Concurrent validation of activity monitors in patients with rheumatoid arthritis.

Background: Physical activity is frequently reported in rheumatology but it is difficult to measure objectively outside the gait laboratory. A new generation of activity monitors offers this potential but it has not yet been evaluated in patients with rheumatoid arthritis. This study aimed to evaluate three types of activity monitors in patients with rheumatoid arthritis.

Methods: The Step-N-Tune, Activ4Life Pro V3.8, and the Intelligent Device for Energy Expenditure and Activity activity monitors were tested concurrently in 12 patients with rheumatoid arthritis as well as in a healthy control group of 12 volunteers. Participants walked at a self selected speed for two minutes and were filmed for later review. Temporal and spatial gait parameters were also validated against the GAITRite walkway and the total number of steps recorded by each activity monitor was compared to a gold standard derived from half speed video replays.

Findings: Activity monitor performance varied between devices but all showed poorer performance when used in the group with rheumatoid arthritis. Bland-Altman plots demonstrated wider 95% limits of agreement in the group with rheumatoid arthritis and a systematic decrease in agreement between activity monitors and the gold standard with decreasing functional ability.

Interpretation: Despite some variation between devices, all the activity monitors tested performed reasonably well in healthy young volunteers. All except the Activ4Life showed a marked decrease in performance in patients with rheumatoid arthritis, suggesting Activ4Life could be the most suitable for use in this patient group. The marked between group difference in functional ability, and systematic decrease in device performance with deteriorating gait, indicate that activity monitors require specific validation in target clinical populations.