Intra-rater reliability of electromyographic recordings and subjective evaluation of neck muscle fatigue among helicopter pilots Marcus Thuresson a, * , Bjo ¨rn A ¨ ng a , Jan Linder c , Karin Harms-Ringdahl a,b a Section of Physiotherapy, Neurotec Department, Karolinska Institutet, Stockholm, Sweden b Karolinska University Hospital, Stockholm, Sweden c The Armed Forces Headquarters, Aeromedical Section, Stockholm, Sweden Abstract The aim was to evaluate the reliability of a method of measuring neck muscle fatigue among helicopter pilots. Method: Surface EMG from three areas in the neck region, bilaterally, was recorded among 10 male helicopter pilots while they were performing isometric contractions in flexion and extension for 45 s, sustaining a force representing 75% of maximum strength in a seated position. Perceived fatigue was rated using the Borg CR-10 scale. The test was repeated twice the first day and then two additional times with one-week intervals. Variables analyzed were the slope of the median frequency change, the normalized slope, and the ratings after 15, 30 and 45 s; and also the initial median frequency (IMDF). The intra-class correlation (ICC) and the mea- surement error (S w ), intra- and inter-day were calculated statistically. Results: The best reliability for the slope was found for the 45 s intra-day analysis taking all measurements into account (ICC 0.65–0.83). The reliability after 30 s was poorer but still acceptable (ICC 0.52–0.71). For the subjective ratings, the highest reliability was found after 30 s inter-day (ICC 0.86–0.88). IMDF showed generally high reliability for the intra-day analyses (ICC 0.63–0.80). Conclusion: The method is reliable for use in further research. Since performing a contraction of 75% of maximum was quite strenuous, we recommend that the protocol be shortened to 30 s. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: Borg scale; EMG; ICC; Measurement error; Repeatability 1. Introduction The prevalence of neck pain among helicopter pilots is reportedly between 29% and 48% [2,8,38]. Knowledge of the etiology of the high levels of reported neck pain is still incomplete. However, unfavorable positions with static neck and body angles, due to helicopter pilotsÕ working positions in the cabin, whole-body vibration and enlarged head worn equipment have been suggested as risk factors [22,38]. One and one these risk factors have also been sug- gested as causing muscle fatigue over time. Pope et al. [33] showed that helicopter pilotsÕ lower-back muscle fa- tigue increased after exposure to whole-body vibration. Phillips and Petrofsky [30–32], showed that increased neck load increased fatigue of the neck muscles. 1.1. Neck muscle fatigue Evaluation of selective muscle fatigue in the cervical muscles using electromyography is somewhat compli- 1050-6411/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jelekin.2004.11.001 * Corresponding author. Present address: Department of Rehabil- itation Medicine, S1:01 Karolinska Hospital/Institutet, SE-171 76 Stockholm, Sweden. Tel.: +46 8 517 754 38; fax: +46 8 517 766 41. E-mail address: marcus.thuresson@neurotec.ki.se (M. Thuresson). Journal of Electromyography and Kinesiology 15 (2005) 323–331 www.elsevier.com/locate/jelekin