Hemodynamic and Metabolic Responses to Moderate and Vigorous Cycle Ergometry in Men Who Have Had Transtibial Amputation

More than two million Americans have experienced limb loss, and 28 million face an increased risk of having an amputation surgery [1]. Adults who have had an amputation complete minimal physical activity (PA) [2] and have challenges engaging in PA [3]. This inactive lifestyle may attenuate their muscle mass and fitness level which reduces their health status [4]. It is evident that the maximal oxygen uptake (VO₂max) is lower in adults who have had an amputation versus non-injured controls (CONs). Chin et al. [5] showed a significantly lower VO₂max and maximal workload (Wmax) in amputees versus CONs, which is important as exercise capacity is related to the activities of daily living and the ability to walk with a prosthetic in amputees [6].

Prior studies explored effects of PA on changes in aerobic fitness in adults who have had an amputation. Thirty sessions of one-leg cycling at an intensity equivalent to the ventilatory threshold (VT) significantly increased the VO₂max and Wmax [5]. Results from a study by Chin et al. [7] revealed 36 and 26% increases in these outcomes after 6 wk of training, which were significantly different versus a non-exercise control group. These adaptations were attendant with high values of a preferred walking speed, suggesting that endurance training improves ambulation.

Studies in amputees have primarily employed one-leg cycling [5,7], upper-body cycling [8], or walking [6,9] as the exercise modality. However, little is known about responses to bilateral leg cycling, which has a larger working muscle mass which increases calorie expenditure and trains the musculature above the prosthetic leg. Cycling is accessible in fitness centers and can be performed at home. Kurdibaylo [10] required adults with a lower-limb amputation to perform graded exercise on an upper-body ergometer, and results showed attenuated cardiac function versus CONs. Exercise capacity is related to the cardiovascular system’s ability to transport O₂, which encompasses the hemodynamic responses typically represented by changes in heart rate (HR), stroke volume (SV), and cardiac output (CO). Therefore, a diminished capacity to deliver O₂ may reduce daily function and in turn, the health status of amputees.

The American College of Sports Medicine recommends 150 min/week of moderate intensity continuous exercise (MICE) including walking, running, cycling, or swimming, to increase fitness and health [11]. This prescription also applies to individuals with disabilities, including amputees. High intensity interval exercise (HIIE), repeated brief (5 s–5 min) efforts completed at workloads at or near VO₂max, leads to significant increases in VO₂max [12], glycemic control [13], and reductions in body fat [14] which are similar or superior to MICE [15,16]. To our knowledge, no study has examined the acute physiological response to HIIE of adults who have had an amputation or compared these responses to a bout of MICE.

This novel study compared hemodynamic and cardiometabolic responses between MICE and HIIE in men who have had a TTA versus non-injured CONs. Results showed higher HR, CO, and VO₂ values during HIIE versus MICE, supporting data from non-amputees [24]. There was no difference in HR or VO₂ during exercise between groups, suggesting that the two-leg cycling performed by men who have had a TTA elicits similar cardiovascular responses versus non-amputees. Lastly, men reported “good” affective valence and high enjoyment, suggesting that cycling-based MICE and HIIE are not viewed as unpleasant for men who have had a TTA.

Our study has several unique methodological elements versus prior studies in amputees. First, we prescribed exercise using a metabolic outcome (VT) rather than the %HR/VO₂max, which reflects the cardiovascular demands of exercise. Exercise prescription using %HR/VO₂max places participants at different levels of metabolic strain, exhibited by discrepancies in BLa which reduce exercise tolerance and elicit premature fatigue [29]. Second, we examined exercise responses below (MICE) and above the VT (HIIE) to better characterize the acute hemodynamic and metabolic response to varied exercise intensities, which can be used by clinicians for proper exercise prescription. Third, we selected two-leg cycling as it activates a large amount of muscle mass and is accessible in gyms or at home.

HIIE is unique in is its ability to elicit a near-maximal HR which is important to substantially increase VO₂max long term [30]. The peak and mean HR in response to HIIE were equal to 88 and 80%HRmax in men who have had a TTA and CONs. Similar values were reported in non-amputees [31] and adults with spinal cord injuries [32] (SCIs) and reflect the substantial cardiovascular stimulus imposed by HIIE. Our results show no effect of group on HR/VO₂ responses to MICE and HIIE, which opposes those of prior work in amputees. Jarvis et al. [33] reported higher VO₂ values (+24%) in bilateral transfemoral amputees versus CONs during 5 min of self-selected walking; however, there were no differences in those who have had unilateral transtibial or transfemoral amputations. A meta-analysis [34] showed no differences in VO₂ between CONs and adults who have had a TTA. Nevertheless, significantly higher VO₂ values occurred in adults who have had a lower limb amputation completing walking compared to CONs [35]. Yet, other results [36] showed similar VO₂max values between adults who have had a traumatic amputation and CONs, suggesting that the origin and region of an amputation exert significant effects on the VO₂ response to exercise. Our data, albeit speculative due to the small and homogeneous sample, suggest that men who have had a TTA attain a near-maximal HR during HIIE, similar in magnitude to that observed in CONs, which could potentiate increases in VO₂max values if performed long-term.

