunteered for this study.
This work comprised a portion of the PhD studies of Dr Hasnan under a University of Sydney—University of Malaya Cotutelle agreement. Partial financial support was provided by the University of Sydney internal grants schemes and partial financial support was provided by University of Malaya UM.C/HIR/MOHE/ENG/39 Program Grant.
No external funding was received for this work. No commercial company or manufacturer has any professional relationship with any of the authors involved in this work, and the results of this work will not confer any commercial benefit on any of the authors involved.
The results of the present study do not constitute endorsement by the American College of Sports Medicine.
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Keywords:©2013The American College of Sports Medicine
HYBRID EXERCISE; CARDIORESPIRATORY RESPONSES; MAXIMAL AND SUBMAXIMAL TESTS; OXYGEN UPTAKE