Motor unit recruitment and EMC power spectra during ramp contractions of a bifunctional muscle

Surface electromyograms (EMGs) were analysed on the short and long head of the biceps brachii (BBSH and BBLH) during single (F and S) or dual (F+S) flexion and supination tasks. It was confirmed, by the analysis of EMG root-mean-square (RMS) values, that the highest activations of BBSH and BBLH were obtained during a maximal dual task. This study was essentially concerned with the analysis of power spectra data obtained during progressive or ramp contractions (RCs). The shape of the power spectra established during the first second of the RCs differs between F, S and F+S tasks. Differences in mean power frequency (MPF) calculated during RCs would be representative of a recruitment of motor units (MUs) that is, at least partly, task-dependent. In order to compare MPF values calculated from RCs performed under different mechanical conditions (F, S and F+S), MPF-RMSPSD relationships have been established (RMSPSD being defined as the power spectrum density RMS). Both BBSH and BBLH exhibited initial MPF values higher in supination RC than in flexion RC. Because of plateau values reached at the same level of muscle activation whatever the task performed, the slope of the MPF-RMSPSD relationship was lower in S than in F. These results are in favour of MU recruitment that is, at least partly, different in F and in S conditions. Dual submaximal tasks seem to mix the activation of the F and S subpopulations of MUs as revealed by the spectral parameters obtained during F+S ramp contractions. This study could find some implication in the field of muscle rehabilitation or reinforcement. (C) 2000 Elsevier Science Ltd. All rights reserved.