Impact of shoulder position and fatigue on the flexion–relaxation response in cervical spine Ashish D. Nimbarte ⁎, Majed Zreiqat, Xiaopeng Ning Department of Industrial and Management Systems Engineering, PO Box 6070, West Virginia University, Morgantown, WV 26506-6107, USA abstract article info Article history: Received 12 June 2013 Accepted 3 December 2013 Keywords: Cervical spine Flexion–relaxation phenomenon Fatigue Shoulder posture Background: Neck pain is common among general population with a high prevalence among the people who are routinely exposed to prolonged use of static head–neck postures. Prolonged static loading can cause localized muscle fatigue which may impact the stability of the cervical spine. In this study, flexion–relaxation phenomenon was used to study the post fatigue changes in the stability of cervical spine by evaluating the synergistic load sharing between muscles and viscoelastic elements. Methods: Thirteen male participants were recruited for data collection. The variables that influence cervical flex- ion–relaxation were studied pre- and post-fatigue using neutral and shrugged shoulder postures. The Sorensen protocol was used to induce neck extensor fatigue. Surface electromyography and optical motion capture sys- tems were used to record neck muscle activation and head posture, respectively. Findings: The flexion–relaxation phenomenon was observed only in the neutral shoulder position pre- and post- fatigue. The flexion relaxation ratio decreased significantly post-fatigue in neutral shoulder position but remained unchanged in shrugged shoulder position. The onset and offset angles and the corresponding durations of the silence period were significantly affected by the fatigue causing a post-fatigue expansion of silence period. Interpretation: The muscular fatigue of neck extensors and shoulder position was found to modulate the cervical flexion–relaxation phenomenon. Early shifting of load sharing under fatigued condition indicates increased de- mands on the passive tissues to stabilize the cervical spine. Shrugging of shoulder seems to alter muscular de- mands of neck extensors and make cervical flexion–relaxation phenomenon disappear due to continuous activation of the neck extensors. © 2013 Elsevier Ltd. All rights reserved. 1. Introduction Musculoskeletal disorders (MSD) of the neck are common. Among the general population, an annual prevalence of neck pain was reported to be between 30 and 50% (Hogg-Johnson et al., 2008a). It is also estimated that about 67% of the people suffer neck pain at some point in their life (Côté et al., 2004). Recent Unites States Bureau of Labor Statistics (US BLS) data indicated that work-related neck pain requires a median of 11 days away from work to recuperate compared to 5 days for all other body parts combined. The economic impact of work related neck pain may vary considerably between the working populations; the social im- pact is enormous in terms of human suffering and morbidity. The occupations that see relatively high prevalence of neck pain in- cludes video display terminal (VDT) user, surgeons, sewing machine operator, and dentist (Côté et al., 2009; Hagberg and Wegman, 1987). The work activities by these occupations demand use of static and/or awkward head–neck postures for sustained durations which can cause static loading of the musculoskeletal structures. Prolonged static loading leads to localized muscle fatigue which may impact the stability of the cervical spine. A less stable spine can be both a cause and consequence of spinal pain (Ariens et al., 2000; Ariëns et al., 2001; Palmer et al., 2001). Spinal stability is achieved by the highly coordinated interaction of active and passive components of the neuromuscular systems. Viscoelas- tic behavior of the neck system, baseline muscle activation and reflexive activation of muscles stabilize the cervical spine (Simoneau et al., 2008). The viscoelastic behavior contributes to the passive stiffness, whereas ac- tive stiffness is provided by both baseline muscle activation and reflexive activation of muscles (Hendershot et al., 2011). A few studies on the lum- bar spine have reported a reduction in passive support of the spine fol- lowing a prolonged trunk flexion (Sánchez-Zuriaga et al., 2010; Shin and Mirka, 2007) and proposed that the active stiffness plays an impor- tant role to maintain spinal stability (Hendershot et al., 2011). Baseline activation is typically measured using the flexion–relaxation phenome- non (FRP) and sudden perturbation protocols are used to quantify the muscle reflex responses (Olson et al., 2004; Solomonow et al., 2003). Prior cervical spine studies based on the sudden perturbation protocol primarily looked at the effect of load, torque and torso acceleration to better understand the mechanism of whiplash injuries (Forbes et al., 2012; Simoneau et al., 2008; Tangorra et al., 2003). The FRP explains the synergistic load sharing between the muscles and viscoelastic elements (ligaments, disks, capsules, and fascia). For the cervical spine, during head flexion, cervical extensors gradually increase Clinical Biomechanics 29 (2014) 277–282 ⁎ Corresponding author. E-mail address: nimbarte.ashish@gmail.com (A.D. Nimbarte). 0268-0033/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.clinbiomech.2013.12.003 Contents lists available at ScienceDirect Clinical Biomechanics journal homepage: www.elsevier.com/locate/clinbiomech