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 as well as the force generated by the muscle. The relationship between force and instantaneous integrated EMG could not be fitted by a single linear function or a parabola. The muscle was then considered as a system (A) presenting a maximum force and (B) in which the EMG activity represented the input and the contraction force the output, that is F = f(iEMG) and not, as is usually done, iEMG = f(F). In addition the summed iEMG from the beginning of the contraction was used since the actual force depends on all preceding events. These findings let to testing for a non-linear and asymptotic function relating F to iEMG. A double exponential function: (formula: see text) could account for the two mechanisms by which the force of a voluntary contraction is increased.