Background/Purpose: Swimming is a non-weight-bearing exercise often recommended to people with knee OA because it is viewed as being less harmful to the knee despite that no epidemiologic studies have evaluated the effect of swimming on knee pain or OA.  Therefore, we aimed to evaluate the relationship of a history of swimming with symptomatic knee OA in the Osteoarthritis Initiative (OAI), a community based cohort not recruited based on swimming status.

Methods: This is a cross-sectional study of OAI participants with knee x-ray readings, symptom assessments, and completed lifetime physical activity surveys where participants identified 3 most frequently performed physical activities (≥ 20 times in life) from a list of 37 for ages 12 – 18, 19 – 34, 35 – 49 and ≥50 years old. Those indicating swimming as an activity were defined as a swimmer in that time period. Any history of swimming included swimmers from all time periods. Data on number of swimming bouts performed were ascertained. Posterior-Anterior semi-flexed knee radiographs were obtained at 48-month visit and scored for Kellgren-Lawrence (KL) grade (0-4). Radiographic OA (ROA) was defined as KL ≥ 2. Frequent knee pain within a person required at least one knee with symptoms. Symptomatic radiographic OA (SOA) required that at least one knee had both ROA and frequent knee pain. Anyone with a total knee replacement was classified as having all outcomes. We performed logistic regression analyses where the predictor was any history of swimming and in the specific age ranges using dichotomous groups and groups based on tertiles of swimming bouts. The outcomes were ROA, frequent knee pain, and SOA; adjusted analyses included covariates age, sex, BMI, prior history of injury, and all other activities correlated with swimming, including walking, in the respective age ranges. Trends were tested using the Cochrane-Armitage test. 

Results: The activities positively correlated with swimming were walking and other water-based activities, such as sailing.

Conclusion: We did not find an increased prevalence of symptoms or ROA in people with a history of swimming compared to those without.  There may be a benefit to swimming for all these outcomes which could be tempered due to confounding by indication, particularly in older age groups. In the 19-34 year old group, a younger age group, where the result is strongest, there is less likely to be reverse causation.  People who swim tend to also walk and participate in other water-based leisure activities. This is the first epidemiologic study to support that swimming does not appear detrimental and may be beneficial towards knee health.