Motor unit correlates of stress: preliminary observations

Motor unit activity was measured during 20 repetitions of [aepae] and 20 repetitions of [aepaé] with emphatic stress placed on the second syllable. A definite pattern of recruitment and discharge reorganization was observed during stressed productions characterized by (1) an increase in motor unit discharge rate, (2) recruitment of additional motor units, (3) a reduction in time intervals separating successively activated motor units, (4) an earlier onset of mandibular lowering for the open vowel in relation to initiation of motor unit activity, (5) fewer discharges of a given motor unit per utterance, and (6) sharply reduced variability in recruitment intervals. The shortened and highly stable latencies between motor unit events and motor unit-articulator events point to a more carefully orchestrated motor program for heavily stressed productions as a more forceful attainment of target specification is carried out.