Recently, there has been interest in using RPE to guide exercise testing (2,16) and prescription (15,31,32) in individuals with spinal cord injury (SCI), as typical markers of exercise intensity are confounded by both somatic and autonomic impairment unique to each injury. This problem is further compounded in tetraplegia as exercise-induced increases in HR are blunted above T6 injuries because of autonomic impairment (13), which limits the use of HR to monitor training intensity (39). Rather than monitoring physiological parameters prone to significant variation between individuals, RPE may be a more appropriate marker of exercise intensity in SCI, as it is better able to target cardiorespiratory adaptations after exercise training programs.
Although global RPE is used as a gestalt fatigue response (6), RPE can also be separated into differentiated regional scores, including fatigue associated with the peripheral working limbs (peripheral RPE [PRPE]) and the central cardiorespiratory sensations (central RPE [CRPE]) (28). It is generally thought that peripheral fatigue dominates the end stages of upper limb maximal exercise tests (5,10,12,19). Previously, both Borg CR10 and 6–20 scales have indicated greater PRPE than CRPE at high exercise intensities during arm cycling in individuals with paraplegia (25,32), although this finding is not universal (33). It is unknown whether PRPE mirrors CRPE responses in individuals with tetraplegia, or how these responses can be modeled throughout the duration of an incremental test rather than at a single end-exercise time point.
Previous studies have demonstrated an uncoupling between global RPE and cardiorespiratory parameters measured during graded arm cycling tests (26). Lewis et al. (26) examined 42 adults with motor-complete paraplegia and tetraplegia and reported poor correlations between global RPE as measured by the Borg 6–20 RPE scale and traditional cardiorespiratory markers (i.e., V˙O2, V˙E, and HR) at three discrete intervals during a graded arm cycling test. These authors suggested that the assessment of regional indicators of exertion might be more sensitive for assessing the relationship between RPE and cardiorespiratory outcomes. Differentiated assessments of RPE have been shown to offer additional value above that of global RPE alone (3,38) and may explain discrepancies in the relationship between RPE and cardiorespiratory markers of performance during arm cycling. In addition, although previous studies have observed altered affective responses after a critical intensity level (i.e., ventilatory threshold; VT) in able-bodied individuals (29), it is unknown whether the differentiated RPE response is also altered at a similar breakpoint in SCI.
Therefore, the purpose of this study is twofold: 1) to describe the differentiated RPE responses to graded exercise testing in individuals with paraplegia and tetraplegia and 2) to examine the relationship between cardiorespiratory indices (i.e., V˙O2 and HR) and differentiated RPE during graded arm exercise. Given the previously reported dissociation between global RPE and cardiorespiratory parameters during fitness testing, we sought to examine the unique response of PRPE and CRPE at regular intervals during exercise. We hypothesized that PRPE would increase at a faster rate than CRPE, and that only PRPE would be related to physiological cardiorespiratory markers during the later stages of exercise. Furthermore, although individuals with tetraplegia will have blunted increases in HR and oxygen consumption during exercise, changes in RPE throughout exercise will be more comparable to individuals with paraplegia.
Thirty-six recreationally active individuals with SCI (lesion level C1-T11, AIS A-D, 13 ± 10 yr postinjury) were recruited to participate in this study. Before the exercise test, participant height and waist circumference were determined in the supine position with an anthropometric tape measure. Body mass was determined using a wheelchair scale (Detecto BRW-1000 Digital Bariatric Wheelchair Scale; DETECTO, Webb City, MO), with the weight of the chair being subtracted while the participant was resting supine. The study procedures were approved by the Hamilton Integrated Research Ethics Board in Hamilton, Canada, and adhered to the Declaration of Helsinki. All participants gave verbal and written informed consent before enrollment in the study.
