Quadriceps activation during maximal isometric and isokinetic contractions : The minimal real difference and its implications

BACKGROUND:A method of measurement of voluntary activation (VA, percent of full muscle recruitment) during isometricand isokinetic concentric contractions of the quadriceps femoris (QF) at 60◦/s and 120◦/s was previously validated.OBJECTIVE:This study aimed to quantify the test-retest minimal real difference (MRD) of VA during isometric (ISOM) andisokinetic concentric contractions of QF (100◦/s, ISOK) in a sample of healthy individuals.METHODS:VA was measured through the interpolated twitch technique. Pairs of electrical stimuli were delivered to the QF at40◦of knee flexion during maximal voluntary contractions. Twenty-five healthy participants (20–38 years, 12 women, 13 men)completed two testing sessions with a 14-day interval. VA values were linearized through logit transformation (VAl). The MRDwas estimated from intraclass correlation coefficients (model 2.1).RESULTS:The VA (median, range) was 84.20% (38.2–99.9%) in ISOM and 94.22% (33.8−100%) in ISOK. MRD was 0.78 and1.12 logit for ISOM and ISOK, respectively. As an example, in terms of percent VA these values correspond to a change from 76%to 95% and from 79% to 98% in ISOM and in ISOK, respectively.CONCLUSIONS:The provided MRD values allow to detect significant individual changes in VA, as expected after training andrehabilitation programs.