Reliability and Validity of the Six-Minute Arm Test for the Evaluation of Cardiovascular Fitness in People With Spinal Cord Injury

Methods 

Measures 

We assessed cognitive impairment with the Cognitive Capacity Screening Examination.25, 26 No subjects were identified as having a cognitive impairment (as indicated by a score of <24/30). The American Spinal Injury Association (ASIA) assessment was used to classify the completeness of each subject’s SCI, as well as to determine their motor and sensory function.27 We assessed the physical activity level of all subjects by using the Physical Activity Scale for Individuals with Physical Disabilities (PASIPD).28 The PASIPD is a self-report questionnaire that uses an estimate of the number of days per week and hours per day spent in different leisure, household, and occupational activities over the past 7 days. Total scores are calculated as the sum of the average hours per day multiplied by a metabolic equivalent for all 12 items. Independence in wheeled mobility was described using 8 categories of wheeled mobility, modified from categories of functional walking developed by Perry et al.29 The categories in this wheeled mobility scale range from full time power wheelchair users to those who can walk independently for most activities but rely on a manual wheelchair for wheeling long distances (table 1).

Table 1. Wheeled Mobility Scale

	Category	Description
	1	Relies fully on power wheelchair
	2	Relies primarily on a power wheelchair for community mobility; uses manual wheelchair for exercise only
	3	Relies primarily on a power wheelchair for community mobility; uses manual wheelchair for some household activities
	4	Relies primarily on manual wheelchair; relies on power wheelchair or assistance with long distances or uphill
	5	Relies fully on manual wheelchair and is independent in all home and community activities
	6	Relies primarily on manual wheelchair; able to walk for exercise only at home
	7	Able to use walking for some household activities, but uses a wheelchair in the community
	8	Can walk independently for most activities; relies on a manual wheelchair for long distances

Procedures 

For both the 6-MAT and cardiovascular fitness test, subjects wore a face mask while respiratory variables (oxygen consumption [V̇o2], carbon dioxide output, ventilation [V̇e], respiratory exchange ratio [RER]) were continuously measured by a portable metabolic unit performing breath-by-breath gas analysis (Cosmed K4b2 system).a Subjects were asked to empty their bladders prior to commencing the exercise tests to minimize any episodes of autonomic dysreflexia. Subjects rated their level of perceived exertion using Borg’s 15-point ratings of perceived exertion (RPE) scale24 immediately following the tests. Blood pressure was recorded at rest, immediately following exercise and throughout recovery to ensure that it returned to baseline values following the exercise tests. Blood lactate measurements were taken at rest and at the end of the exercise tests using a drop of blood collected from a finger-prick. For subjects with insufficient hand grip, elastic straps were used to secure their hands to the handles of the arm ergometer.

Subjects completed a single, 6-minute stage of submaximal exercise on a standard arm cycle ergometerb commonly found in rehabilitation settings. An individual PO was initially selected for each subject based on their manual muscle strength, ASIA motor score, and physical activity level (as reported in table 2). These parameters were used following discussions with physical therapists and physicians who worked in SCI rehabilitation as they felt these were typical variables that were accessible for these clients. The PO was chosen with the aim of eliciting either a steady state heart rate of 60% to 70% of age-predicted maximum heart rate (for subjects with low-level paraplegia) or a rating of 11 to 15 on the Borg RPE scale24 (for subjects with tetraplegia or high-level paraplegia). Heart rate was continually recorded using a chest heart rate monitor.c Approximately 1 week after the initial 6-MAT, subjects were invited to return to complete a second 6-MAT. One week after the second test, each subject underwent a Vo2peak test on an arm ergometer.

Table 2. Subject Characteristics (N=30)

	Variables	N	Mean ± SD	Range
	Sex (male/female)	25/5
	Age (y)		36.3±9.3	19–49
	Time since injury (y)		12.0±9.8	1–34
	Tetraplegia/paraplegia	17/13
	ASIA grade (A/B/C/D)	22/7/0/1
	ASIA motor score (0–100)		41.2±16.7	19–75
	PASIPD score		16.5±9.8	1.0–38.7
	Wheeled mobility category
	1	2
	3	1
	4	11
	5	15
	6	1

Abbreviation: SD, standard deviation.

