Wrist muscle activation patterns and stiffness associated with stable and unstable mechanical loads.

The objectives of this study were to examine the effects of load mechanical characteristics and agonist-antagonist muscle cocontraction, on the stretch reflex response of wrist flexor muscles, and to measure the associated wrist stiffness. Subjects were required to maintain a constant wrist angle while operating against flexor loads with different stability characteristics (constant, elastic or unstable). We measured the stretch reflex responses and joint stiffness by applying step displacements of 3 degrees and 10 degrees. Subjects used very little cocontraction of wrist flexor and extensor muscles when the load was constant or elastic, but increased cocontraction dramatically when the load was unstable, in order to increase the wrist stiffness. Although the magnitude of stretch reflex responses also increased with cocontraction, this simply reflected the level of tonic flexor muscle activity. We found no evidence to suggest that phasic stretch reflexes contributed significantly to the joint stiffness in this task. Clear differences in flexor muscle synergy were observed in the presence and absence of cocontraction, particularly when comparing the FCR and FCU muscles.