Motor unit recruitment and EMC power spectra during ramp contractions of abifunctional 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) fl exion and supination tasks. It was confirmed, by the analysis of EMG root-m ean-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-dependen t. In order to compare MPF values calculated from RCs performed under diffe rent mechanical conditions (F, S and F+S), MPF-RMSPSD relationships have be en established (RMSPSD being defined as the power spectrum density RMS). Bo th 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 muscl e activation whatever the task performed, the slope of the MPF-RMSPSD relat ionship was lower in S than in F. These results are in favour of MU recruit ment that is, at least partly, different in F and in S conditions. Dual sub maximal tasks seem to mix the activation of the F and S subpopulations of M Us as revealed by the spectral parameters obtained during F+S ramp contract ions. This study could find some implication in the field of muscle rehabil itation or reinforcement. (C) 2000 Elsevier Science Ltd. All rights reserve d.