Muscle strength, mass, and quality in older men and women with knee osteoarthritis.

Osteoarthritis (OA) of the knee is a major cause of disability among the elderly (1–3), and its prevalence is projected to grow substantially with the aging of the U.S. population (2,4). Despite the high prevalence of this condition and the high personal and economic impact of osteoarthritis-related disability, there are currently no specific therapies that can prevent the onset or progression of joint damage caused by OA. Medical therapies designed to control pain and swelling can be associated with side effects, and the withdrawal of certain COX-2 inhibitors from the market due to safety concerns has further limited therapeutic options (5). With limited therapeutic modalities and concerns about long-term safety of existing modalities, it is imperative to better understand the physiological and biological relationships underlying knee OA in order to devise and promote better preventive and therapeutic options. Muscle weakness, and in particular quadriceps weakness, has been implicated in knee OA and quadriceps strengthening exercises are a therapeutic approach recommended by some experts (6–9). However, reports on the relationship between muscle strength and knee OA have presented conflicting data, due in part, perhaps, to an emphasis on gross measurements of strength that do not adequately control for other intrinsic muscle properties. A generalized loss of skeletal muscle mass with aging, known as sarcopenia, contributes to muscle weakness. In OA patients as well as in aging, the decrement in strength is greater than would be expected for reduction in size of the muscle (10). This suggests that alterations in the intrinsic characteristics of muscle, reflecting a diminished muscle quality, may also contribute to impaired muscle function of the elderly and of those with knee OA. However, there is little information concerning the association of skeletal muscle composition with the prevalence of OA. Lower limb muscle composition and quality is likely related to overall body composition, and obesity has also been implicated as a risk factor for OA, particularly in women (10,11). Many patients with knee OA are obese, and will be stronger than those who are less obese because they have a higher muscle mass as part of a higher total mass. Strength is highly correlated with lean or muscle mass (12). Thus, strength differences in those with and without OA cannot be adequately examined without adjusting for lean mass. One of the most intruiging reports relating body composition and muscle strength to knee OA was that of Slemenda and colleagues (10), who showed that reduced quadriceps strength relative to body weight and lean muscle mass (measured by Dual Energy X-ray Absorptiometry (DXA)) was a risk factor for development of knee OA in women. These results suggest that the quality of muscle is impaired; however, there has been little recent literature to follow-up or confirm these provocative findings. A more recent report by Sharma and colleagues (14) suggested that in some cases greater quadriceps strength may be associated with higher risk of progressive knee OA in patients with greater laxity in their knee. Unfortunately, this study did not include any measures of muscle mass or composition and controlled for body composition only through BMI. However, these results emphasize that the relationship of muscle strength and knee OA may be modified by other factors. Advanced imaging techniques such as computed tomography (CT) can be used to give accurate estimates of lower extremity muscle mass. Lower extremity strength per unit muscle mass, or specific torque, is a measure of muscle quality. In addition, CT can be used to measure muscle attenuation, suggestive of an increased muscle fat content (15), is a marker of muscle quality that contributes to muscle weakness independent of the age-associated loss in muscle mass (16). In the current study, we examined the association of radiographic and symptomatic knee OA with muscle strength, muscle area, intermuscular fat infiltration, and muscle quality using CT in a large, biracial cohort of elderly participants. We hypothesized that both radiographic and symptomatic knee OA would be associated with poorer muscle quality and higher muscle fat content.