Mechanomyogram and force relationship during voluntary isometric ramp contractions of the biceps brachii muscle

The aim of the present study was to examine the non-stationary mechanomyogr am (MMG) during voluntary isometric ramp contractions of the biceps brachii muscles using the short-time Fourier transform, and to obtain more detaile d information on the motor unit (MU) activation strategy underlying in the continuous MMG/force relationship. The subjects were asked to exert ramp co ntractions from 5% to 80% of the maximal voluntary contraction (MVC) at a c onstant rate of 10% MVC/s. The root mean squared (RMS) amplitude of the MMG began to increase slowly at low levels of force, then there was a slight r eduction between 12% and 20% MVC. After that, a progressive increase was fo llowed by a decrease beyond 60% MVC. As to the mean power frequency (MPF), a relatively rapid increase up to 30% MVC was followed by a period of slow increment between 30% and 50% MVC. Then temporary reduction at around 50% M VC and a further rapid increase above 60% MVC was observed. The interaction between amplitude and MPF of the MMG in relation to the MU activation stra tegy is discussed for five force regions defined on the basis of their infl ection points in the RMS-amplitude/force and MPF/force relationships. It wa s found that the MMG during ramp contractions enables deeper insights into the MU activation strategy than those determined during traditional separat e contractions. In addition, this contraction protocol is useful:not only t o ensure higher force resolution in the MMG/force relationship, but also to markedly shorten the time taken for data acquisition and to reduce the ris k of fatigue.