IntroductionThe goal of this study is to describe precisely orofacial muscle activities during labial stop consonant production (/p/, /b/), as a function of the vowel context and level of articulatory effort. This work raises several methodological problems: First, the high muscle density of the lip region raises some concerns with the use of surface electromyography to investigate lip movements, since there may be some signal crosstalk between the different pairs of electrodes positioned on different lip muscles. Second, for the production of a labial stop consonant the precise lip movement during the closing and opening gestures is crucial for speech acoustics. We propose to decompose these gestures into several basic phases, lip closing, lip compression and lip opening, during which the precise muscle coordination will be investigated.Material and methodsFour adult subjects without speech disorders were recorded in laboratory conditions while producing the logatoms /lepa/, /leba/, /lepi/, /lebi/, /lepu/ and /lebu/ with 5 levels of articulatory effort. Twelve repetitions of each logatom were recorded for each level of articulatory effort. Three types of signals were simultaneously recorded : the audio signal (at 44.1kHz), high-speed images of the lips (at 200Hz) and the EMG signal of 6 different orofacial muscles recorded at 200Hz with bipolar surface electrodes. The seven pairs of surface electrodes were placed on the superior orbicularis oris (OOS), the inferior orbicularis oris (OOI), the zygomatic (ZYG), the buccinator (BUC), the depressor labii inferioris (DLI), the mentalis (MNT) and the anterior belly of the digastric (DIG)The external and internal lip contours, extracted from the high speed images, were used to segment the speech gesture into four time intervals corresponding to 1- lip closing, 2- lip compression increase, 3- lip compression decrease and 4- lip re-opening. The activity of each muscle was analyzed during these 4 times intervals, considering the full-wave rectified EMG signal.Results Preliminary results suggest that the mentalis plays a major role in the lip closing phase, while the orbicularis oris is crucial for holding the compression. Simultaneous signals observed of electrodes informing on agonist/antagonist muscles suggest the existence of co-activation patterns that will be further investigated. The amplitude of muscle activation depends on the level of effort in a speaker-dependent way..