Assessing physical activity in persons with knee osteoarthritis using accelerometers: data from the osteoarthritis initiative

Abstract

Objective: Physical activity measured by accelerometers requires basic assumptions to translate the output into meaningful measures. We used accelerometer data from the Osteoarthritis Initiative to investigate in the context of knee osteoarthritis (OA) the following data processing assumptions derived from the general US adult population: nonwear (a period the monitor was removed), based on zero activity exceeding 60 minutes; and a valid day of monitoring, based on wear time evidence exceeding 10 hours.

Methods: We examined the influence of nonwear thresholds ranging from 20 to 300 minutes of zero activity on mean daily activity minutes (counts>0), mean daily activity counts, and mean daily moderate to vigorous physical activity minutes. The effect of selecting minimums of 8, 10, or 12 wear hours to signify a valid day of monitoring on data retention was examined.

Results: Our sample of 3,536 days of accelerometer data from 519 persons with knee OA showed that mean daily activity minutes increased with the nonwear threshold until stabilizing at 463 minutes per day, corresponding to the 90-minute nonwear threshold. Similar patterns were observed for mean daily activity counts. Varying the nonwear threshold had no effect on mean daily moderate to vigorous physical activity minutes. Choosing the 90-minute nonwear threshold and a minimum of 10 wear hours to constitute a valid day provided 94% data retention.

Conclusion: Data supported applying the 90-minute nonwear threshold to the knee OA population instead of the 60-minute threshold for the general population, while retaining the 10-hour valid day threshold.

Figure 1

An example of minute-by-minute accelerometer counts graphed over a 24 hour period

Figure 2

Figure 2A Relationship of non-wear parameter values to basic outcomes: Mean daily wear hours and mean daily activity counts from 519 persons with knee osteoarthritis contributing 3536 days of accelerometer monitoring Figure 2B Relationship of non-wear parameter values to physical activity intensity outcomes: activity minutes (nonzero counts) and moderate-vigorous physical activity (MVPA) minutes from 519 persons with knee osteoarthritis contributing 3536 days of accelerometer monitoring

Figure 2

Figure 2A Relationship of non-wear parameter values to basic outcomes: Mean daily wear hours and mean daily activity counts from 519 persons with knee osteoarthritis contributing 3536 days of accelerometer monitoring Figure 2B Relationship of non-wear parameter values to physical activity intensity outcomes: activity minutes (nonzero counts) and moderate-vigorous physical activity (MVPA) minutes from 519 persons with knee osteoarthritis contributing 3536 days of accelerometer monitoring

Figure 3

BMI stratification: Relationship of non-wear parameter values to mean daily activity minutes (nonzero counts) from 519 persons with knee osteoarthritis contributing 3536 days of accelerometer monitoring

Figure 4

Percentage of monitored days with daily wear hours exceeding 8, 10, and 12 hours by non-wear parameter values from 519 persons with osteoarthritis contributing 3536 days of accelerometer monitoring