Background/Purpose: People with knee osteoarthritis (OA) are thought to be less physically active than those without OA.  This includes time in moderate-to-vigorous physical activity (MVPA), a critical level of intensity associated with numerous health benefits, and walking, the most common type of physical activity in older adults.  However, little empirical data exists to support these trends.  The purpose of this study was to compare objectively measured physical activity and walking among people with symptomatic knee OA (SxOA) with people from the US population of the same sex and similar age.  We hypothesized that those with SxOA will be less active than the US population.

Methods: We used data from the Osteoarthritis Initiative (OAI), a large prospective cohort study of knee OA (n=486), and the National Health and Nutrition Examination Survey (NHANES), a nationally representative sample from the United States (n=1455).   Physical activity was measured in both cohorts with an accelerometer-enabled monitor worn for ≥ 10 hours/day for ≥ 4 days (NHANES – Actigraph 7124; OAI – Actigraph GT1M). Data were collected in OAI from 2008-2010, and in NHANES from 2005-2006.  SxOA was defined as those with Kellgren–Lawrence grade ≥ 2 and knee pain on most of the last 30 days. MVPA was defined as ≥ 2020 activity counts/min.  We also classified those who walked ≥10 minutes/week at ≥80 steps/min to reflect purposeful walking.   Within age- and sex-specific strata, we compared mean time in MVPA between cohorts with independent t-tests and calculated the likelihood that people with SxOA walked purposefully ≥10 min/day compared with the US population using odds ratios (OR). 

Results: Within age- and sex-specific strata, physical activity levels were generally similar (p>0.05) among people with SxOA and the general US population. People with SxOA spend on average 4-29 minutes in MVPA, while the US population spent 2-26 minutes in MVPA (Figure 1). The proportion of people who walked at least 10 min/day was 8-60% for those with SxOA and 10-54% in the US population (Table 1).  The odds of walking at least 10 minutes/day was similar for individuals with SxOA relative to the US population.

Conclusion: Strikingly low levels of physical activity were observed in both people with SxOA and people in the general US population. The presence of pain and OA is not likely a primary reason for low levels of physical activity among people with SxOA. Figure 1. Mean minutes per day (± 95% CI) in MVPA for the US population (NHANES) and people with SxOA (OAI SxOA).

Table 1. Proportions and odds ratio (± 95% CI) for walking purposefully at least 10 minutes/day for people with SxOA (OAI) relative to the US population (NHANES).