Relations between required accuracy and muscle synergy in isometric contraction tasks

Humans modify their movement characteristics according to the requirement of the movement accuracy. The relations between the required accuracy and muscle activities were previously determined by varying the size of the targets to be reached in isotonic contraction tasks. However, in isometric contraction tasks, such relations have not been studied yet. In this study, we examined whether there was a difference in the movement characteristics depending on the sizes of the targets being introduced. We observed that the distribution of the forces at the end of the movement varied with the target sizes, similar to results of previous studies based on isotonic contractions. Furthermore, we noted a trade-off relation between the required accuracy and co-contraction of the muscles. When we examined the muscle synergies, we observed that for some subjects, the number of synergies increased with the increase in target size. This result suggested that the strategy of motor control might be modified depending on the requirement of accuracy of the isometric reaching task considered herein.