Minimal detectable change in quadriceps strength and voluntary muscle activation in patients with knee osteoarthritis

OBJECTIVE: To examine the test-retest reliability and quantify the minimal detectable change (MDC) in quadriceps strength and voluntary activation in patients with knee osteoarthritis (OA). DESIGN: Repeated measures over a 1-week interval. SETTING: Tertiary care center. PARTICIPANTS: A convenience sample of patients (N=20) diagnosed with knee OA. INTERVENTION: Isokinetic and isometric quadriceps strength testing and voluntary quadriceps activation testing using interpolated twitch technique. MAIN OUTCOME MEASURES: Peak isokinetic and isometric knee extension torque (Nm) and percentage of voluntary quadriceps activation (%). RESULTS: The mean differences with 95% confidence intervals between the 2 test sessions for quadriceps isokinetic strength, isometric strength, and percent of voluntary activation were -4.34Nm (-14.01 to 5.34Nm), 1.56Nm (-5.56 to 8.68Nm), and 1.34% (-.53 to 3.22%), respectively. The intraclass correlation coefficients for all measures ranged from .93 to .98. The standard errors of measurement (SEMs) for quadriceps isokinetic and isometric strength were 14.57Nm and 10.76Nm, respectively. The SEM for percentage of voluntary activation was 2.84%. Based on these values, the MDCs were 33.90Nm, 25.02Nm, and 6.60% for quadriceps isokinetic strength, isometric strength, and percentage of voluntary activation, respectively. CONCLUSIONS: Maximal quadriceps isokinetic strength, isometric strength, and percentage of voluntary activation measures demonstrate excellent test-retest reliability in patients with knee OA. In addition to research applications, the present findings suggest these measures are appropriate for use when evaluating change in neuromuscular function of the quadriceps in individual patients.