Inter-muscular coordination of forearms and hands during sustained grip contraction

The purpose of this study was to explore the relationship between grip strength and inter-muscular coordination of the forearms and hands. Twenty-four healthy young volunteers participated in the experiment. The subjects were instructed to grip hydraulic hand dynamometer with five force levels-maximal voluntary contraction (MVC), 70% MVC, 50% MVC, 30% MVC and without any contraction. Muscle activity was described by sEMG simultaneously collected from the brachioradialis (BR), flexor carpi ulnaris (FCU), flexor carpi radialis (FCR), extensor digitorum communis (EDC), flexor digitorum superficialis (FDS), abductor pollicis brevis (APB), first dorsal interosseous (FDI) and abductor digiti minimi (ADM). Results showed that strong correlations existed between the grip force and the sEMG amplitude. Furthermore there were increased inter-muscular coordination with increasing grip force. As the force level increased, the fluctuations of the grip forces during stable contraction (the force levels of 70% MVC, 50% MVC and 30% MVC) increased, whereas the inter-muscular coordination showed similar patterns. In contrast to the relationship between the force and the sEMG amplitude, the grip force is negatively correlated with the median frequency (MF) of sEMG. Besides, Forearm muscles correlation between the grip force and the MF of individual muscles was significantly higher than the hand muscles.