The reliability of the interpolated twitch technique during submaximal and maximal isometric muscle actions

The purpose of this study was to examine the test-retest reliability of the percent voluntary activation (%VA) vs. force relationships. Fourteen healthy men (mean 6 SD age = 21 6 2.6 years) and 8 women (age = 21 6 1.8 years) completed 4 maximal voluntary contractions (MVCs) and 9 randomly ordered submaximal isometric plantar flexions from 10 to 100% of the MVC. Transcutaneous electrical stimuli were delivered to the tibial nerve using a highvoltage constant-current stimulator (DS7AH; Digitimer, Herthfordshire, United Kingdom). The %VA was calculated for each maximal and submaximal MVC. Paired-samples t-tests were used to quantify systematic variability, whereas the intraclass correlation coefficients (ICCs), standard error of the mean (%SEM), and minimum differences (%MD; expressed as a percentage of the means) were used for test-retest reliability. Systematic variability was not present at any of the contraction intensities (p . 0.05). The ICCs ranged from 0.52 to 0.84, whereas the %SEM ranged from 6.75 to 38.45%, and the %MD ranged from 18.71 to 106.58%. The ICCs were $0.74 at contraction intensities ranging from 40 to 100% MVC (6.75-16.78% SEM), whereas the ICCs were #0.65 (20.95-38.45% SEM) for the contraction intensities #30% MVC. Although not statistically tested, the ICCs tended to be higher, whereas the %SEMs lower for contractions $40% MVC. Future research using %VA during submaximal contraction intensities to predict a true maximal force may want to exclude contraction intensities ,40% MVC. In addition, caution is warranted when interpreting the changes in the % VA during MVCs after an experimental intervention.