Repeatability of surface EMG variables during voluntary isometric contractions of the biceps brachii muscle

The repeatability of initial value and rate of change of mean spectral freq uency (MNF), average rectified values (ARV) and muscle fiber conduction vel ocity (CV) was investigated in the dominant biceps brachii of ten normal su bjects during sustained isometric voluntary contractions. Four levels of co ntraction were studied: 10%, 30%, 50% and 70% of the maximal voluntary cont raction level (MVC), Each contraction was repeated three times in each of t hree different days for a total of nine contractions/level/subject and 90 c ontractions per level across the ten subjects. Repeatability was investigat ed using the Intraclass Correlation Coefficient (ICC) and the standard erro r of the mean (SEM) of the estimates for each subject. Contrary to observat ions in other muscles, CV estimates appeared to be very repeatable both wit hin and between subjects. CV showed a small but significant increase when c ontraction force increased from 10% to 50% MVC but no change for further in crease of force. As force increased, MNF showed a slight decrease possibly related to a wider spreading of the CV values. The rate of time decrement o f MNF and CV increased with the level of contraction. The normalized decrem ent (% of initial value per second) was in general higher for MNF than for CV and was more repeatable between subjects at 10% MVC than at 70% MVC. A f inal observation is that a resting time of 5 minutes may not be sufficient after a contraction at 50% or 70% MVC. (C) 1999 Published by Elsevier Scien ce Ltd. All rights reserved.