Coupling dynamics interlip coordination in lower lip load compensation.

PURPOSE To study the effects of lower lip loading on lower and upper lip movements and their coordination to test predictions on coupling dynamics derived from studies in limb control. METHOD Movement data were acquired using electromagnetic midsagittal articulography under 4 conditions: (a) without restrictions, serving as a baseline; (b) with a small carrier device attached to the lower lip; (c) with a 50-g weight added to the device; and, at the end of the session (d) with the weight and device removed. For all conditions, 8 participants repeated nonwords at 2 speaking rates. Movement data were used to derive discrete kinematic measures, a cyclic index of spatiotemporal variability, phase deviations, and standard deviations of relative phase for interlip coupling. RESULTS Kinematic variables were not systematically affected by lower lip load. Phase deviations also showed no change, but in contrast, phase variability showed a significant increase for the lower lip load condition at fast rates. CONCLUSION Lower lip load effects are comparable to the reported impact of homologous limb loading, showing evidence for a tight coupling between both lips in line with predictions from coordination dynamics accounts in the literature.