Researchers have recently advocated for a 2MW rather than a 6MW as a walking performance test in persons with MS, and this is based on a strong correlation between distances and similar correlations with external criteria [9, 10]. This evidence alone is not sufficient for selecting a single test duration given the likely different physiological and metabolic demands of the 2MW and 6MW and the heterogeneous disability of individual MS patients. Accordingly, for this study we adopted a physiological perspective for understanding the appropriate duration of a walking performance test in MS, and we provide data on the pattern of change in V·O2 over the 6MW at various degrees of disability using the EDSS as gold standard. V·O2 changed in a curvilinear pattern during the first 3 minutes of the 6MW, and then reached a steady-state characterized by no further changes in V·O2 over the remainder of the 6MW. Such a pattern of change in V·O2 occurred despite similar pacing over the entire 6MW based on cadence measured by an accelerometer. This is explained by the lag in aerobic oxidation during the initiation of steady-state, submaximal exercise [11, 12]. Overall, the pattern of change in V·O2 suggests that there are different metabolic systems providing energy for ambulation during phases of the 6MW. Both aerobic and anaerobic systems are providing energy for walking in the first part of the 6MW, whereas aerobic metabolism is the primary energy system used for walking during the second part of the 6MW. This observation might be further examined by administering tests of anaerobic and aerobic power and examining the association with 2MW and 6MW performance in persons with MS. Anaerobic power could be measured using a Wingate Test (i.e., pedal a manually braked bicycle ergometer for 30 seconds with an all out pace), whereas aerobic power could be measured using a maximal, incremental exercise test (i.e., pedal an electronically braked ergometer over 10-12 minute period with increasing resistance until volitional termination). Such tests can be administered in persons with MS and would provide further information on the physiological and metabolic systems underlying 2MW and 6MW performance. This would be important for identifying the possibility that these tests measure different features of walking and therefore have different interpretations or applications as clinical outcome variables.
We compared our data on the pattern of change in V·O2 over the 6MW with previous research involving persons with MS . That previous research included a small sample of persons with MS (n = 11) and reported that V·O2 increased over the first 3 minutes of the 6MW and then reached steady-state over the remainder of the test. This pattern of change was consistent with that observed during submaximal exercise in non-diseased, healthy adults [11, 12], but the level of increase in V·O2 during the 6MW is higher in controls than persons with MS ; this is expected based on the difference in walking speed, but has not been examined across levels of disability status in MS versus controls. We confirmed that V·O2 increases over the first 3 minutes of the 6MW and then reaches "steady state," but in a nearly 10 times larger sample of persons with MS. We further reported on the novel result that the pattern of change over the first 3 minutes was generally consistent across mild, moderate, and severe disability status, although the slope was steeper and the absolute level of V·O2 was higher in those with mild disability compared with moderate and severe disability. This is caused, in part, by the difference in walking speed and its associated effect on V·O2. In particular, if we expressed V·O2 relative to walking speed (i.e., oxygen cost of walking), there would be a difference between the moderate and severe disability groups given similar energy expenditure, but different walking speed. Nevertheless, there appears to be consistent evidence for a curvilinear pattern of change in V·O2 over the first 3 minutes of the 6MW in MS, and this has implications for identifying the primary source of energy for walking during the 2MW and 6MW. The pattern of change in V·O2 would suggest that the 2MW represents a test of both aerobic and anaerobic aspects of walking performance, whereas the 6MW represents a test of primarily aerobic or endurance aspects of walking performance in MS. Conceptually, 2MW performance might be associated with everyday tasks that require brief, but intense bouts of ambulation (e.g., stair climbing or crossing the street), whereas 6MW performance might be associated with everyday tasks that require sustained, submaximal bouts of ambulation (e.g., grocery shopping).
