Effect of baseline quadriceps activation on changes in quadriceps strength after exercise therapy in subjects with knee osteoarthritis.

Knee osteoarthritis (OA) is a common chronic condition affecting more than 4.3 million older adults in the United States (1). It is a major cause of pain and functional impairment including difficulty with several activities of daily living (1–3). Weakness of the quadriceps muscle is well documented in subjects with knee OA(4–6), is strongly associated with pain, and is an important determinant of disability(4;7). Quadriceps strength is an important factor to target because weakened quadriceps muscles may increase joint stresses from decreased ability to attenuate loads across the joint(8). Additionally, quadriceps weakness may play a role in the etiology and progression of OA(6;8;9). Multiple factors play a role in the etiology of muscle weakness. In addition to pain and disuse atrophy, reduced quadriceps activation (QA) has been suggested to contribute (4;8;10). The presence of reduced QA (failure to fully activate the muscle) is well established in subjects with knee joint effusions (11;12) and traumatic injuries(10;13–17), and also can be present in knee OA even in the absence of pain and effusion (4;5;7;18). The proposed mechanism for the etiology of reduced QA in knee OA is a damaging cycle. Degenerative changes in the joint may damage articular mechanoreceptors which lead to abnormal processing of sensory information and inhibition of muscle activation. This, in turn, predisposes the quadriceps to weaken which increases risk for more joint damage (4;8). Recently, it has been suggested that reduced QA may have an impact on physical function in subjects with knee OA by moderating the relationship of quadriceps strength to function (19). Subjects with low strength and reduced QA had lower function than those with similar strength but high QA (19). While exercise leads to increased quadriceps strength along with improved pain and function (20;21), strength deficits remain and overall effect sizes have been variable (10;22). It is possible that the presence of reduced QA could contribute to this lack of robust response. A few studies have suggested some improvement in QA with exercise alone, but results have been inconclusive, and there has not been an attempt to examine how pre-therapy level of QA might affect or predict the degree of response to therapeutic exercise (10;23;24). It may be possible that reduced QA might not allow an individual the capacity to produce enough tension in the muscle during exercise to achieve an exercise training effect, which in turn may limit their responsiveness to an exercise program. We are specifically interested in looking at how pre-therapy QA levels may affect response to therapeutic exercise in terms of quadriceps strength, which is a key determinant of function in subjects with knee OA. Therefore, the study aim was to examine whether pretreatment magnitude of QA helps predict changes in quadriceps strength after exercise therapy in subjects with knee OA. We hypothesized that subjects with lower magnitudes of QA (greater failure of muscle activation) would have smaller gains in strength compared to those with higher magnitudes of QA following exercise therapy.