To describe the associations between interindividual (between-person) and intraindividual (within-person) variability in physical activity (PA) and knee pain and functional limitation among older adults. We also investigated the potential bidirectional association of between-person and within-person variability in knee pain and functional limitation with PA.
Participants (N = 1064; 51% women; mean age 63 ± 7.4 years) were measured at baseline, 2.5, and 5 years. PA was measured using pedometers. Knee pain and functional limitation were assessed using the WOMAC scale. A two-part hurdle model, with adjustment for confounders, estimated the association of between-person and within-person variability in PA with knee pain/functional limitation (as the outcome). Linear mixed effect regression models described the association of between-person and within-person variability in knee pain and functional limitation with PA (as the outcome).
Between-person effects showed that participants with a higher 5-year average PA had lower average WOMAC scores (β= -1.17, 95% CI: -1.82, -0.51). Within-person effects showed that at time-points when participants had a higher PA level than average, they also had lower WOMAC scores (β= -0.85, 95% CI: -1.36, -0.35). Conversely, both between-person (β= -15.6, 95% CI: -22.5, -8.8) and within-person increase (β= -7.4, 95% CI: -13.5, -1.4) in WOMAC scores were associated with lower PA.
These findings suggest that PA and knee pain/dysfunctional contribute to the development of one another. Pain can lead to changes in inter- and intraindividual PA levels, but the reverse is also possible - changes in PA results in changes in inter- and intraindividual pain/dysfunctional levels.