Adaptations to isolated shoulder fatigue during simulated repetitive work. Part I: Fatigue Calvin T.F. Tse, Alison C. McDonald, Peter J. Keir ⇑ Occupational Biomechanics Laboratory, Department of Kinesiology, McMaster University, Hamilton, ON L8S 4K1, Canada article info Article history: Received 2 January 2015 Received in revised form 7 July 2015 Accepted 7 July 2015 Available online xxxx Keywords: Repetitive work Muscle fatigue Shoulder Scapular kinematics Control strategy abstract Upper extremity muscle fatigue is challenging to identify during industrial tasks and places changing demands on the shoulder complex that are not fully understood. The purpose of this investigation was to examine adaptation strategies in response to isolated anterior deltoid muscle fatigue while performing simulated repetitive work. Participants completed two blocks of simulated repetitive work separated by an anterior deltoid fatigue protocol; the first block had 20 work cycles and the post-fatigue block had 60 cycles. Each work cycle was 60 s in duration and included 4 tasks: handle pull, cap rotation, drill press and handle push. Surface EMG of 14 muscles and upper body kinematics were recorded. Immediately fol- lowing fatigue, glenohumeral flexion strength was reduced, rating of perceived exertion scores increased and signs of muscle fatigue (increased EMG amplitude, decreased EMG frequency) were present in ante- rior and posterior deltoids, latissimus dorsi and serratus anterior. Along with other kinematic and muscle activity changes, scapular reorientation occurred in all of the simulated tasks and generally served to increase the width of the subacromial space. These findings suggest that immediately following fatigue people adapt by repositioning joints to maintain task performance and may also prioritize maintaining subacromial space width. Ó 2015 Elsevier Ltd. All rights reserved. 1. Introduction Numerous industrial jobs consist of low load, repetitive tasks, which have been identified as increasing risk for developing work- place injuries (Nordander et al., 2009). Along with tissue damage, altered movement patterns have been observed with repetitive work in animal models. Movement pattern changes have the potential to increase, or decrease, exposure over time, and also pro- vide opportunity for rest and recovery (Barbe et al., 2003; Coq et al., 2009; Elliott et al., 2008). The mobility afforded by the human shoulder and the upper extremity may allow workers opportunities to use kinematic and muscle recruitment strategy changes to adapt to the demands of repetitive work, especially when compromised with fatigue. The shoulder complex achieves its large range of motion through the simultaneous motion of its three joints (Inman et al., 1944). Vital to maintaining this range of motion is proper motion of the scapula (Inman et al., 1944; Picco et al., 2010; van der Helm et al., 1995). Scapular position also impacts the space between the acromion and the humeral head that encompasses rotator cuff tendons, known as the subacromial space (SAS) (Banas et al., 1995). The SAS is highly variable between individuals and affected by arm position, scapular rotation, and muscle activity (Banas et al., 1995; Chopp and Dickerson, 2012; Graichen et al., 2005). Muscle attachment sites on the scapula cause its orientation to be affected by changes in muscle activity patterns and fatigue (Ebaugh et al., 2005). For example, a fatiguing external rotation protocol has been shown to lead to increased scapular external rotation, upward rotation, and decreased posterior tilt during hum- eral elevation (Ebaugh et al., 2006a,b; Tsai et al., 2003). The SAS can also be affected with rotator cuff muscle fatigue and humeral head migration during humeral elevation (Chen et al., 1999; Chopp et al., 2011). Repetitive work and muscle fatigue can lead to other kinematic changes in the upper extremity as well. Following fatigue proto- cols, scapulothoracic and glenohumeral changes have been found to be sensitive to elevation angle (Ebaugh et al., 2006a,b; Tsai et al., 2003). Adaptations have also been observed in more complex tasks involving multiple joints and specific performance demands. For example, with repetitive pointing, participants changed their wrist and elbow movements to compensate for altered shoulder position (Fuller et al., 2009). People appear to prioritize http://dx.doi.org/10.1016/j.jelekin.2015.07.003 1050-6411/Ó 2015 Elsevier Ltd. All rights reserved. ⇑ Corresponding author at: McMaster University, Department of Kinesiology, Ivor Wynne Centre, room 212, 1280 Main Street West, Hamilton, ON L8S4K1, Canada. Tel.: +1 905 525 9140x23543. E-mail address: pjkeir@mcmaster.ca (P.J. Keir). Journal of Electromyography and Kinesiology xxx (2015) xxx–xxx Contents lists available at ScienceDirect Journal of Electromyography and Kinesiology journal homepage: www.elsevier.com/locate/jelekin Please cite this article in press as: Tse CTF et al. Adaptations to isolated shoulder fatigue during simulated repetitive work. Part I: Fatigue. J Electromyogr Kinesiol (2015), http://dx.doi.org/10.1016/j.jelekin.2015.07.003