Myoelectric manifestations of fatigue at low contraction levels in subjects with and without chronic pain Laura A.C. Kallenberg a, * , Elke Schulte b , Catherine Disselhorst-Klug b , Hermie J. Hermens a,c a Roessingh Research and Development, 7500 AH Enschede, P.O. Box 310, The Netherlands b Helmholtz-Institute–Chair for Applied Medical Engineering, Aachen, Germany c University of Twente, Faculty of electrical engineering, mathematics and informatics, Enschede, The Netherlands Received 12 June 2005; received in revised form 15 April 2006; accepted 26 April 2006 Abstract The aim of the present study was to investigate differences in myoelectric responses to fatigue development between cases with chronic neck-shoulder pain (n = 10) and healthy controls (n = 10) during a low force level sustained contraction. Subjects performed a 15-min isometric shoulder elevation at a force level of 40 N (sustained contraction), preceded and followed by a step contraction, consisting of five force levels from 20 to 100 N. EMG recordings were made with a two-dimensional electrode array on the upper trapezius of the dominant side. Root-mean-square (RMS G ), median power frequency (FMED G ), conduction velocity (CV), number of motor unit action potentials per second (MUAP Rate) and MUAP shape properties were estimated. Changes over time and differences between the groups were statistically evaluated with a linear mixed model. During the sustained contraction, cases showed less increase in RMS G than controls (controls: 58.5%, cases: 33.0%). FMED G and CV decreased in controls (FMED G : À6.3%, CV: À5.3%) and stayed constant (FMED G ) or slightly increased (CV, 3.15%) in cases. Overall, cases showed a less pronounced myoelectric response to the fatiguing task than controls, which may be related to additional recruitment of higher-threshold MUs. A possible explanation might be that cases were already (chronically) fatigued before the experiment started. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Fatigue; Chronic pain; Multi-channel electrode arrays; Electromyography 1. Introduction Chronic muscular pain is becoming an increasingly important problem in western countries. The Third Euro- pean Survey on working conditions in acceding and candi- date EU countries in 2000 (Paoli and Parent-Thirion, 2003) revealed that 23% of the workers in the EU countries report muscular pain in the neck-shoulder region. Muscu- lar pain is the third important work-related health problem in the EU, only preceded by backache (34%) and stress (28%). Chronic pain has been studied in relation to muscle activity patterns by surface electromyography (EMG). Sev- eral authors reported that EMG activity during work tasks was increased in patients with neck-shoulder complaints (Veiersted et al., 1990; Veiersted, 1994; Lundberg et al., 1999; Madeleine et al., 2003). Other studies demonstrated an inability to relax in trapezius myalgia patients (Elert et al., 1992). Furthermore, a decreased percentage of mus- cle rest, measured as short silent periods in the EMG (‘gaps’), has been demonstrated in chronic pain patients (Veiersted, 1994; Ha ¨gg and A ˚ stro ¨m, 1997). In another 1050-6411/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.jelekin.2006.04.004 * Corresponding author. Tel.: +31 53 4875777; fax: +31 53 4340849. E-mail address: l.kallenberg@rrd.nl (L.A.C. Kallenberg). Journal of Electromyography and Kinesiology 17 (2007) 264–274 www.elsevier.com/locate/jelekin