Synergistic and antagonistic interactions in the rat forelimb: acute effects of coactivation.

The goals of the present study were 1) to assess effects of antagonist coactivation on mechanical interactions between synergistic muscles, and 2) to quantify the extent of epimuscular myofascial force transmission between synergistic and antagonistic muscles in the rat forelimb. Connective tissues enveloping the muscle bellies in the antebrachium were left intact. Forces exerted at the distal tendons of flexor carpi ulnaris (FCU), palmaris longus (PL), and extensor carpi ulnaris (ECU) muscles were measured at various FCU lengths for two different stimulation protocols: 1) simultaneous stimulation of ulnar/median nerve complex (exciting all wrist flexors, including synergistic FCU and PL) and radial nerve (exciting all wrist extensors, including antagonistic ECU); and 2) stimulation of the ulnar/median nerve exclusively. PL and ECU were kept at a constant length. In addition, muscle forces were measured during stimulation of one of the indicated nerves, with later addition of stimulation of the second nerve during the maintained tetanic contraction. Coactivation of antagonistic muscles increased FCU isometric forces (on average, by 10% of optimal force) and PL forces (on average, by 13% of maximal force), but mechanical interaction between FCU and PL was unchanged. Changing the length and relative position of FCU significantly affected PL (by 20%) as well as ECU forces (by 8%). In addition, distal tetanic force of FCU kept at a constant high length was determined by the order of nerve stimulation onset. These results indicate effects of myofascial pathways between synergistic and antagonistic muscles in the rat forelimb. Coactivation may enhance the stiffness of connective tissues between muscles, but the present data suggest that activation of all wrist flexors already preloaded the myofascial pathways to the greatest extent. The stimulation order effects were explained by dynamic features of muscle and connective tissues (i.e., length-history dependence and viscoelasticity).