To examine whether the associations of physical performance with isometric hip and knee extensors strength and passive hip flexion and internal rotation range of motion (ROM) are nonlinear in community-dwelling adults with hip osteoarthritis (OA).
Participants were 100 adults (mean age 62 years) with radiographically confirmed hip OA. Physical performance measures included gait speed test and timed stair tests. Piecewise regression models of muscle strength and hip ROM on physical performance measures were used to identify possible breakpoints. Receiver operating characteristic (ROC) curve analysis was used to identify participants with functionally inadequate (≤1.0 meters/second) gait speed and optimal cut points were identified.
Muscle strength and hip ROM were nonlinearly associated with stair measures but not with gait speed measures. The optimal breakpoints and ROC-derived cut points were 1.8–2.5Nm/kg for knee extensors, 1.5–2.1Nm/kg for hip extensors, 25–26° for hip internal rotation ROM, and 109–115° for hip flexion ROM. Muscle strength breakpoints increased monotonically with increasing movement demand.
In individuals with hip OA, the associations between physical performance and measures of muscle strength and hip ROM are less straightforward than previously assumed. These breakpoints and ROC-derived cut points may be useful in identifying individuals for whom interventions that improve muscle strength or hip ROM would be most beneficial.