Intensity of physical activity as a percentage of peak oxygen uptake, heart rate and Borg RPE in motor-complete para- and tetraplegia

Objective The aims were to describe VO2peak, explore the potential influence of anthropometrics, demographics and level of physical activity within each cohort; b) to define common, standardized activities as percentages of VO2peak and categorize these as light, moderate and vigorous intensity levels according to present classification systems, and c) to explore how clinically accessible methods such as heart-rate monitoring and Borg rating of perceived exertion (RPE) correlate or can describe light, moderate and vigorous intensity levels. Design Cross sectional. Setting Rehabilitation facility and laboratory environment. Subjects Sixty-three individuals, thirty-seven (10 women) with motor-complete paraplegia (MCP), T7-T12, and twenty-six (7 women) with motor-complete tetraplegia (MCT), C5-C8. Interventions VO2peak was obtained during a graded peak test until exhaustion, and oxygen uptake during eleven different activities was assessed and categorized using indirect calorimetry. Main outcome measures VO2peak, Absolute and relative oxygen consumption, Borg RPE. Results Absolute VO2peak was significantly higher in men than in women for both groups, with fairly small differences in relative VO2peak. For MCP sex, weight and time spent in vigorous-intensity activity explained 63% of VO2peak variance. For MCT sex and time in vigorous-intensity activity explained 55% of the variance. Moderate intensity corresponds to 61–72% HRpeak and RPE 10–13 for MCP vs. 71–79% HRpeak, RPE 13–14 for MCT. Conclusion Using current classification systems, eleven commonly performed activities were categorized in relative intensity terms, (light, moderate and vigorous) based on percent of VO2peak, HRpeak and Borg RPE. This categorization enables clinicians to better guide persons with SCI to meet required physical activity levels.


Introduction
Physical activity (PA) is important in spinal cord injury (SCI) rehabilitation for improving physical function, to increase energy expenditure, but also for the prevention and treatment of the increasingly prevalent cardiovascular risk factors [1][2][3][4][5][6]. Despite this, PA level and physical fitness are generally low in SCI populations compared to the general population [7,8]. Therefore, it's essential for individuals with SCI to be more physically active. To facilitate this, it is of interest to define intensity levels as relative values and to describe a variety of common physical activities by intensity level. The intensity of an activity is, especially in populations with low VO 2peak consumption [9], best described in relation to peak capacity, often expressed as a percentage of oxygen consumption and heart rate at peak effort (here defined as VO 2peak and HR peak , respectively), or percentage of HR peak reserve. Rating of perceived exertion according to the Borg RPE-scale is another common and easily accessible method [10]. The perceived intensity and percentage of VO 2peak of activity (e.g. wheelchair wheeling at 5 km/h or ergometer arm cycling at 50 watts) constitute one of the variables closely connected to an individual's VO 2peak , and this may vary widely between individuals.
However, it is hard in clinical practice to assess VO 2peak or HR peak . This makes it difficult to assess relative intensity level accurately in people with an SCI, especially motor-complete tetraplegia [11]. Previous studies have reported intensity levels according to the Borg RPE scale and percentage of HR peak from arm-ergometer VO 2peak tests [12][13][14]. Other studies have used percentage of HR peak to describe time spent at different intensity levels during rehabilitation [7,15]. More easily accessible methods to describe relative intensity levels would facilitate clinical work and allow better guidance towards a physically active lifestyle. Moreover, there are one extensive studie that have collected data from standardized activities and used SCI METs [16]; however the study didn't relate the VO 2 to peak or to Borg RPE or HR to describe intensity levels.
The present study of two defined cohorts of persons with tetraplegia (C5-C8) AIS A-B and paraplegia (T7-T12) AIS A-B, aimed a) to describe peak oxygen uptake (VO 2peak ) and explore the potential influence of anthropometrics, demographics and level of physical activity within each cohort; b) to define common, standardized activities as percentages of VO 2peak and categorize these as light, moderate and vigorous intensity levels according to present classification systems, and c) to explore/describe how clinically accessible methods such as heart-rate monitoring and rating of perceived exertion correlate, or can describe light, moderate and vigorous intensity levels.

