EMG changes in agonist muscles during practice of a multijoint throwing skill

The purpose of this investigation was to determine, for a planar multijoint throwing skill, if electromyographic (EMG) amplitudes of eccentric and concentric muscle activity for two agonist muscles (i.e. agonists for the concentric phase) changed over the course of practice. Eighteen men threw a weighted ball with their dominant arm at a target while the motion was restrained to a horizontal plane. EMG and kinematic data were collected for selected practice trials. EMG signals of the triceps brachii (lateral head) and posterior deltoid were full-wave rectified and integrated (IEMG) over two portions of the movement: the countermovement and the throw. Performance showed an expected improvement over trial blocks. IEMG activities of both muscles did not change across trial blocks for the countermovement (eccentric phase) or for the throw (concentric phase) even though several kinematic and kinetic descriptors changed significantly (e.g. during the throw, peak elbow angular velocity and mechanical work by the elbow extensor moment increased during practice). Results suggest that EMG amplitudes of assistive or auxiliary muscles may change rather than primary agonists or that limitations inherent in skill acquisition research and EMG signal processing may be the reasons why no changes were observed in EMG amplitudes.