Motor coordination in a multi-muscle system as revealed by principal components analysis of electromyographic variation

The variation in electromyographic output of twelve trigeminal muscles of t he rabbit was studied to test the hypothesis that they are under the contro l of a small number of independent neural factors. Jaw muscle electromyogra ms (EMGs) of eight animals were recorded in 95 chewing sequences, each cons isting of 40-75 chewing cycles. The within-sequence correlations of the EMG burst amplitudes (integrated per cycle) and burst onsets were calculated b etween the muscles. The correlation matrix was subjected to a principal com ponents analysis. This method aims at describing the variation in EMG ampli tude and timing by means of the smallest possible set of newly defined vari ables, or principal components. Of the variation in EMG amplitude values of the twelve muscles, 75-90% could be accounted for by only three principal components. Each principal component was characterized by a group of muscle s with high mutual positive correlations; they had zero correlation with ot her principal components. The first component represents the jaw dosers: mo st of the bilateral masseter and the medial pterygoid muscles. The second r epresents the openers: the bilateral digastric and lateral pterygoid muscle s. This demonstrates the tight control of both the jaw openers and dosers, each by a single neural factor; these two factors are independent of one an other. They most likely originate from the specific inputs from primary aff erents to the opener and closer motoneurons. Unexpectedly, a third independ ent principal component appeared to control the closing activity of the non -chewing side, posterior deep masseter muscle. It was hypothesized that thi s muscle acts independently of the other dosers to disengage the teeth and resets the jaw for a new chewing cycle. Principal components analysis of va riation in timing of EMG onset revealed a grouping of all masticatory muscl es in a single cluster, independent of EMG amplitude. This supports the hyp othesis that timing and amplitude of masticatory EMG patterns are controlle d independently.