Participants performed a graded arm cycling test on an electronically braked wall-mounted Lode arm ergometer (Angio V2; Lode BV, Groningen, The Netherlands), height adjusted so that the shoulder joint was aligned with the crank axis of the ergometer. Participants were asked to refrain from caffeine, alcohol, smoking, and moderate-vigorous physical activity 12 h before the test as well as void their bladders before exercise to minimize the risk of autonomic dysreflexia. Single-lead electrocardiogram (PowerLab 26T [LTS]; ADInstruments, Colorado Springs, CO), blood pressure, and respiratory measures were collected with the participants at seated rest 2 min before exercise. Expired gas and ventilatory data were obtained using a commercially available metabolic cart (MOXUS Metabolic System; AEI Technologies, Pittsburgh, PA). HR was also monitored with a Polar HR monitor (Polar T31; Polar Electro Inc., Woodbury, NY). For participants with tetraplegia with limited handgrip function, tensor bandages were used to secure their hands to the ergometer handles. After a 1-min warm-up at 0 W, power output was increased every minute (5 W·min−1 for tetraplegia; 10 W·min−1 for paraplegia), or as required to reach an exercise test duration of 8–12 min at a cadence between 60 and 70 rpm. The test was terminated at volitional exhaustion, or when the cycling rate dropped less than 30 rpm. V˙O2peak was defined as the highest 30-s average of V˙O2 obtained during the duration of the test. VT was determined using the ventilatory equivalents method (14) to ascertain if participants were able to achieve intensities near peak cardiorespiratory limits. In brief, V˙E/V˙O2 was plotted against V˙E/V˙CO2, and VT was identified at the point where the V˙E/V˙O2 began to rise without a concomitant increase in V˙E/V˙CO2.
Participants were asked to identify both CRPE and PRPE on the Borg category ratio (CR-10) RPE scale (28) during the last 15-s of every minute until the completion of exercise. Before the exercise test, participants were given standardized instructions and were oriented to the Borg CR-10 scale. Participants were specifically instructed to report first how hard they were working with their “heart and lungs” for CRPE and then “arms and shoulders” for PRPE. A large printed copy of the RPE scale was held in front of the participants during exercise, and they were asked to nod their head at the appropriate level as the investigator pointed to successively greater values on the scale.
All data were linearly interpolated to 10 points at 10% increments to facilitate comparisons at different exercise test durations and between participants. Data were checked for normality with the Kolgomorov–Smirnov test. Two 2 × 10 (RPE–time) repeated-measures ANOVAs were used to assess differences between CRPE and PRPE within each participant group (tetraplegia and paraplegia). Tukey's HSD was used to evaluate significant interactions. Bivariate Pearson's correlation coefficients were used to assess the relationship between cardiorespiratory outcomes with differentiated RPE at 40%, 60%, 80%, and 100% of the graded exercise test.
Curve analysis was performed to assess the development of CRPE and PRPE throughout exercise. Using the linearly interpolated points, F tests were used to best fit the data to a linear, quadratic, or cubic function, where the simpler function was used unless the model reached statistical significance. F tests were also used to determine whether the best-fit curves differed between measures of RPE or subpopulations (paraplegia vs tetraplegia). Analyses were repeated on the subgroup of individuals who were determined to have an identifiable VT during the graded exercise test. Statistical analyses were performed using IBM SPSS Statistics for Macintosh (version 20.0; IBM Comp., Armonk, NY) and GraphPad Prism (Prism 5.0a for Mac; Graph Pad Software, San Diego, CA). Statistical significance was set at α = 0.05.
Participant characteristics can be found in Table 1. Individuals with tetraplegia had lower resting systolic blood pressure, diastolic blood pressure, and mean arterial pressure (P < 0.01) compared with individuals with paraplegia.
All participants were able to complete the V˙O2peak test to volitional fatigue. Cardiorespiratory end points can be found in Table 2. V˙O2peak was significantly greater in individuals with paraplegia compared with tetraplegia (P < 0.01), which was accompanied by greater end-exercise HR (P < 0.01) and ventilatory rate (P < 0.01). VT could only be determined in 23 participants (paraplegia: 11/13; tetraplegia: 12/23) where individuals with paraplegia had a higher V˙O2 at VT (P = 0.02), although there was no difference in %V˙O2peak at VT (P = 0.67). Peak CRPE (paraplegia = 7.8 ± 1.7 vs tetraplegia = 6.5 ± 2.5; P = 0.11) and PRPE (paraplegia = 8.8 ± 1.5 vs tetraplegia = 8.7 ± 1.6; P = 0.78) at end exercise were not different between paraplegia and tetraplegia. In 100% of participants, peripheral fatigue (i.e., shoulders, back, and forearms) was reported as the main cause of exercise cessation of the graded exercise test.