We compared our data on the 6MW with those of previous research  for further understanding the veracity of this performance test for differentiating disability status in MS. That previous research  observed a reduction in walking distance (mean ± standard deviation in ft) across three levels of disability status, namely mild (1958 ± 155 ft; EDSS = 0-2.5; n = 15), moderate (1636 ± 332 ft; EDSS = 3-4; n = 19), and severe (1254 ± 259 ft; EDSS = 4.5-6.5; n = 6) disability. We observed a similar reduction in walking distance across the three levels of disability status of mild (1723 ft; EDSS = 2-3.5; n = 29), moderate (1389 ft; EDSS = 4-5.5; n = 29), and severe (979 ft; EDSS = 6.0-6.5; n = 37) disability. Interesting, the coefficient of variability was larger for persons with moderate and severe disability than mild disability in the present study and previous research . We further note that all participants walked for the entire 6-minute period, and we had no falls or other adverse events. This too is similar with the observations of previous research . Overall, we replicated the capacity of the 6MW for differentiating disability status, particularly in the severe group that had a small sample size in previous research , and recommend that the 6MW continue to be considered as a performance assessment of walking endurance in MS as it is in other neurological conditions such as Parkinson's disease [20, 21].
We measured cadence during the 6MW using an accelerometer as an initial method of examining spatiotemporal parameters of pacing; this has not been undertaken in previous research. Interestingly, we observed that cadence (i.e., steps per minute) did not change over the 6MW for any of the groups who differed in disability status, but it did differ significantly among the three groups. Those with mild disability had a greater cadence during the 6MW than those with moderate and severe disability, and those with moderate disability had a greater cadence during the 6MW than those with severe disability. This indicates that the spatiotemporal parameters of pacing do not change over the 6MW in persons with MS (i.e., persons select and maintain a stable cadence over the entire 6MW) perhaps to optimize energy expenditure, and future researchers might consider other temporal and spatial parameters of gait such as step length during the 6MW given that there has been an observation of possible differences in minute-by-minute walking speeds during the 6MW across disability status . We note that there might be different neural systems controlling step rate and length in MS, and perhaps this is important for monitoring in clinical trials of MS. Of note, the sample in the present study has a different disability composition than that of the previous publication examining changes in performance pattern during the 6MW . This issue could be further explored through application of devices such as the Intelligent Device for Energy Expenditure and Physical Activity or IDEEA® or GAITRite™ electronic walkway and might further our understanding of gait parameters that are associated with long distance walking performance and identify targets of rehabilitation for improving this valued function.
We did not administer an independent 2MW in this study and have no information on V·O2 over the actual 2MW test. Nevertheless, we suspect that V·O2 would actually be similar during the 2MW compared with the first 2 minutes of the 6MW. This is based on the similar distance walked during the 2MW and first 2 minutes of the 6MW in persons with MS . By extension, walking speed would be comparable between the 2MW and first 2 minutes of the 6MW and result in a similar metabolic demand. This observation is important as it minimizes the concern that our results regarding change in V·O2 over the initial part of the 6MW are not applicable for the 2MW. However, even if the change in V·O2 was not applicable it would not undermine our argument that there are different metabolic processes underlying the 2MW and 6MW, as one would expect a steeper slope of the V·O2 response if the intensity was actually greater when performing the 2MW test. Nevertheless, such assumptions should be confirmed in subsequent research along with the recommendation of administering tests of anaerobic and aerobic power in MS.
There are notable strengths of the current study including the standardized administration of the 6MW protocol, measurement of V·O2 and cadence over the 6MW, and relatively large sample of persons with MS and subsamples with a proportionate range of disability status. The limitations include (a) lack of a control group for further comparing V·O2, cadence, and distance over the 6MW, (b) lack of measurement of V·O2 and cadence over the 2MW, (c) the cross-sectional design and lack of information on the comparative sensitivity and responsiveness for capturing change in 2MW and 6MW performance over time and in the context of an intervention trial, (d) lack of external criteria for validating the 2MW and 6MW against anaerobic and aerobic power, and (e) lack of control for fatigue levels and exact types of symptomatic therapies that might impact ambulation, V·O2, and cadence.