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Open Access Research article

Evidence for the different physiological significance of the 6- and 2-minute walk tests in multiple sclerosis

Robert W Motl*, Yoojin Suh, Swathi Balantrapu, Brian M Sandroff, Jacob J Sosnoff, John Pula, Myla D Goldman and Bo Fernhall

BMC Neurology 2012, 12:6  doi:10.1186/1471-2377-12-6

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Comment on BMC Neurol 2012 12: 6

Ulrik Dalgas   (2012-06-18 10:39)  Aarhus University email

Dalgas U, Kjoelhede T,
Department of Public Health, Section of Sport Science, Aarhus University, Denmark

de Groot V,
Department of Rehabilitation Medicine, VU University Medical Center, Amsterdam, The Netherlands

Eijnde BO and Feys P
BIOMED, Hasselt University, Diepenbeek and REVAL Research Institute, PHL University College, Hasselt, Belgium



In a recent paper in this journal Motl et al.[1] question a recent recommendation of replacing the 6 minute walk test (6MWT) with the 2 minute walk test (2MWT) when assessing walking performance in clinical trials evaluating patients with multiple sclerosis (MS)[2]. In the paper the presented argument for maintaining the 6MWT as an outcome measure in clinical trials is based on the difference in the physiological demand (i.e. oxygen consumption and metabolic processes) between the two tests.
Motl et al. showed a curvilinear increase in VO2 during the first three minutes of the test, followed by a plateau (`steady-state��) during the last three minutes. Based on this finding the authors state, that in the first part (equal to the 2MWT) of the 6MWT energy is produced by a combination of anaerobic and aerobic metabolism, whereas in the last part of the 6MWT energy is produced by aerobic metabolism. The authors conclude that the 2MWT and 6MWT measure different metabolic features of walking and have different interpretations as clinical outcome variables.
However, in order to investigate the actual contribution of different energy systems, Motl et al. should have provided additional information regarding the relative intensity of the test by performing a maximal oxygen uptake test (or an estimation of VO2max) and also by reporting values of the respiratory exchange ratio (RER). The RER value must have been obtained given that CO2-secretion was measured simultaneously with O2-uptake by the K4b2 mobile equipment (Cosmed, Italy). This would likely have shown that the test is performed at a moderate intensity (approximately 69% of maximal heart rate), as is shown in other studies[3].
Although there might be a slight difference in metabolic demand between the first and the last minutes of the 6MWT, we argue that, neither the aerobic nor the anaerobic energy system is functioning near its maximal capacity, because multiple studies have indicated that the test is very likely performed at sub-maximal intensities [3-5]. This means, that walking performance in neither of the two tests is limited by the metabolic systems providing energy. Therefore, we question the suitability of the 6MWT to test the function of these metabolic systems, and, consequently, the conclusion of Motl et al. that the 6MWT adds significant information in addition to the 2MWT.
However, we would agree that other arguments can be presented in the debate of choice of walking test. Comprehensive norm-data already exist for the 6MWT, while those of the 2MWT first need to be collected, what can easily be done if the 2MWT earns consensus. Another argument may relate to the responsiveness of the 2MWT compared to the 6MWT, which at present is unknown.
The purpose of applying the 2MWT and the 6MWT in clinical trials is to monitor degree of walking disability in a clinical setting, and not to monitor sub-maximal energy metabolism. Because the 6MWT performance can be precisely predicted by the 2MWT performance[2; 6], while both are similarly related to habitual walking performance in MS patients[7], we still think that it is justified to conclude, that the 2MWT can replace the 6MWT in clinical practice, when measuring walking disability in persons with MS.

References

1. Motl, RW, Suh, Y, Balantrapu, S, Sandroff, BM, Sosnoff, JJ, Pula, J et al. Evidence for the different physiological significance of the 6- and 2-minute walk tests in multiple sclerosis. BMC Neurol 2012 12: 6.
2. Gijbels, D, Dalgas, U, Romberg, A, de, G, V, Bethoux, F, Vaney, C et al. Which walking capacity tests to use in multiple sclerosis? A multicentre study providing the basis for a core set. Mult Scler 2011
3. Chetta, A, Rampello, A, Marangio, E, Merlini, S, Dazzi, F, Aiello, M et al. Cardiorespiratory response to walk in multiple sclerosis patients. Respir Med 2004 98: 522-529.
4. Bosnak-Guclu, M, Gunduz, AG, Nazliel, B, Irkec, C. Comparison of functional exercise capacity, pulmonary function and respiratory muscle strength in patients with multiple sclerosis with different disability levels and healthy controls. J Rehabil Med 2012 44: 80-86.
5. Paltamaa, J, Sarasoja, T, Leskinen, E, Wikstrom, J, Malkia, E. Measuring deterioration in international classification of functioning domains of people with multiple sclerosis who are ambulatory. Phys Ther 2008 88: 176-190.
6. Gijbels, D, Eijnde, BO, Feys, P. Comparison of the 2- and 6-minute walk test in multiple sclerosis. Mult Scler 2011 17: 1269-1272.
7. Gijbels, D, Alders, G, Van, HE, Charlier, C, Roelants, M, Broekmans, T et al. Predicting habitual walking performance in multiple sclerosis: relevance of capacity and self-report measures. Mult Scler 2010 16: 618-626.

Competing interests

None

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