Table 3 shows significantly higher CO values in response to HIIE versus MICE, as revealed in non-amputees completing cycling [24]. CO values increased approximately 4-fold in response to MICE and 5-fold during HIIE. In addition, HIIE induced higher CO values versus that of MICE, which was explained by higher SV values. Interval exercise elicited near-maximal SV values, suggesting that vigorous exercise elicits substantial central oxygen delivery in amputees. In addition, HIIE elicited an 86 and 89%COmax in men who have had a TTA and CONs. Kurdibaylo [10] measured changes in cardiovascular outcomes in response to wheelchair ergometry up to 75% of the predicted VO₂max in amputees. Compared to CONs, amputees showed lower SV values and higher HR values and attenuated end diastolic and end systolic volumes. In amputees, SV declined at the highest intensity, whereas, it continued to increase in CONs, suggesting superior hemodynamic function. Yet, the VO₂max was not determined, so it is unknown if the fitness level differed across groups, which could partially explain the discrepancies reported in hemodynamic function. Vella and Robergs [37] stated that SV plateaus in untrained adults at 50–60% VO₂max, an intensity similar to MICE, which may explain the lack of difference in the SV between MICE and HIIE shown by men who have had a TTA despite the differences in intensity between sessions. However, CONs showed higher peak SV and CO values versus those who had a TTA, which may be due to a higher body mass (+11 kg) which is attendant with a higher BV and SV. Recent data show a causal impact of BMI on cardiovascular function [38], as participants with a higher BMI reveal a higher CO, supporting this potential explanation. Unfortunately, our study cannot identify the origin of this discrepancy in SV between groups, so further testing is merited to compare hemodynamic responses to exercise in amputees versus age-, mass-, and fitness-matched controls.

Blood lactate concentration reflects the balance between lactate production through glycolysis and its removal [39]. Compared to MICE, vigorous exercise including HIIE significantly increases BLa due to a greater activation of fast twitch fibers [39]. Figure 3 revealed that BLa peaked at 8.5 mM after HIIE, supporting values from non-amputees [31] and adults with SCIs [32] completing HIIE on the cycle or arm ergometer. A peak BLa equal to 6 mM was shown in amputee soccer players [40]. Nevertheless, the peak BLa is equal to 14 mM in able-bodied elite soccer players [41], with this difference being due to the dramatically lower peak velocities attained in amputee compared to conventional soccer. Our data show no difference in BLa in response to MICE and HIIE between groups, which may be related to their similar baseline VO₂max and VT values. Studies in non-amputees exhibit that BLa serves as an energy sensor [42], so we recommend measuring the BLa of amputees to better portray their metabolic response to acute or chronic exercise.

The perceptual response to exercise in amputees is poorly understood, which is unfortunate considering the relationship between changes in outcomes including FS and long-term exercise adherence in non-injured adults [43]. It is likely that no form of physical activity, irrespective of its efficacy, is feasible for adults unless it is well-tolerated. Data exhibit similar changes in RPE and FS values, as there was no main effect of group nor a groupXtime interaction. These results may be related to similar VO₂max, PA, and BLa values between groups. HIIE revealed lower FS values versus those of MICE, which is related to the higher BLa attendant with exercise above the VT [31]. However, this aversive response was not coincident with lower enjoyment, as our results show no differences in enjoyment between sessions. Adults cite a lack of enjoyment as a primary barrier to regular PA [44], which necessitates designing exercise regimens which not only promote health and fitness-related benefits, but also a positive perceptual response.

This study has a few limitations. Our data do not apply to men who have had upper-body or bilateral amputations, and our sample had a VO₂max similar to that of non-amputees, so results apply primarily to habitually active amputees. Infinite permutations of MICE and HIIE exist, and different responses would occur if the duration, intensity, or modality were modified. Our participants comprised a small convenience sample, so additional testing is needed to verify these results in a larger and more diverse group of adults who have had an amputation. Nevertheless, our results are strengthened by matching participants by age, physical activity, and VO₂max values, so discrepancies in physical characteristics may not affect our results. Also, prescribing exercise according to a VT standardizes the acute metabolic response, which better characterizes whole-body responses to MICE and HIIE versus the use of %HR/VO₂max [29]. Lastly, this is the first study documenting the cardiometabolic response to HIIE of men who have had an amputation, and our results may apply to clinicians and scientists who implement physical activities for this population.