Neck flexor muscle fatigue is side specific in patients with unilateral neck pain Deborah Falla a, * , Gwendolen Jull a , Alberto Rainoldi b,c , Roberto Merletti b a Department of Physiotherapy, The University of Queensland, Brisbane, Qld 4072, Australia b Dipartimento di Elettronica, Centro di Bioingegneria, Politecnico di Torino, Italy c Department of Physical Medicine and Rehabilitation, University of Tor Vergata and Fondazione Don Gnocchi, Roma, Italy Received 6 February 2003; accepted 29 May 2003 Abstract Despite the evidence of greater fatigability of the cervical flexor muscles in neck pain patients, the effect of unilaterality of neck pain on muscle fatigue has not been investigated. This study compared myoelectric manifestations of sternocleidomastoid (SCM) and anterior scalene (AS) muscle fatigue between the painful and non-painful sides in patients with chronic unilateral neck pain. Myoelectric signals were recorded from the sternal head of SCM and the AS muscles bilaterally during sub-maximal isometric cervical flexion contractions at 25% and 50% of the maximum voluntary contraction (MVC). The time course of the mean power frequency, average rectified value and conduction velocity of the electromyographic signals were calculated to quantify myoelectric manifestations of muscle fatigue. Results revealed greater estimates of the initial value and slope of the mean frequency for both the SCM and AS muscles on the side of the patientÕs neck pain at 25% and 50% of MVC. These results indicate greater myoelectric manifestations of muscle fatigue of the superficial cervical flexor muscles ipsilateral to the side of pain. This suggests a specificity of the effect of pain on muscle function and hence the need for specificity of therapeutic exercise in the management of neck pain patients. Ó 2003 European Federation of Chapters of the International Association for the Study of Pain. Published by Elsevier Ltd. All rights reserved. Keywords: Neck pain; Muscle fatigue; Surface electromyography; Sternocleidomastoid; Anterior scalenes 1. Introduction Greater myoelectric manifestations of muscle fatigue of the sternocleidomastoid (SCM) and anterior scalene (AS) muscles have been demonstrated in chronic neck pain patients with respect to asymptomatic controls (Falla et al., 2003). These results support other clinical studies which have identified a reduction in the strength and endurance capacity of the neck flexor muscles in patients with various neck pain syndromes (Barton and Hayes, 1996; Placzek et al., 1999; Silverman et al., 1991; Treleaven et al., 1994) as well as the findings of muscle biopsy studies of patients with neck pain undergoing spinal surgery, which established an increase in the number of type-IIC transitional fibres suggesting transformation of slow-twitch oxidative type-I fibres to fast-twitch glycolytic type-IIB fibres (Uhlig et al., 1995). Despite the evidence of greater fatigability of the cervical flexor muscles in neck pain patients, the effect of unilaterality of neck pain on muscle fatigue has not been investigated. There is some evidence available to indi- cate that unilateral pain may have a particular effect on muscle function ipsilaterally. Larsson et al. (1998) demonstrated a significant reduction in trapezius muscle blood flow on the painful side concomitant with reduced electromyographic amplitude and mean power fre- quency ipsilateral to symptoms in patients with chronic cervico-brachial pain. A reduction in muscle cross- sectional area has also been demonstrated ipsilateral to the side of pain in patients with unilateral back pain (Dangaria and Naesh, 1998; Hides et al., 1994). These results suggest a specific bias in the effect of pain on European Journal of Pain 8 (2004) 71–77 www.EuropeanJournalPain.com * Corresponding author. Tel.: +61-7-3365-4529; fax: +61-7-3365- 2775. E-mail address: d.falla@shrs.uq.edu.au (D. Falla). 1090-3801/$30 Ó 2003 European Federation of Chapters of the International Association for the Study of Pain. Published by Elsevier Ltd. All rights reserved. doi:10.1016/S1090-3801(03)00075-2