The impact of gait disability on the calibration of accelerometer output in adults with multiple sclerosis.

Accelerometer activity counts have been correlated with energy expenditure during treadmill walking among ambulatory adults with multiple sclerosis (MS). This study examined the effects of gait disability on 1) the association between rates of energy expenditure and accelerometer output in overground walking and 2) the calibration of accelerometer output for quantifying time spent in moderate-to-vigorous physical activity (MVPA) in people with MS. The sample consisted of 24 individuals with MS, of whom 10 reported gait disability based on Patient-Determined Disease Steps (PDDS) scores. The participants undertook three 6-minute periods of overground walking while wearing an accelerometer and a portable metabolic unit (K4b2, Cosmed, Rome, Italy). In the first period of walking, the participants walked at a self-selected, comfortable speed. In the two subsequent walking periods, participants walked at speeds above and below (±0.5 mph) the comfortable walking speed, respectively. Strong linear relationships were observed between rates of accelerometer activity counts and energy expenditure during walking in the overall sample (R(2) = 0.90) and subsamples with (R(2) = 0.88) and without gait disability (R(2) = 0.91). The slope of the relationship was significantly steeper in the subsample with gait disability (β= 0.0049) than in the subsample without gait disability (β= 0.0026). The difference in slopes resulted in a significantly lower cut-point for MVPA (1886 vs. 2717 counts/min) in those with gait disability. These findings provide a metabolic cut-point for quantifying time spent in MVPA in people with MS, both with and without gait disability.