RELATIONSHIP BETWEEN FORCE AND INTEGRATED EMG ACTIVITY DURING VOLUNTARY ISOMETRIC ANISOTONIC CONTRACTION

The surface electromyogram (EMG) was recorded for the extensor carpi radiales of normal human subjects during voluntary isometric anisotonic contractions and the force generated by the muscle. The relationship between force [F] and instantaneous integrated EMG [iEMG] could not be fitted by a single linear function or a parabola. The muscle was then considered as a system presenting a maximum force and in which the EMG activity represented the input and the contraction force the output, i.e., F = f(iEMG) and not, as is usually done, iEMG = f(F). The summed iEMG from the beginning of the contraction was used since the actual force depends on all preceding events. These findings led to testing for a non-linear and asymptotic function relating F to iEMG. A double exponential function could account for the 2 mechanisms by which the force of a voluntary contraction is increased.