Participants
We studied a convenience sample including 63 persons with SCI; 37 (27 men and 10 women) with motor-complete paraplegia (MCP) AIS A-B (T7-T12), and 26 persons (19 men and 7 women) with motor-complete tetraplegia (MCT) AIS A-B (C5-C8). All were recruited through SCI-specific websites or by word of mouth. Inclusion criteria were SCI injury level C5-C8 and T7-T12; AIS A and B motor-complete, �1 year post-injury, age �18 years, with minimal spasticity (Baclofen treated), as reported on the spasm frequency scale (Penn) [17]. All participants used manual wheelchairs except for one who used power-assisted wheels indoors the same participant did none of the outdoor activities due to the rain. Exclusion criteria were coronary artery disease, angina pectoris and chronic congestive heart failure, chronic obstructive pulmonary disease, or shoulder pain. Persons were excluded if they were on pharmaceutical treatment such as beta blockers or hormone replacement therapy (thyroid hormone). However, those on Baclofen were included. All data were collected in a rehabilitation setting. The participants were asked to avoid heavy exercise 12 hours before the testing, to refrain from coffee and nicotine and to empty the bladder before testing. Each provided written informed consent, and ethical approval was given by the Stockholm region ethics committee, reference number 2011/1989-31/1. We choose to not include persons with an injury level between T1-T6, to have a clear difference between MCP with sufficient ANS functioning versus MCT with reduced or lack of response from ANS. This is based on that the group T1-T6 produces fuzzy results on HR due to shady differences in ANS function, whereas persons with injury level below T7 have an almost normal functioning cardiovascular response from ANS [18,19].

Assessment of VO 2 during the standardized activities
The VO 2 assessment procedure has been described previously [20,21]. In brief, VO 2 was assessed using the same mobile system as for the VO 2peak test. The system was calibrated using built-in automated procedures. All data were collected the same day (except for seven participants who did the VO 2peak test on a different day). The sedentary activities included televisionwatching and desk-based computer work; light intensity included setting a table following a standardized procedure, wheeling a manual wheelchair indoors (in a training hall, wooden floor, 25-meter track with two turns) at their own individual pace perceived as 10-11 (light exertion) on the Borg RPE scale, and individually-paced wheelchair wheeling outside on asphalt also perceived as . The exercise activities included wheeling the wheelchair outside on asphalt at exercise pace  Arm ergometer work at 60 rpm was performed at low level (10W or 15W for MCT and 18W or 24W for MCP) and high level (20W or 25W for MCT and 36W or 42W for MCP). For weight training, the instruction was to select a weight that the participant was able to lift ten times, comfortably at an even and controlled pace. This was tested/practiced before data collection began. The participants rested 15-45 seconds between each machine (including transfer and set-up times to the next gym machine). Circuit-resistance training was performed by one second for the concentric phase and two seconds for the eccentric phase, (rowing machine, pulldowns, Pec Dec for MCP and, rowing machine, pulldowns, external, internal rotation for MCT) [22]. The VO 2peak tests were performed as the last activity of the day.
Information about each activity was given as a standardized verbal instruction together with (Borg RPE 10-11 for light and 13-14 for moderate) and weight selection in weight training. Each test lasted for 6-7 minutes, and the time between each activity varied between a few minutes (sedentary) and >30 minutes (exercise activities). Before testing started the tire pressure was checked to be between 7-10 BAR depending on manufacturer.