To measure Vo2peak, subjects performed a symptom-limited graded arm cycle ergometer test on an electronically braked arm ergometerd in the presence of a physician and a kinesiologist. Cardiac stability and heart rate were monitored by a physician using a 12-lead electrocardiogram (Quark C12).a Subjects initially sat quietly for 2 minutes while resting values of heart rate and respiratory variables were collected. Arm cycling began without resistance at a comfortable, self-selected cadence (between 60 and 80rpm). After a brief warm up, PO increased in a step protocol by either 5 or 10W/min (5W/min for subjects with tetraplegia,30 10W/min for subjects with paraplegia12). Subjects continued to arm cycle until they reached volitional fatigue (ie, they were not able to maintain a cycling rate of 30rpm). American College of Sports Medicine Guidelines9 were used to determine whether the test should be terminated early (eg, ST-segment depression >2mm, increasing nervous system symptoms [ie, blurred vision, dizziness], sustained ventricular tachycardia, chest discomfort).

Results 

Descriptive data for the subject characteristics can be found in table 2. A total of 31 subjects volunteered to participate in this study, but 1 subject was excluded after reporting a previous myocardial infarction. Of the remaining 30 subjects that participated in this study, there were 13 with paraplegia and 17 with tetraplegia. The distribution of lesion levels is presented in figure 1. All subjects, including those with C4 and C5 lesions, had a minimum manual muscle testing score of grade 4 for wrist extension. Subjects with C4 and C5 lesions had impaired sensation below their stated lesion level; however, muscle function was partially preserved in 1 to 2 segments below their lesion level, and this allowed them to have adequate wrist extension strength to allow arm cycling. Subjects had a mean age ± standard deviation (SD) of 36.3±9.3 years and a mean time since SCI of 12.0±9.8 years. Eighty-three percent of subjects were men, which is comparable to global statistics.32 Subjects in this study ranged from full-time power wheelchair users to persons that were able to ambulate for exercise.

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Fig 1. Distribution of subject lesion levels.

Maximal Test: Vo2peak 

The mean Vo2peak ± SD value for all subjects was 18.6±8.4mL·kg−1·min−1, with the range extending from 6.5 to 38.1mL·kg−1·min−1 (table 3). The peak heart rate averaged 129±29 beats/min, and the mean peak PO attained was 60.2±36.0W. Subjects in this study had a wide range of fitness levels, and there was overlap in the physiologic responses to exercise between subjects with paraplegia and tetraplegia. During both the Vo2peak test and the 6-MAT, 15 subjects required their hands to be secured to the handles of the arm ergometer with elastic straps. No Vo2peak tests were terminated early because of adverse effects. One subject with T3 level paraplegia experienced self-reported “mild” autonomic dysreflexia midway through the test, which presented as a rapid decrease in heart rate of 25 beats/min (exercising heart rate of 152 beats/min decreased to 127 beats/min; within 30 seconds, heart rate returned to 155 beats/min). Seven subjects reported mild light-headedness on cessation of cycling, and 2 experienced muscle spasms at low POs that briefly interrupted their cycling cadence.

Table 3. Physiologic Values During the Vo2peak Test

	Variables	Mean ± SD	Range
	Peak PO (W)	60.2±36.0	20–160
	Peak heart rate (beats/min)	129±29	75–183
	Percentage heart rate maximum⁎	70.1±14.0	43.6–97.3
	Peak Ve (L/min)	42.8±19.5	18.0–113.1
	Vo2peak (mL·kg−1·min−1)	18.6±8.4	6.5–38.1
	Vo2peak (L/min)	1.33±0.52	0.74–2.81
	Peak RER	1.14±0.09	0.97–1.34
	Blood lactate (mmol/L)†	6.6±2.8	2.3–15.1

⁎ Based on 220 − age prediction equation. † Only 27 of 30 subjects had their blood lactate tested.

Submaximal Test: 6-MAT 

All subjects were able to complete the 6-MAT. Four subjects experienced mild muscle spasms during their cycling that briefly interrupted their cycling cadence. The mean steady-state V̇o2 value ± SD was 13.1±4.2mL·kg−1·min−1, with the range extending from 6.3 to 22.9mL·kg−1·min−1 (table 4). Heart rate averaged 103±21 beats/min, and the mean PO was 27.8±17.0W. Similar to that seen in the Vo2peak test, there was overlap between the subjects with paraplegia and tetraplegia for both the PO and physiologic responses.

