The structure, reliability and validity of pain expression: Evidence from patients with shoulder pain Kenneth M. Prkachin a, * , Patricia E. Solomon b a Department of Psychology, University of Northern British Columbia, 3333 University Way, Prince George, BC, Canada V2N 4Z9 b School of Rehabilitation Science, McMaster University, Hamilton, Ontario, Canada Received 23 November 2007; received in revised form 14 April 2008; accepted 15 April 2008 Abstract The present study examined psychometric properties of facial expressions of pain. A diverse sample of 129 people suffering from shoulder pain underwent a battery of active and passive range-of-motion tests to their affected and unaffected limbs. The same tests were repeated on a second occasion. Participants rated the maximum pain induced by each test on three self-report scales. Facial actions were measured with the Facial Action Coding System. Several facial actions discriminated painful from non-painful move- ments; however, brow-lowering, orbit tightening, levator contraction and eye closing appeared to constitute a distinct, unitary action. An index of pain expression based on these actions demonstrated test–retest reliability and concurrent validity with self- reports of pain. The findings support the concept of a core pain expression with desirable psychometric properties. They are also consistent with the suggestion of individual differences in pain expressiveness. Reasons for varying reports of relations between pain expression and self-reports in previous studies are discussed. Ó 2008 International Association for the Study of Pain. Published by Elsevier B.V. All rights reserved. Keywords: Pain; Shoulder pain; Pain expression; Facial expression; Facial Action Coding System 1. Introduction Pain is often accompanied by changes in behavior, among which facial expressions have attracted signifi- cant attention [3,28]. Like expressions of emotion [13], facial expressions during pain are thought to play a critical role communicating information about the experience [5,21,25]. Furthermore, they provide an alternative to common indices of pain, such as verbal reports, and the information they provide may be more useful and valid in certain circumstances [16]. Research into pain expression was stimulated by the development of the Facial Action Coding System (FACS) [4] a technique that allows any visible facial expression to be decomposed into its constituent move- ments. Studies that have applied the FACS have identi- fied several actions that increase in likelihood or intensity during pain. Based on a study of facial expres- sions during four different types of experimental pain, and a review of existing evidence, Prkachin [19] pro- posed that four actions – brow-lowering, tightening the eyelids or raising the cheeks (orbit tightening), nose wrinkling or upper-lip raising (levator contraction) and eye closure – showed sufficient consistency to be consid- ered a ‘‘core” expression of pain. Several studies have reported other actions to be associated with pain. For example, LeResche [17] reported that horizontal lip stretching produced by the risorius muscle, was associated with pain. Likewise, Craig and colleagues [2,18] reported that oblique lip raising, the action of 0304-3959/$34.00 Ó 2008 International Association for the Study of Pain. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.pain.2008.04.010 * Corresponding author. Tel.: +1 250 960 6633; fax: +1 250 960 5744. E-mail address: kmprk@unbc.ca (K.M. Prkachin). www.elsevier.com/locate/pain Pain 139 (2008) 267–274