Assessment of VO 2peak
A Monark arm ergometer (Ergomedic 891E Monark, Sweden) was used for the VO 2peak test. It was attached to a height-adjustable and the participants were seated in their own wheelchair. Prior to the test, placement and positioning of table and arm ergometer were individually adjusted. Individuals with poor hand function brought their own gloves to be able to hold on to the handlebars. All were asked if they wanted to be strapped to the wheelchair to retain upper-body balance. The test began with approximately three minutes of warm-up, followed by a short break, and was then incremented until exhaustion [23,24]. The protocol used to achieve VO 2peak was individually designed according to international laboratory procedures and previous studies [24] with self-paced cadence starting between 70 and 90 revolutions per minute (rpm) and finishing around 100 to 120 rpm [25]. The starting resistance was chosen depending on level of injury and exercise status based on the resistance during the "low" and "high" arm ergometer activity", which were performed at the beginning of the test-day. The resistance was subsequently increased every minute by 0.25kg for MCT subjects with low (10W or 15W) resistance during arm ergometer work and 0.5kg for those with high (20W or 25W) resistance. For MCP participants the resistance was increased by 0.5kg for those with 36W resistance during arm ergometer exercise and by 0.75kg for those with 42W. The increase during the last 2-3 minutes was individually managed according to the participant's state of exhaustion as evaluated from visual/auditory contact. This gave an anticipated time to exhaustion of between 6 and 12 min. VO 2 and HR were measured continuously during the test using a mobile open-circuit system (Jaeger Oxycon Mobile system (Hoechberg, Germany). VO 2 was analyzed as the average of 10-second averages. VO 2peak was determined as the mean of the highest 30 seconds. Criteria for acceptance of the VO 2peak measurement were: "levelling off" of VO 2 despite increased resistance, RPE above 16, test time more than 6 minutes, supported by a respiratory quotient (RQ/RER) greater than 1.1. All participants reached the criteria of acceptance. None of the participants wore leg wraps and/or abdominal binders.

Other measurements
Body weight was measured to the nearest 0.1 kg, and height was self-reported, with subsequent calculation of BMI ¼ Weight ðkgÞ Height ðcmÞ2 . Self-reported PA was assessed using a validated questionnaire [26], where time spent in moderate and vigorous PA was reported and subsequently calculated by multiplying the minutes (reported as 15 min bouts) each person spent in moderate, vigorous and leisuretime activity (LTA) by the number of days per week. The questionnaire was further dichotomized according to current SCI guidelines for cardiorespiratory fitness (0-44 or 45-450 minutes per week) [27]. Heart rate was measured with chest-strap (Polar) connected to the Oxycon Mobile.

Statistical analysis
Statistical analysis used SPSS (SPSS for Windows Version 23.0; Inc. Chicago, IL, USA). All data was tested for normality using Shapiro-Wilk tests and Q-Q plot analyses. For descriptive statistics, mean, SD or median and range were used. An independent sample two-tailed t-test for group comparisons and statistical significance was set at α = 0.05. Non-normally-distributed data is presented as median interquartile range, Q1-Q3 (IRQ). Categorical data was analyzed with the Mann-Whitney U test. Spearman's rho correlation coefficient was assessed to describe the association between VO 2peak and anthropometrics (body weight and height), demographics (age, level of injury and gender) and PA questionnaire (level, duration and intensity), within the cohorts. Further, stepwise (both forward and backward) multiple linear regressions were used to identify factors that could explain variance in VO 2peak within each cohort, since VO 2peak on group level was normally distributed. The probability for entry into the model was set to F value = p<0.05 and the probability for removal was F value = >0. 10. The collinearity statistics variance inflation factor (VIF) was 1.0-1.53 and the variables' correlation to each other was below 0.6, with a correlation to VO 2peak above 0.48. All data for percentage of VO 2peak were stratified into standardized levels of activity according to ACSM [9,28]. The levels for MCP were light (37-45%VO 2peak ), moderate (46-63% VO 2peak ) and vigorous (64-91%VO 2peak ) [9,28,29]. The levels for MCT were light (44-51% VO 2peak ), moderate (52-67%VO 2peak ) and vigorous (68-94%VO 2peak ). For an activity to be categorized into an intensity all variables, % of VO 2peak , Borg RPE, % of HR peak and speed (kmh), were analyzed and median and interquartile range was used to classify each activity. The MET-value for VO 2peak , calculated with help of the individual REE from the same cohort, previously published by our research group, shows a MET �8 for MCP and �5 for MCT [20]. Categorization of the activities was made by examining how many persons (in each activity) that were categorized by stratifying for % of VO 2peak Borg RPE and HR. At least 66% of the participants in each activity needed to be categorized correct.