Table 4. Steady-State Physiologic Values During the 6-MAT

	Variables	Mean ± SD	Range
	6-MAT PO (W)	27.8±17.0	10–60
	Heart rate (beats/min)	103±21	61–142
	Percent heart rate maximum⁎	56.0±10.2	35.7–75.7
	V̇e (L/min)	25.0±5.7	14.7–39.8
	V̇o2 (mL·kg−1·min−1)	13.1±4.2	6.3–22.9
	V̇o2 (L/min)	0.95±0.28	0.55–1.61
	Percentage Vo2peak	74.5±13.0	51.0–97.8
	RER	0.89±0.07	0.76–1.04
	Blood lactate (mmol/L)†	3.0±1.3	1.0–6.0

⁎ Based on 220 − age prediction equation. † Only 28 of 30 subjects had their blood lactate tested.

One of the difficulties of conducting the 6-MAT was determining an appropriate submaximal PO for each subject. In this study, PO for the 6-MAT was selected based on subjects’ ASIA motor score, manual muscle strength, and physical activity level. If the initial PO was too high (ie, subjects could not complete 6min, RPE >15, and/or heart rate >70% of age-predicted maximum) or too low (ie, RPE <11 and/or heart rate <60% age-predicted maximum), subjects were given a 15-minute rest period before they attempted a second 6-MAT at an adjusted PO. Subjects’ heart rate and V̇o2 had to return to resting values before testing would recommence. Typically it was evident within the first 2 minutes of arm cycling if the PO selected was too high or low. After testing all subjects with the 6-MAT, manual muscle strength scores and physical activity levels were reviewed to develop clinical guidelines for the selection of appropriate starting PO (table 5). Using these criteria, 11 of the 13 subjects with paraplegia (85%) and 11 of the 17 subjects with tetraplegia (65%) would have had their PO set appropriately on the first attempt.

Table 5. PO Selection for the 6-MAT

	For Subjects With Tetraplegia
	Set PO to 10W if:	Set PO to 15W if:	Set PO to 20W if:
	Power wheelchair user OR ≤ grade 4 wrist extension	Manual wheelchair user	Manual wheelchair user AND grade 5 wrist extension AND physically active⁎

	For Subjects With Paraplegia
	Set PO to 30W if:	Set PO to 40W if:	Set PO to 50W if:	Set PO to 60W if:
	Female: inactive	Female: active OR male: inactive	Female: competitive athlete OR male: active	Male: competitive athlete

⁎ Engaged in physical activity at least 3 times a week as assessed by the PASIPD.

Reliability of the 6-MAT 

Twenty of the 30 subjects (8 with paraplegia, 12 with tetraplegia) completed the 6-MAT on a second occasion, approximately 1 week after the first test. Steady-state ICCs for heart rate and V̇o2 were .90 (SE of measurement, 7.12; 95% CI, .75–.96) and .81 (SE of measurement, 1.62; 95% CI, .58–.92), respectively (fig 2). The 20 subjects who completed the reliability portion of the study did not differ from the other 10 subjects with respect to age, ASIA motor score, Vo2peak, or distribution of tetraplegia and paraplegia. The Bland-Altman method31 showed minimal differences between mean heart rate ± SD at time 1 (104±23 beats/min) and time 2 (104±20 beats/min) for the reliability sample. The mean difference between time 1 and time 2 was 0±10 beats/min. As can be seen in figure 3A, all but 1 data point fell within 2 SDs of the mean difference, and the data points are equally distributed above and below the mean difference line (11 above, 8 below, 1 on the line). The outlying data point was a subject with tetraplegia. Test-retest reliability was recalculated without this outlier, and the ICC increased to .92 (95% CI, .81–.97). Similarly, during the 6-MAT minimal differences were seen between mean V̇o2 ± SD at time 1 (12.64±3.71mL·kg−1·min−1) and time 2 (12.26±3.10mL·kg−1·min−1) for the reliability sample. The mean difference between time 1 and time 2 was −0.44±2.07mL·kg−1·min−1 (fig 3B). Once again, there was only 1 data point outside 2 SDs of the mean difference, and the data points are equally distributed above and below the mean difference line (11 above, 9 below). This outlier was once again a subject with tetraplegia, but a different subject. With this outlier removed, the ICC for V̇o2 increased to .86 (95% CI, .67–.94).

