Muscle coordination and force variability during static and dynamic tracking tasks.

This study examined muscular activity patterns of extensor and flexor muscles and variability of forces during static and dynamic tracking tasks using compensatory and pursuit display. Fourteen volunteers performed isometric actions in two conditions: (i) a static tracking task consisting of flexion/pronation, ulnar deviation, extension/supination and radial deviation of the wrist at 20% maximum voluntary contraction (MVC), and (ii) a dynamic tracking task aiming at following a moving target at 20% MVC in the four directions of contraction. Surface electromyography (SEMG) from extensor carpi ulnaris, extensor carpi radialis, flexor carpi ulnaris and flexor digitorum superficialis muscles and exerted forces in the transverse and sagittal plane were recorded. Normalized root mean square and mutual information (index of functional connectivity within muscles) of SEMGs and the standard deviation and sample entropy of force signals were extracted. Larger SEMG amplitudes were found for the dynamic task (p<.05), while normalized mutual information between muscle pairs was larger for the static task (p<.05). Larger size of variability (standard deviation of force) concomitant with smaller sample entropy was observed for the dynamic task compared with the static task (p<.01 for both). These findings underline a rescaling of the muscles' respective contribution influencing force variability relying on feedback and feed-forward control strategies in relation to display modes during static and dynamic tracking tasks.