Association of functional status with changes in physical activity: insights from a behavioral intervention for participants with arthritis.

OBJECTIVE: To analyze change over 6 months in accelerometer-measured physical activity for participants with arthritis in a physical activity promotion trial. We tested the hypothesis that participants with the highest baseline functional capacity, regardless of their intervention status, experienced the greatest increases in physical activity levels at 6-month follow-up.
DESIGN: At baseline, participants were interviewed in person, completed a 5-minute timed walk, and wore a biaxial accelerometer for 1 week, with a subsequent week of accelerometer wear at 6 months. We present data on the changes in accelerometer-measured physical activity across baseline function quartiles derived from participants' walking speed. Analyses were controlled for sociodemographic, health status, and seasonal covariates as well as exposure to the study's behavioral intervention.
SETTING: A Midwest academic medical center. PARTICIPANTS: Participants (N=226) with knee osteoarthritis or rheumatoid arthritis currently enrolled in the Improving Motivation for Physical Activity in Persons With Arthritis Clinical Trial. INTERVENTION: Counseling by physical activity coaches versus control group physician advice to exercise. MAIN OUTCOME MEASURE: Change in average daily counts between baseline and 6-month follow-up.
RESULTS: Contrary to our hypothesis, and after controlling for other predictors of change, the lowest quartile function participants had the largest mean absolute and relative physical improvement over baseline, regardless of intervention group status.
CONCLUSIONS: Participants at a higher risk of immanent mobility loss may have been more committed to improve lifestyle physical activity, reflecting the wisdom of targeting older adults at risk of mobility loss for physical activity behavior change interventions.