NORMALIZING UPPER TRAPEZIUS EMG AMPLITUDE - COMPARISON OF RAMP AND CONSTANT FORCE PROCEDURES

The present study compared three procedures for normalization of upper trapezius surface electromyographic (EMG) amplitudes: (a) a ramp proc edure (providing data in per cent of maximal voluntary contraction, MV C); (b) a constant force procedure based on two reference contractions (two-force procedure) (%MVC) and (c) a procedure expressing muscle ac tivation in per cent of a reference voluntary electrical activity (%RV E). The study also evaluated the repeatability of the ramp and the RVE procedures and estimated the force exertion (%MVC) corresponding to t he RVE. To illustrate the ergonomic effect of different normalization procedures, trapezius EMG during two work tasks was compared after nor malization by the two-force and the RVE procedures. Fifteen subjects p articipated in the whole study. We found that force estimates obtained by the ramp procedure equation could be translated to force estimates obtained by the two-force procedure by the equation: %MVC(2force) = - 0.6 + 0.9 %MVC(ramp), although with a considerable imprecision due t o large inter-individual differences. In the ramp procedure, the intra -individual test-retest coefficient of variation (CV) depended on the force level; it was 45% at 5% MVC and 10% at 30% MVC. The CV of the RV E was 15%. The reference contraction used in the RVE procedure corresp onded from 13-79% MVC (median 33%MVC). The load reducing effect of an ergonomic intervention was less obvious with the RVE procedure than wi th the two-force procedure due to a larger inter-individual variation. The advantages and disadvantages of the different procedures are disc ussed.