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

BACKGROUND Researchers have recently advocated for the 2-minute walk (2MW) as an alternative for the 6-minute walk (6MW) to assess long distance ambulation in persons with multiple sclerosis (MS). This recommendation has not been based on physiological considerations such as the rate of oxygen consumption (V·O2) over the 6MW range. OBJECTIVE This study examined the pattern of change in V·O2 over the range of the 6MW in a large sample of persons with MS who varied as a function of disability status. METHOD Ninety-five persons with clinically-definite MS underwent a neurological examination for generating an Expanded Disability Status Scale (EDSS) score, and then completion of the 6MW protocol while wearing a portable metabolic unit and an accelerometer. RESULTS There was a time main effect on V·O2 during the 6MW (p=.0001) such that V·O2 increased significantly every 30 seconds over the first 3 minutes of the 6MW, and then remained stable over the second 3 minutes of the 6MW. This occurred despite no change in cadence across the 6MW (p=.84). CONCLUSIONS The pattern of change in V·O2 indicates that there are different metabolic systems providing energy for ambulation during the 6MW in MS subjects and steady state aerobic metabolism is reached during the last 3 minutes of the 6MW. By extension, the first 3 minutes would represent a test of mixed aerobic and anaerobic work, whereas the second 3 minutes would represent a test of aerobic work during walking.