CONTRIBUTION OF THE DIGASTRIC MUSCLES TO THE CONTROL OF BITE FORCE INMAN

The contribution of the (co-contracting) digastric muscles to the rapi d decline in bite-force magnitude after unloading of a static bite was investigated by asking participants to perform two different biting t asks with sudden unloading, and correlating the degree of co-contracti on of the digastrics (as derived from their electromyograms) with the impact force, the impact velocity (as measured after a travel distance of 5 mm), and the residual force when the jaw system was in static co nditions again after the impact. Co-contraction of the digastrics was varied by asking participants to perform the biting task while control ling bite force (force-controlled experiments) or jaw position (positi on-controlled experiments). In half of the experiments, participants c o-contracted their digastrics more strongly in the position-controlled than the force-controlled experiments. However, there was no clear re lation between the level of co-contraction and the magnitude of the im pact force, the impact velocity and the residual force. The results im ply that co-contraction of the digastric muscles is not sufficient to explain the reduction in bite force and the low impact velocity after an unexpected jaw-closing movement. Two other possible mechanisms that reduce forces in an unloaded jaw system are: (1) force-velocity prope rties of the activated jaw muscles in conjunction with creep of the ap oneurotic sheets of the jaw muscles, resulting in a slow partial recov ery of the biting force after impact; (2) force-length properties of j aw-opening muscles, an activity not recorded here. (C) 1997 Elsevier S cience Ltd.