Objective. To determine if time spent in light intensity physical activity is related to incident disability and disability progression.
Design. Prospective cohort study.
Setting and participants. This study uses a subcohort from the Osteoarthritis Initiative, a longitudinal study that enrolled 4796 men and women aged 45 to 79 years with or at high risk of developing knee osteoarthritis. Inclusion criteria for the main cohort study were: (1) presence of osteoarthritis with symptoms in at least 1 knee (with a definite tibiofemoral osteophyte) and pain, aching, or stiffness on most days for at least 1 month during the previous 12 months; or (2) presence of at least 1 from a set of established risk factors for knee osteoarthritis: knee symptoms in the previous 12 months; overweight; knee injury causing difficulty walking for at least a week; history of knee surgery; family history of a total knee replacement for osteoarthritis; Heberden’s nodes; repetitive knee bending at work or outside work; and age 70–79 years. The subcohort of the current study draws from the 2127 participants that enrolled in the substudy with accelerometer monitoring, included those without disability at study onset; exclusion criteria include insufficient baseline accelerometer monitoring, incomplete outcome or covariate data, decedents and those lost to follow up. A total of 1680 were included in the main analysis, and an additional 134 participants (for a total of 1814) with baseline mild or moderate disability were included in a secondary analysis. between September 2008 to December 2012 at 4 sites (Baltimore, Pittsburgh, Columbus, Ohio, and Pawtucket, Rhode Island)
Main outcome measure . Disability at the 2-year follow-up visit among those without disability at baseline. Disability was ascertained by using a set of questions asking if participants have any difficulty performing each basic or instrumental activity of daily living because of a health or memory problem. Basic activities include walking across a room, dressing, bathing, eating, using the toilet and bed transfer. Instrumental activities of daily living include preparing hot meals, grocery shopping, making telephone calls, taking drugs, and managing money. Disability levels were defined as none, mild (only instrumental activities limitations), moderate (1–2 basic activities limitations), and severe (more than 2 basic activities limitations).
Statistical analysis . Main predictor variable was physical activity monitored using accelerometers measured at baseline. Participants wear the accelerometer for 7 consecutive days on a belt from arising in the morning until retiring, except during water activities. Participants also recorded on a daily log the time spent in water and cycling. Intensity thresholds were applied on a minute by minute basis to identify non-sedentary activity of light intensity and moderate to vigorous intensity. The primary variable was the accelerometer assessment of physical activity measured as daily minutes spent in light or moderate-vigorous activity. The time spent was divided in quartiles; the quartile cut-points for light activity were 229, 277, and 331 minutes, and the cut-points for moderate-vigorous activity were 4.3, 12.2, and 28.2 average minutes per day. Other covariates were socioeconomic factors including race and ethnicity, age, sex education and income, health factors including chronic conditions by self report, body mass index, knee-specific health factors and symptoms, smoking, and gait speed. The main analysis of the relationship between baseline physical activity and the development of disability was done using survival analysis techniques and hazard ratios. Secondary analysis using the larger cohort evaluated hazard ratios for disability progression as defined by progression to a more severe level among the 1814 participants.
Main results . In the main analysis, with 1680 participants without disability at baseline, 149 participants had new disability over the 2 years of follow-up. Average age of the cohort was 65 years, the majority (85%) were white, and approximately 54% were female. The cohort averaged 302 minutes a day of non-sedentary activity, the majority of which was light-intensity activities (284 minutes). Older age was associated with lower physical activity ( P < 0.001), as was male sex ( P < 0.001), higher body mass index, a number of chronic medical conditions (cancer, cerebrovascular disease, congestive heart failure), lower extremity pain, and higher grade of knee osteoarthritis severity. Onset of disability was associated with daily light-intensity activity times, even after adjusting for covariates. Using the group with the lowest quartile of light intensity activity time as reference, groups with higher quartiles of activity level had lower hazard ratios for onset of disability—hazard ratios were 0.64, 0.51, and 0.67 for the second, third, and highest quartile, respectively. Using daily moderate to vigorous activity time–defined quartile, longer duration of moderate-vigorous activity time was associated with delayed onset of disability. In the secondary analysis using the cohort with and without disability at baseline (n = 1814), similar results were found. Participants who spent more time in light intensity activity were associated with less incident disability.
Conclusion. Greater daily time spent in light intensity physical activity was associated with lower risk of onset and progression of disability among adults with knee osteoarthritis and those with risk factors for knee osteoarthritis.
Disability, such as the inability to dress, bathe, or manage one’s medications, is prevalent among older adults in the United States [1,2]. The development of such disability among older adults is often complex and multifactorial. One significant contributor is osteoarthritis of the knee . Although prior observational and randomized controlled trials have established that moderate to vigorous physical activity reduces disability incidence and progression [4,5], less is known about light intensity physical activity—activities that may be more realistically introduced for adults with symptomatic knee arthritis.
The current prospective cohort study included adults with and at risk for knee osteoarthritis; the authors found that physical activity, even if it is of light intensity, is associated with lower risk of disability onset and progression. A major strength of the study is the objective measurements of physical activity using an accelerometer rather than relying on recall or diaries, which are more subject to bias. Another strength is the long follow-up period, which allowed for the examination of incident disability or disability progression over 2 years. The results confirm that even light intensity activity is associated with reduced risk of incident disability.
It is important to note that causation cannot be inferred in this study. As the authors stated, those who can do longer periods of physical activity may be at lower risk of developing incident disability because of factors other than the physical activity itself. A different study design, such as a randomized trial, is needed to demonstrate that light intensity physical activity, when introduced to adults with or at risk for knee arthritis, may lead to reduced risk of disability.
Applications for Clinical Practice
Prior studies suggest that introducing regular exercise have significant health benefits. The recommendation for exercise for adults with knee arthritis remains the same. Whether introducing light intensity activity, particularly for those who are unable to perform more vigorous exercises, yields similar benefits will need further studies that are designed to determine therapeutic effect.
—William Hung, MD, MPH