COUPLING ELECTRICAL AND MECHANICAL OUTPUTS OF HUMAN JAW MUSCLES UNDERTAKING MULTIDIRECTIONAL BITE-FORCE TASKS

Previous studies have identified both linear and curvilinear relations between increasing bite-force magnitude and the integrated electromyo gram (EMG) of jaw-closing muscles. In an attempt to explain the discre pancy, bite forces of incrementally increasing magnitude were produced on the right-hand side in five specified directions by eight humans. Linear regression lines were fitted to nor malized EMG activities of t he left and right masseter and temporalis muscles against increasing b ite-force magnitude in each direction. The grand mean of linear correl ation coefficients was 0.79 (+/-0.11 SD), suggesting an overall linear relation. Each set of individual data was fitted with polynomial line s up to the third order. The 'best' fit was selected by statistical si gnificance of coefficients and the least square analysis of the sum of residues for each filled line; 62% of individual data-sets were best fitted with linear regression lines, 31% with quadratic lines and the remaining 7% with cubic lines. Repeated analysis of residue variance o f the pooled data showed that either a linear or quadratic line fitted every data set except one, for which a cubic line had the best fit. W orking-side muscles had significantly larger linear correlation coeffi cients than corresponding balancing-side muscles for most bite-force d irections. Analysis of variance of linear correlation coefficients rev ealed that the degree of linearity often depended upon the roles playe d by a muscle in producing forces in different directions. It appears that linearity or non-linearity of the EMG-force relation is a determi nant, among other variables, of the direction of the resultant force. (C) 1997 Elsevier Science Ltd.