Investigation of the activation of the temporalis and masseter muscles in voluntary and spontaneous smile production Jessica E. Steele a, *, Ian R. Woodcock b , Adrian D. Murphy a , Monique M. Ryan b , Tony J. Penington a , Christopher J. Coombs b a Department of Plastic and Maxillofacial Surgery, Royal Children’s Hospital, Flemington Road, Parkville, Melbourne, Australia b Department of Neurology, Royal Children’s Hospital, Flemington Road, Parkville, Melbourne, Australia Received 13 July 2017; accepted 20 February 2018 KEYWORDS Facial Palsy; Smile restoration; Spontaneous smile; Masseter; Temporalis Summary Introduction: Masticatory muscles or their nerve supply are options for facial reanimation surgery, but their ability to create spontaneous smile has been questioned. This study assessed the percentage of healthy adults who activate the temporalis and masseter muscles during voluntary and spontaneous smile. Methods: Healthy volunteer adults underwent electromyography (EMG) studies of the temporalis and masseter muscles during voluntary and spontaneous smile. Responses were repeated three times and recorded as negative, weakly positive, or strongly positive according to the activity observed. The best response was used for analysis. Results: Thirty healthy adults (median age: 34 years, range: 25–69 years) participated. Overall, 92% of the masseter muscles were activated during voluntary smile (22% strong, 70% weak). Seventy-seven percent of the masseter muscles were activated in spontaneous smile (12% strong, 65% weak). The temporalis muscle was activated in 62% of responses in voluntary smile (15% strong, 47% weak) and in 45% of responses in spontaneous smile (13% strong, 32% weak). No significant difference was found for males vs females or closed vs open mouth smiles. There was no significant difference in responses between voluntary and spontaneous smiles for the temporalis and masseter muscles, and their use in voluntary smile did not predict activity in spontaneous smile. Financial Disclosure Statement: The authors have nothing to disclose. Presentations: Australian Association of Plastic Surgeons Conference, Gold Coast, June 2017. Facial Nerve Symposium, Beverly Hills, Los Angeles, August 2017. * Corresponding author. 48, Ravenswood Road, Balham, London SW12 9PJ, UK. E-mail address: Jessief@doctors.net.uk (J.E. Steele). Journal of Plastic, Reconstructive & Aesthetic Surgery (2018) ■■, ■■–■■ https://doi.org/10.1016/j.bjps.2018.02.013 1748-6815/© 2018 Published by Elsevier Ltd on behalf of British Association of Plastic, Reconstructive and Aesthetic Surgeons. ARTICLE IN PRESS