When interpolated to 10% intervals, all cardiorespiratory data increased throughout exercise in a linear fashion: PO = ax + b (vs quadratic P = 0.11); HR = ax + b (vs quadratic P = 0.25); and V˙O2 = ax + b (vs quadratic P = 0.44) (Fig. 1). For all cardiorespiratory data, paraplegia was associated with a faster increase in responses as compared with tetraplegia (P < 0.05).
Perceived exertion interpolation
No differences were observed between participant groups for either CRPE (P > 0.05) or PRPE (P > 0.05) at any point during the graded exercise test.
In individuals with paraplegia, no differences were observed between CRPE and PRPE during the initial and middle stages of the exercise test (10% to 60%; P > 0.05) (Fig. 2); however, responses were found to diverge during the later stages (70% to 100%; P < 0.05) when PRPE was higher than CRPE. In individuals with tetraplegia, no differences between CRPE and PRPE were observed in the initial stages of the exercise test (10% to 30%; P > 0.05); however, responses were found to diverge during the middle-to-late stages (40% to 100%; P < 0.01), with PRPE increasing at a faster rate than CRPE. When these analyses were repeated on only the subset of individuals who reached VT, PRPE and CRPE responses diverged in individuals with both tetraplegia and paraplegia from 80% to 100% of test duration.
Perceived exertion modeling
When modeled separately, CRPE and PRPE were best described by quadratic models (ax2 + bx + c) with positively accelerating growth in both participant groups (CRPE: paraplegia P < 0.01 and tetraplegia P = 0.05; PRPE: paraplegia P < 0.01 and tetraplegia P < 0.01) (Fig. 2). When analyses were repeated on the data from only those individuals who demonstrated a VT, RPE models remained best explained by quadratic models.
Curve analysis revealed that RPE developed differently between individuals with paraplegia and tetraplegia for both CRPE (P = 0.04) and PRPE (P = 0.02), where individuals with paraplegia had an accelerated perception of fatigue in both differentiated responses. However, when analyses were repeated on data from only those individuals who reached VT, differences were no longer found between groups, indicating both CRPE and PRPE developed similarly over the graded exercise test.
When grouped by lesion level, there were also differences between the development of CRPE and PRPE for both paraplegia (P < 0.01) and tetraplegia (P < 0.01), where peripheral fatigue developed faster than central fatigue in both groups. When these analyses were repeated on only individuals who demonstrated a VT, differences between CRPE and PRPE remained for individuals with tetraplegia (P < 0.01) and strongly trended for individuals with paraplegia (P = 0.07).
Relationship to cardiorespiratory measures
Throughout the initial and middle stages of the exercise test, there were no correlations between differentiated RPE and cardiorespiratory outcomes (40%, 60%, and 80%; P > 0.05) (Table 3). At 100% of the graded exercise test, there was a moderate overall relationship between CRPE and V˙O2 (r = 0.35; P = 0.04), with no relationship within each lesion level group.
We observed a nonlinear rise in both CRPE and PRPE in individuals with paraplegia and tetraplegia despite linear increases in cardiorespiratory outcomes during arm cycling. A divergence between CRPE and PRPE responses during the later stages of the exercise test was found in all participants; however, this divergence occurred earlier in the test for individuals with tetraplegia and those individuals who lacked an observable VT. In line with our hypothesis, we found that PRPE developed faster than CRPE only in individuals with tetraplegia, although with a strong trend for a similar relationship in individuals with paraplegia. These results demonstrate an uncoupling between the development of physiological and subjective indicators of fatigue, which is subsequently modified by lesion level and the ability to reach VT during a standardized graded exercise test.