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Fig 2. (A) Scatterplot comparing heart rate during 6-MAT test 1 and test 2. (B) Scatterplot comparing V̇o2 during 6-MAT test 1 and test 2.

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Fig 3. (A) Bland-Altman plot of difference in 6-MAT heart rate (HR) between time 1 and time 2 versus average heart rate from time 1 and time 2. (B) Bland-Altman plot of difference in 6-MAT V̇o2 between time 1 and time 2 versus average V̇o2 from time 1 and time 2.

Validity of the 6-MAT 

On visual inspection of the bivariate scatterplots, there was considerable overlap in values between subjects with tetraplegia and paraplegia (fig 4). Given the overlap of values between subjects with tetraplegia and paraplegia, in addition to the similar coefficients (slope and intercept) of the bivariate scatterplots between the subjects with tetraplegia and paraplegia, all subjects were pooled. Using Pearson correlations, an excellent, positive, linear correlation was found between the 6-MAT V̇o2 and Vo2peak (r=.92) (see fig 4) and good correlations were found between 6-MAT PO and Vo2peak (r=.73) and 6-MAT heart rate and Vo2peak (r=.63).

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Fig 4. Scatterplot comparing V̇o2 during the 6-MAT and Vo2peak (r=.92).

Discussion 

Demographic variables (age, sex, time since injury, level of injury) were comparable to global statistics.33 Because we excluded people who were ambulatory or could not use a manual wheelchair, the majority of our participants had lower cervical injuries (C4-7) with a smaller representation of thoracic and lumbar injuries. Such a distribution reflects the large representation of lower cervical injuries among all SCI levels reported by the National Spinal Cord Statistical Centre Database.34 We expected the majority of participants to have ASIA grade A and B injuries (as represented in the sample); however, we did not have any ASIA grade C injuries, which is a limitation to the generalizability of the sample. The PASIPD is a fairly new measurement tool used for the evaluation of physical activity level of people with physical disabilities, so limited data are available for comparison. The mean PASIPD score of the subjects in this study was comparable to that reported by Washburn et al28 in people with a variety of disabilities (including SCI).

Because this study only recruited wheelchair users who were able to independently cycle an arm ergometer, it is appropriate that the majority of subjects were described by categories 4 and 5 of the wheeled mobility scale (independent manual wheelchair users who do and do not require assistance with wheeling long distances or uphill). No subjects were described by categories 7 or 8, because these categories describe primarily ambulators, and not people who mainly use a wheelchair.

A portable metabolic system was used to measure V̇o2. Some studies35, 36 have reported excellent reliability, but small systematic biases in V̇o2 values of a portable system compared with a standard metabolic cart. These differences are modest and others have concluded that this small discrepancy was acceptable for measuring V̇o2 across a wide range of exercise intensities.37 Such small systematic biases would not affect the reliability or the correlations attained in this study. The Vo2peak values found in this group of SCI subjects were similar to others previously reported in the literature,38, 39 indicating that this sample is representative of the general population of people with SCI. This study included subjects with diverse activity levels ranging from sedentary people using power wheelchairs to international caliber wheelchair athletes. Using normative categories that were developed from the 20th percentiles of Vo2peak values from 146 men with SCI,39 the subjects from this study were categorized as follows: for subjects with tetraplegia, 1 poor, 1 fair, 6 average, 6 good, and 3 excellent; for subjects with paraplegia, 2 poor, 4 fair, 3 average, 1 good, and 3 excellent. The Vo2peak values used by Janssen et al39 to develop the normative categories are from only male subjects, with 40% of them being athletes. It is likely that the “normative” Vo2peak values used to categorize individuals are higher than the true values of the general population of people with SCI; thus Vo2peak classification of subjects in this study may be underestimated.

All subjects were able to complete 6 minutes of arm ergometry exercise at an individually selected submaximal PO. During the 6-MAT, subjects exercised at an average of 74.5% of Vo2peak and at 56% of their age-predicted maximum heart rate, indicating that the exercise was aerobic. Blood lactate immediately after the 6-MAT averaged 3.0mmol/L. The onset of blood lactate accumulation, or the transition to anaerobic exercise, is said to take place when blood lactate concentrations rise above 4.0mmol/L,40 so our value of 3.0mmol/L provides further confirmation that aerobic exercise was occurring during the 6-MAT.

