Development of chewing in children from 12 to 48 months: longitudinal study of EMG patterns

Abstract

Developmental changes in the coordinative organization of masticatory muscles were examined longitudinally in four children over 49 experimental sessions spanning the age range of 12-48 mo. Electromyographic (EMG) records were obtained for right and left masseter muscles, right and left temporalis muscles, and the anterior belly of the digastric. Two independent analytic processes were employed, one that relied on identification of onset and offset of muscle activation and a second that used pairwise cross-correlational techniques. The results of these two analyses, which were found to be consistent with each other, demonstrated that the basic chewing pattern of reciprocally activated antagonistic muscle groups is established by 12 mo of age. Nevertheless, chewing efficiency appears to be improved through a variety of changes in the chewing pattern throughout early development. Coupling of activity among the jaw elevator muscles was shown to strengthen with maturation, and the synchrony of onset and offset of these muscles also increased. Coactivation of antagonistic muscles decreased significantly with development. This decrease in antagonistic coactivation and increase in synchrony among jaw elevators, and a parallel decrease in EMG burst duration, were taken as evidence of increased chewing efficiency. No significant differences in the frequency of chewing were found across the ages studied. Additional considerations include the appropriateness of this coordinative infrastructure for other developing oromotor skills, such as speech production. It is suggested that the relatively fixed coordinative framework for chewing exhibited by these children would not be suitable for adaptation to speech movements, which have been shown to rely on a much more variable and adjustable coordinative organization.

FIG. 1

Electromyographic (EMG) activity during chewing by the same subject at 12 and 48 mo of age. Left: raw EMG activity. Right: rectified and filtered signals. Modulation of the EMG signals at 12 mo is much more poorly defined than in the later samples. RMass, right masseter; LMass, left masseter; RTemp, right temporalis; LTemp, left temporalis; ABD, anterior belly of digastric.

FIG. 2

Top: cross-correlation (RMass × ABD) and autocorrelation (RMass) functions of chewing data collected at 48 mo. Peak coefficient and associated lag were extracted from cross-correlation function for subsequent analysis. Bottom: chewing rate was calculated by identifying the dominant peak in the spectrum of the autocorrelation.

FIG. 3

Horizontal bars: EMG activity from (top to bottom in each group of bars) RMass, LMass, RTemp, LTemp, and ABD averaged across all repetitions and all subjects. Error bars: average within-subject SD. EMG onsets and offsets were calculated relative to onset of RMass.

FIG. 4

Duration of overlap between antagonistic muscle pairs averaged across all repetitions, all antagonistic muscle pairs, and all subjects. Duration of overlap was found to decrease with age, as did variability of overlap.

FIG. 5

Vertical bars: peak coefficients obtained from pairwise cross-correlations, averaged across all repetitions and all subjects. Error bars: average within-subject SD. For each age, from left to right, 1st 6 bars represent synergistic muscle pairs; the last 4 bars represent antagonistic pairs. Greater values were interpreted as representing increases in coupling of activation. Increases in coupling were observed in both synergistic and antagonistic muscles pairs.

FIG. 6

Vertical bar height corresponds to absolute lag value averaged across all repetitions and subjects for a given muscle pair. Error bars: average within-subject SD. For each age, from left to right, 1st 6 bars represent synergistic muscle pairs; the last 4 are antagonistic muscle pairs. Increase in synchrony among synergists with age was statistically significant.

FIG. 7

Chewing rate averaged across all repetitions and all subjects within each age level. Variability of chewing rate decreased with age, although chewing frequency itself did not vary substantially across ages.