It is well established that V˙O2 and HR increase linearly during graded exercise in the able-bodied population (9,18) as well as among individuals with SCI (20,26,36), although there is some variability in individuals with tetraplegia (39). We report a similar linear increase in the present study, with lower peak cardiorespiratory responses for individuals with tetraplegia compared with paraplegia. According to previously published normative fitness values (21), groups would be considered to have “fair” and “good” fitness, for individuals with paraplegia and tetraplegia, respectively. Although traditional indices of exertion have been well documented (21), we are not aware of published comparisons of these outcomes to subjective indices of differentiated exertion during graded arm exercise in SCI. We observed a moderate overall relationship between CRPE and V˙O2 at end-stage exercise (100% test duration: r = 0.35; P = 0.04). This finding is likely mediated by muscle mass, indicating that individuals with greater upper limb function are able to overcome peripheral fatigue leading to higher CRPE as well as higher cardiorespiratory exercise capacity. Previously, Lewis et al. (26) demonstrated a moderate relationship between global RPE with HR in individuals with tetraplegia (n = 10) and with V˙O2 in individuals with paraplegia (n = 32) at low-to-moderate exercise intensities; however, contrary to our hypotheses, we were unable to replicate these findings. As graded versus ramp peak exercise tests have been shown to produce different end-test peak V˙O2 and HR with similar peak RPE in individuals with paraplegia (1), differences in exercise protocols may have limited direct comparison between our findings. Furthermore, day-to-day variability in peak exercise testing may be greater in individuals with tetraplegia (24), which should be a consideration in our study as we recruited twice the number of tetraplegics compared with Lewis et al. (26).
Given the relatively small active muscle mass associated with upper limb exercise, it has been suggested that a differentiated, rather than global, RPE may offer more valuable information to perceptions of exertion and intensity during exercise in SCI (26,30). Although there is evidence of the separation of peak CRPE and PRPE during lower limb graded exercise (5,10,12), the growth of this response and the response during upper limb exercise has not been characterized. In the present study, we observed nonlinear growth in both CRPE and PRPE for upper limb graded exercise tests in groups of individuals with tetraplegia and paraplegia. The Borg CR10 scale has previously demonstrated a similar positively accelerating growth in lower limb exercise (28). Modeling approaches may yield additional information beyond that of single measurements during exercise. Of note, we did not observe any group differences in interpolated CRPE or PRPE scores throughout the duration of the test. However, when the entire response was compared between groups, we found differences in how CRPE and PRPE developed between individuals with paraplegia and tetraplegia. Although these differences did not persist when excluding participants who did not reach VT, the modeling process was better able to detect group differences than single values alone.
CRPE and PRPE may be differentially perceived throughout a maximal exercise test in individuals with tetraplegia as demonstrated both when including or excluding those participants who did not reach VT. The rationale for the differentiated perception of exertion may include physiological differences postinjury. Smaller active muscle mass in individuals with tetraplegia may result in altered perception of exertion (22,37), especially at higher exercise intensities. Perceived exertion has been correlated most notably with HR and peripheral lactate concentration in the able-bodied population (4,17,34). Autonomic dysfunction after SCI is known to blunt the HR response to aerobic exercise with injuries above T6 (7). As HR has been shown to be a major correlate of RPE (34), reduced feedback from the central cardiovascular system may have resulted in reduced perceptions of central fatigue as workloads increased in our study. Differences in blood lactate are likely not governed by autonomic dysfunction (35), although work in wheelchair athletes by Leicht et al. (23) suggests that maximum blood lactate concentrations are limited in tetraplegics with small upper limb muscular mass. Further research is needed to describe the relationship between autonomic dysfunction and metabolism with RPE in sedentary or recreationally-active individuals with tetraplegia.