Results
The study participants' characteristics showed significant gender differences for height, weight and BMI both for persons with MCP and for those with MCT (Table 1).

Relationship between VO 2peak and anthropometric, demographic and PA variables
Persons with MCP that were physically active at vigorous intensity level at least 45 min per week had a significantly higher VO 2peak 1.7 L�min -1 compared to 1.3 L�min -1 p = 0.007. For persons with MCT, the difference was not significant: 0.87 L�min -1 (more than 45 mins/week) vs. 0.68 L�min -1 (p = 0.02).
After controlling for ten different variables (Table 2) sex, body weight, body height and selfreported time spent in vigorous activity per week showed a significant weak-(r = 0.40 to 0.59)-to-moderate (r = 0.60 to 0.79) correlation [30] with VO 2peak for both cohorts. These four variables were further introduced into a stepwise regression analysis that revealed that sex

Standardized activities as percentages of VO 2peak
The categorization of the eleven activities into different intensity levels, expressed as % of VO 2peak , Borg RPE (seven activities) and % of HR peak (Fig 2). Some activities in Fig 2 have a 95% CI within two intensity levels (wheeling indoors, setting table for MCP), which makes them harder to categorize. However, wheeling indoors could be stratified by speed (kmh), which is done in Table 3 and the heart rate for setting table was 51% HR peak which was categorized as light intensity (Fig 2). Moreover, for MCT setting a table and strength training were within the limits for moderate intensity and wheeling indoors and the other non-exercise activities was categorized between moderate and vigorous (Fig 2). Vigorous intensity included all exercise activities for MCP and MCT.

Discussion
Men were observed to have a significantly higher absolute VO 2peak than women while there was a smaller gender difference for relative oxygen consumption. Sex, weight and time spent in vigorous intensity activity explained 63% of VO 2peak variance for MCP. For MCT, sex and time in vigorous intensity activity explained 55% of this variance. Moreover, all of the activities of daily life were categorized as moderate-to-vigorous for MCT. Translation of the main results into the categorization scheme (Table 3) is highly clinically relevant, and may function as a tool for choosing activities at certain intensity by combining the activity, RPE and/or % of HR peak .

VO 2Peak
As in previous literature [31,32], our results show differences in absolute (L�min -1 ) and relative (ml�min -1 �kg -1 ) VO 2peak between MCP and MCT. This is most likely related to the difference in cardiorespiratory response from the autonomic nervous system [13,33] and the individual's functional muscle volume. The results for absolute VO 2peak are comparable to previous results for persons with similar levels of injury and completeness [32,[34][35][36][37][38][39]. Hence our results are lower compared to most male athletes in Olympic sitting disciplines except for shooting [39]. We choose to use arm-ergometer for VO 2peak testing. Recent research suggests no difference in VO 2peak between arm ergometer peak testing and wheelchair tests for individuals with SCI (T3 -L1 and completeness AIS A-C) [36,[40][41][42][43]. Asynchronous arm ergometer was used during testing and has been reported to be produce equal or higher values compared to synchronous during VO 2peak testing [44]. Regardless of method, these two assessments differ from the traditional VO 2peak for the general population, which tests cardiac output. Individuals with MCP and MCT have limited muscle mass, so these tests assess aerobic capacity to a higher degree [36]. Moreover, the loss of descending sympathetic control is not always equal to the lesion level in MCT as shown when comparing high-performance athletes to non-athletes [11].

