Abstract
PURPOSE Quadriceps muscle weakness and inhibition have been implicated in both etiology and progression of knee osteoarthritis (OA). Quadriceps inhibition and weakness have been related to age, effusion, pain, inactivity, and inhibitory afferent input from damaged articular surfaces. The purpose of this investigation was to document the extent of quadriceps inhibition and weakness in patients with end-stage knee OA. METHODS Forty-six subjects with knee OA (ave age = 64) were tested prior to unilateral total knee arthroplasty. Quadriceps strength and inhibition were measured using a burst-superimposition technique where a supramaximal burst of electrical stimulation was superimposed on a maximal voluntary isometric contraction. Knee pain during strength testing was verbally scored from zero to ten. Results were compared to 52 healthy age-matched controls. RESULTS Average normalized knee extension force of the involved limb (19.4 N/BMI) was significantly lower than the uninvolved limb (24.0 N/BMI) and controls (24.2 N/BMI). Average quadriceps inhibition was significantly different between the involved limb (11%) and the uninvolved limb (8%) and both were significantly lower than controls (6%). A direct, linear relationship exists between the muscle activation of the involved and uninvolved limb (r = 0.68, p < 0.01). Knee pain during quadriceps contraction was present in only 24% of the subjects and was not significantly related to inhibition. Age did not significantly influence quadriceps activation. CONCLUSIONS The quadriceps of the knee with symptomatic OA was significantly weaker than the asymptomatic side of the patients with OA and age-matched controls. Inhibition, although larger in the involved quadriceps, was still greater in the uninvolved quadriceps than in the controls and was highly correlated between limbs. Unexpectedly, knee pain during quadriceps contraction did not contribute to quadriceps inhibition. Supported by NIH Grant R01 HD041055 and The Foundation for Physical Therapy
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