Previous studies have identified a difference between CRPE and PRPE at the end of both submaximal (25) and maximal (25,32) exercise in individuals with SCI. Interestingly, in the present study, differences between CRPE and PRPE existed in individuals with tetraplegia after 40% of the graded exercise test, but this effect was lost when considering only participants who achieved VT during the test. VT represents a critical point at which the energy contributions from aerobic pathways cannot meet the metabolic demand and begin to shift to anaerobic pathways, resulting in exercise above this threshold being associated with declines in participant “pleasure” (11,29). In the present study, VT was used as a critical point at which we expected differences in the perception of peripheral and central fatigue. When VT was observed, separation of dissociated RPE occurred after the average VT (80%–100% of test duration), indicating a possible important breakpoint during graded arm exercise. Unfortunately, the measurement of VT is difficult, and it was not possible to observe VT in all participants. It is possible that in those participants who are unable to achieve VT, the central cardiorespiratory response is not great enough to elicit increases in CRPE concurrent with the increases in PRPE, reflecting the dissociation between RPE scores early in the maximal test. This observation was not found in individuals with paraplegia, who reported differences between CRPE and PRPE only at intensities above VT.
Recently, there has been interest in using RPE scales to guide exercise testing and prescription for individuals with SCI (2,15,16,31,32). For example, the current Canadian Physical Activity Guidelines for adults with SCI (27) have been validated to increase aerobic fitness using a target of 3–6 on the global CR10 scale (moderate to vigorous intensity) (32). Our results demonstrated that the dissociation between PRPE and CRPE occurred early during the maximal test in the entire cohort of individuals with tetraplegia, at only 2–4 on the CR10 scale (light-to-moderate intensity exercise). Given the recent interest to use subjective intensity scales in exercise prescription, it should be noted that an index of peripheral fatigue might be a stronger sensory cue than an index of central fatigue starting at moderate-intensity exercise in individuals with tetraplegia. Interestingly, moderate-intensity exercise has been shown to be better regulated with PRPE than either CRPE or global RPE in able-bodied arm cycling, as compared with visual power output feedback (30). Peripheral cues may dominate sensory feedback during moderate- to vigorous-intensity exercise and may bias global RPE for setting intensity in exercise prescription; however, further experimental studies are required to confirm these findings, as well as explore the implications for prescribing exercise for cardiorespiratory adaptations.
Because of the heterogeneity in the SCI population, there are various other factors that we did not consider when evaluating the differentiated RPE response to a graded exercise test. Autonomic function has been demonstrated to account for substantial variability in the cardiorespiratory response to exercise, governing the rise of HR and blood pressure during activity (8,40). Although we did not assess autonomic function in this study, 5/23 participants with tetraplegia exceeded peak HR of 130 bpm, indicating some autonomic variability in our sample. Participants were not anchored to a maximal exercise intensity as part of our study design, which may limit validity of end-stage RPE values. Although we were unable to measure reliability in this study, intraclass correlation coefficients of 0.75 and 0.40 for CRPE and PRPE, respectively, have been previously reported (12), although the validity of RPE during short-stage peak fitness testing has not been examined. Finally, while our results are well detailed for 1-min staged increases in power output, it remains to be determined whether these findings persist for continuous submaximal exercise, or other exercise modalities such as wheelchair ergometry.
In summary, we observed nonlinear increases in the differentiated RPE responses to a graded peak arm cycling test in individuals with SCI. Although there appears to be an early divergence of CRPE and PRPE in individuals with tetraplegia, this effect was driven by participants who were not able to reach VT during the exercise test. Although there is promise to use differentiated RPE monitoring for exercise testing and prescription within the SCI population, our results indicate that there are differences in how individuals with tetraplegia perceive peripheral versus central exertion. These regional differences may have implications for intensity prescription for aerobic exercise, with the goal of promoting cardiorespiratory fitness in individuals with SCI.
This study was supported by funding from the Natural Sciences and Engineering Research Council (DG no. 238819-13, DG no. 238819-08) and the Ontario Neurotrauma Foundation (2011-ONF-RHI-MT-888). The authors thank Michael Hutchinson for his insight during the drafting stages of this manuscript. The authors have no conflicts of interest to declare. The results of the present study do not constitute endorsement by the American College of Sports Medicine. The results of this study are presented clearly, honestly, and without fabrication, falsification, or inappropriate data manipulation.
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