Physical Activity Types, Other Analyses
The researchers assessed total, weight-bearing, and non–weight-bearing physical activity using two validated questionnaires (an adapted version of the Zutphen Physical Activity Questionnaire and the Longitudinal Aging Study Amsterdam physical activity questionnaire) that asked participants about the frequency and duration of various types of physical activity. Activity was quantified as metabolic equivalent of task (MET) hours per week, and weight-bearing activities were defined as those in which the knee joint bears the body’s weight.
Walking, gardening, golf, dancing, and ball sports were among the activities qualifying as weight-bearing activities. Non–weight-bearing activities included cycling, rowing, and swimming.
Sex, body mass index, and follow-up time were among the covariates adjusted for in the primary analysis. Similar results were found when adjustments were also made for educational level, alcohol intake, lipid levels, and diabetes.
While incident radiographic knee OA (measured using the Kellgren & Lawrence grading system) was the primary outcome, the researchers also looked at symptomatic knee OA, as defined by x-ray and a knee pain questionnaire, and found no significant association of its incidence with any of the exercise categories (total, weight-bearing, or non-weight-bearing).
Coauthor Joyce B. J. van Meurs, PhD, of the departments of internal medicine and orthopedics & sports medicine at Erasmus Medical Center, told this news organization that “pain as a subjective, recurrent symptom is more difficult to study … [and] a larger sample size or more precise measurements [of pain] in future studies would help to better understand the true association” of symptomatic knee OA and physical activity.
Similarly, analyses of the 1511 patients (out of 5003) who had knee pain at baseline found no significant association of weight-bearing or non–weight-bearing physical activity with incident radiographic knee OA. The trends were similar to those found in the population without knee pain, however, which suggests the analysis was underpowered, the researchers wrote, noting too that patients with baseline pain had lower activity levels than those without pain. (Low case numbers precluded a stratification analysis on LMI for incident symptomatic OA.)
Thigh Circumference as an Indicator of Muscle Mass
The findings build upon an international meta-analysis published in 2021 that found no association between total physical activity and knee OA and align with other studies suggesting a link between greater mechanical stress/strain and greater OA risk, the researchers wrote. (The meta-analysis couldn’t investigate different types of activity.)
“Although we cannot establish a causal relationship … we hypothesize that the mechanical loading on joints and cartilage could explain the association of weight-bearing activity with osteoarthritis in the low LMI tertile group,” they said.
It is possible that thigh muscle-specific strength or mass may temper the risk of knee OA, they wrote, but the lack of thigh strength data in the Rotterdam Study precluded such evaluation. Still, in everyday practice, the researchers noted, lower limb muscle function could be assessed using thigh circumference.
Dr. Allen agreed. “ ‘Gold standard’ assessment of muscle mass is not common in routine practice, but clinicians can evaluate muscle mass in other ways, such as thigh circumference,” she told this news organization, noting that measurement should align with procedures described by the National Health and Nutrition Examination Survey in its anthropometry procedures manual.
“If low lower-limb muscle mass is suspected, a referral to a physical therapist can be helpful for more formally assessing muscle mass and muscle strength,” she added, “and for instructions for a safe and appropriate exercise program for building muscle and protecting joints.”
Among other limitations of the study, according to the researchers, are an ethnically nondiverse population, the unavailability of knee injury data, and the assessment of physical activity only at baseline.
Moving forward, Dr. van Meurs told this news organization, “the main question regarding physical activity and OA is still, if people already have pain or early OA complaints, what kinds of sports they can do without hurting their joints?” This “should be tested,” she said, “in a real-life, ideally trial-like intervention study.”
The study was funded by the Erasmus Medical Center and Erasmus University as well as through various government grants. Dr. Wu also had study support from the China Scholarship Council. Two of the authors reported relationships with arthritis-related organizations. Dr. Allen reported having no disclosures relevant to her comments.