Based on the description of acceptable reliability values presented by Andresen41 and Fleiss,42 heart rate and V̇o2 measured during the 6-MAT have excellent test-retest reliability. Visual inspection of the Bland-Altman plots reveals fairly equal distribution of test-retest differences above and below 0, suggesting minimal bias with repeated testing. For both heart rate and V̇o2, only 1 of the 20 subjects in the reliability sample fell outside 2 SDs of the mean difference, indicating limited test-retest variation for both variables.31 Both heart rate and V̇o2 have well established relationships with PO, so it is not surprising that reliability was high.

The 6-MAT was designed as a submaximal cardiovascular fitness test, and thus was validated against the criterion standard of cardiovascular fitness testing, the Vo2peak test. The correlation between Vo2peak and 6-MAT V̇o2 was excellent (r=.92), indicating that those subjects with a high submaximal V̇o2 had a high Vo2peak. Submaximal PO and Vo2peak had a lower correlation (r=.73). It is not surprising that this correlation was weaker, because up to 9 subjects cycled at the same PO for the 6-MAT, and all did not have the same absolute Vo2peak. It is not possible to require the same PO from each participant because of their varying abilities. The correlation between submaximal heart rate and Vo2peak was also lower (r=.63). The 6-MAT was designed to have subjects cycle while maintaining a constant PO at 60% to 70% of their age-predicted maximum heart rate or at an RPE level of 11 to 15. Because this target heart rate value is based solely on age, 2 people of the same age but different cardiovascular fitness levels would both be exercising at a similar submaximal heart rate. The same submaximal heart rate would then be associated with potentially different Vo2peak values. Thus the correlation between 6-MAT heart rate and Vo2peak was expected to be significant, but not necessarily high. Similarly, the popular submaximal endurance assessment using the 6-minute walk test has been shown to have moderate association with Vo2peak in a number of populations.43, 44

We considered the use of heart rate reserve, rather than percentage of age-predicted maximal heart rate or RPE, to determine submaximal, but to do this, subjects would have had to have a Vo2max test first to determine their maximal heart rate. Because the 6-MAT test is designed for clinical use where maximal testing may not be possible, the use of RPE and percentage of age-predicted maximal heart rate would be more clinically feasible.

Although there was considerable overlap in Vo2peak values between subjects with tetraplegia and paraplegia, many of the subjects with tetraplegia had lower Vo2peak values. This is likely due to a combination of factors including: decreased muscle innervation in the trunk and upper extremities of those with tetraplegia, the absence of the lower-extremity skeletal muscle pump that aids in venous return to the heart,45 and impairment to the sympathetic nervous system and thus a limited ability to increase heart rate and cardiac output, especially in people with injuries of T5 and higher.46 Subjects with impaired sympathetic nervous system function (those with injuries of T5 and higher, n=21) had lower Vo2peak values, but their test-retest reliability and correlations with Vo2peak were similar to those with injuries below T5 (n=9). The proportion of subjects with injuries below T9, however, was small and a larger, more equally distributed sample would help to determine specific influences associated with injury level.

The 6-MAT is a practical test that can be administered to people of all fitness levels. Unlike the modified arm CAFT,19 all subjects recruited for this study were able to complete the 6-MAT. Functionally, all subjects had a minimum manual muscle testing score of grade 4 for wrist extension. The equipment required to conduct the 6-MAT is minimal (arm ergometer, heart rate monitor, RPE scale), and all pieces are available in most rehabilitation settings. For the 6-MAT to be used clinically, clinicians first need to determine the appropriate PO for a person to be exercising at using the guidelines provided in table 5. Baseline outcome variables would be determined by recording the client’s exercising heart rate during the final 30 seconds of the 6-MAT, and taking their RPE at the end of the test. At a later time (ie, after an intervention aimed at increasing cardiovascular fitness) the 6-MAT should be re-administered at the same PO that was used pre-intervention. A decrease in heart rate and/or RPE may indicate an increase in Vo2peak, whereas an increase in heart rate and/or RPE may indicate a decrease in Vo2peak.

Conclusions 

The steady-state heart rate and Vo2 responses obtained during the 6-MAT are reliable, and the concurrent validity of the 6-MAT for assessing cardiovascular fitness in people with SCI is excellent. It would be useful to test the 6-MAT for responsiveness in the future to enhance its use as a clinical tool for the measurement of cardiovascular fitness in persons with SCI.