Knee osteoarthritis (OA) is associated with substantial functional limitations in older adults [1]. Specifically, knee OA, along with hip OA, accounted for the largest proportion of disability in walking and stair climbing more than any other chronic diseases [2]. In patients with advanced stages of painful knee OA, although a total knee arthroplasty (TKA) can effectively restore function, 1 in 6 patients with TKA continue to have substantial physical function limitations [3]. Clearly, an in-depth understanding of the modifiable predictors of physical function and their aetiological pathways is needed which, in turn, could assist the refinement of interventions to improve function.
Clinically, two common knee impairments before and following a TKA are reduced knee extensor strength and deficits in knee extension range-of-motion (ROM) [4]. Although most [5-12] but not all [13] previous studies have implicated knee strength and knee extension ROM as important predictors of physical function, notably absent from these studies was an evaluation of the interrelation between knee extensor strength and knee extension ROM: previous studies – by their choice of standard regression analyses – assumed that these 2 knee measures have distinct and unrelated etiologies and that they act through different pathways to influence physical function.
Is it possible that knee extensor strength and knee extension ROM are interrelated? Based on what is known about the length-dependent nature of muscle force production, [14,15] we believe it is biologically plausible that knee extensor strength may play a role in the mechanistic pathway connecting knee extension ROM and physical function. Indeed, many activities of daily living – for example, level walking [16] – require the knee extensors to produce forces at lesser degrees (~30°) of knee flexion. Ostensibly, at a given knee joint angle near full extension, a flexion contracture may limit force production because the knee extensors – the monoarticular vastii in particular – are operating at a disadvantaged (lower) position on the ascending limb of their force-angle curves [14,15]. In as much as this concept may make sense and lead to the refinement of theoretical and intervention models, supporting data in patients with TKA are surprisingly sparse.
Because the interrelation between knee extensor strength and knee extension ROM is possible and important to understand clinically, we initiated the present study to examine whether changes in knee extensor strength mediate the association between changes in knee extension ROM and self-report physical function in a large group of patients before and following a TKA.