Intensity of standardized activities
This study presents data on the intensity of different standardized activities for persons with MCP and those with MCT, expressed as % of VO 2peak , % of HR peak and self-reported perceived exertion. Further, in Table 3 we categorize activities based on % of VO 2peak, % HR peak and Borg RPE into intensity levels (sedentary, light, moderate and vigorous). Our results are based on general physiological parameters [29,45] and show that we slightly altered Borg RPE and % HR peak for both groups. The results for % HR peak for persons with MCP are comparable to those in previous studies, where 40% of VO 2peak corresponded to 61-66% HR peak [13,46] compared to the present �59% HR peak . Likewise, 60% of VO 2peak corresponded to 73-77% HR peak [13,46] compared to �72% HR peak in our study. For MCT, relative HR at moderate and vigorous intensity is comparable to that in previous studies [47,48]. However, it's difficult categorize relative HR for MCT due to attenuated heart rate responses due to impairment in descending sympathetic control. However, it's still of interest to describe the HR for MCT on the basis of the recommended intensity levels from American college of sports medicine [9]. Consequently, it was more difficult to categorize the activities for persons with MCT and not as accurate as for MCP. So, we decided to merge the activity columns for moderate and vigorous intensity for MCP, which means that all daily activities and arm-crank 20W can be moderate or vigorous depending on % of HR, Borg RPE and speed km/h. The large variations and relative high intensity level for daily activities for MCT has also been reported earlier and that study used % heart rate reserve (HRR) to describe intensity level [49]. Moreover, we explored the use of % of HRR and it showed a lower correlation coefficient (0.6), and the already small span of HR become even smaller for MCT. The result regarding Borg RPE for MCP for different activity and intensity levels is also comparable [13,38,46]. For MCT there are fewer comparable studies [38,48]. However, Goosey-Tolfrey et al [38] found similar levels for light intensity but also had difficulties to distinguish between moderate and vigorous intensity levels using Borg RPE for MCT. Previous research showing cardio metabolic benefits for SCI used a relative intensity between 50-70% of VO 2peak or 50-80 HR peak to represent MVPA [50], this indicates that wheeling wheelchair outdoors at 4.8-5.8 km/h or arm-cranking 24-36W at Borg 12-13 could be a way of reaching that intensity level for person with MCP. Hence, for person with MCT wheeling outdoors at 2-6 km/h or arm-crank 10-15W at , might be more applicable.

Strengths and limitations
The strengths of this study include the large homogenous groups, for level and severity of injury, and the extensive protocol for several different activities performed by almost all participants. Also, nearly 30% of the study cohorts were women, which enable important and clinically relevant gender comparisons to be made. There was large intra-individual variation for VO 2peak , affecting the activity-related relative VO 2 (% of VO 2peak ). This reflects the heterogeneous sample of individuals included in these homogenous subsamples, which means that the included participants have different backgrounds of physical activity. It also shows the complexity of metabolic processes within persons with motor-complete SCI.
The study was limited, however, in that not all activities were represented for both Borg RPE and HR. Additionally, individual PA level may have affected how we categorized the activities. Moreover, variation in mechanical efficiency between the more standardized armcranking and the more variable wheeled mobility may have influenced the variation in relative intensity level of performance between these two different modes of activity. Unfortunately, we were not able to compare mechanical efficiency between the wheeled mobility and armcranking, as valid measurements of the power output. The results apply only to non-elite athletes within the same limits of BMI and levels of injury and injury severity. Another limitation that could influence the result is that nerve roots projecting from T7-T10 innervate the adrenal gland, and this might contribute to varying degrees of catecholamine spillover during the task. This could contaminate the observed heart-rate responses even when they were standardized to a peak performance [51].

Conclusions
Using current classification systems, this study has described 11 standardized activities using percentage of VO 2peak and categorized them into three intensity levels: light, moderate and vigorous. This translation of the main results (Table 3) is highly relevant as it enables rehab professionals to use clinically accessible methods such as HR-monitoring and Borg RPE to describe intensity levels. This provides tools for better guidance of persons with SCI seeking to meet the desirable, recommended target PA levels.