Cervical flexion–relaxation response to neck muscle fatigue in males and females Ashish D. Nimbarte a,⇑ , Majed M. Zreiqat b , Suman Kanti Chowdhury a a Industrial and Management Systems Engineering, West Virginia University, PO Box 6070, Morgantown, WV 26506-6107, United States b Safety Sciences Department, Indiana University of Pennsylvania, 1011 South Drive, Indiana, PA 15705, United States article info Article history: Received 4 July 2014 Received in revised form 9 August 2014 Accepted 1 September 2014 Keywords: Fatigue Gender Flexion–relaxation phenomenon Cervical spine abstract In this study the effect of muscle fatigue on the cervical spine flexion–relaxation response was studied. Twenty healthy participants (10 males and 10 females) were recruited for data collection. The Sorenson protocol was utilized to induce neck muscle fatigue. Surface electromyography and optical motion capture systems were used to measure neck muscle activation and head–neck posture, respectively. A post-fatigue reduction in the Flexion–Relaxation Ratio (FRR) and higher FRR for females compared to males were observed. A post-fatigue decrease was also observed in the onset and offset angles resulting in an expansion of the myoelectric silence period. Gender had no effect on the onset and offset angles of the silence period. Post-fatigue shift in the onset and offset angles and the expansion of the silence period indicate an increased contribution by the passive viscoelastic tissues in stabilizing the cervical spine under fatigued condition. Ó 2014 Elsevier Ltd. All rights reserved. 1. Introduction The work-related musculoskeletal disorders (MSDs) of the neck or cervical spine put a substantial burden on the health and eco- nomics in many industrialized countries. Recent U.S. Bureau of Labor Statistics (BLS) data indicate 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 (BLS, 2012). The symptoms of neck pain often last for prolonged periods of time and reoccurrence is observed for 50–80% of people within five years after the first occurrence (Côté et al., 2008). It is also esti- mated that about 67% of people suffer neck pain at some point in their life (Côté et al., 2004). The etiology of neck or cervical spine MSDs is believed to be multidimensional, with physical work demands, psychosocial stress, and individual characteristics being the major contributing factors. Among the physical demands, work-related exposures such as sustained static postures, sub-maximal repetitive and forceful arm exertions are consistently identified as possible risk factors for cervical spine MSDs. Several studies have shown that the use of static and awkward head–neck postures leads to the fati- gue of neck extensors (Finsen, 1999; Schüldt et al., 1986; Szeto et al., 2002). Repetitive arm and neck exertions are also known to contribute to the fatigue of the neck muscles (Chowdhury et al., 2013; Hagberg, 1981; Hansson et al., 1992). Some studies have also shown an active contribution by the neck muscles during forceful arm exertions leading to the fatigue (Kimura et al., 2007; Nimbarte, 2014; Nimbarte et al., 2012, 2013; Troiano et al., 2008). Among the individual characteristics, gender has been consis- tently identified as a non-modifiable risk factor of MSDs of the neck (Hogg-Johnson et al., 2008). A higher prevalence of work-related neck pain was reported among females than males in several stud- ies. In the United States, one year prevalence of neck pain among the general population was 40% for females compared to 29% for males (Bovim et al., 1994). A similar trend of higher prevalence of neck pain among females was also reported in the studies per- formed among Dutch, Swedish, and Japanese populations (Andersson et al., 1996; Guo et al., 2004; Picavet and Schouten, 2003). A higher prevalence of work-related neck pain for females was also reported in different working populations. Cagnie et al. (2007) reported an annual prevalence of 54.7% and 38.3% for females and males, respectively, among office workers. Fernandes et al. (2011) reported higher neck MSDs among females compared to males in a Brazilian plastics industry. Skov et al. (1996) reported a much higher prevalence of neck pain among the female (76%) sales workers compared to their male (54%) counterparts. The muscle fatigue generated by work-related exertions and its interaction with gender may affect the biomechanical stability of the cervical spine. A less stable spine can be both a cause and http://dx.doi.org/10.1016/j.jelekin.2014.09.002 1050-6411/Ó 2014 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. Tel.: +1 (304) 293 9473; fax: +1 (304) 293 4970. E-mail address: Ashish.Nimbarte@mail.wvu.edu (A.D. Nimbarte). Journal of Electromyography and Kinesiology 24 (2014) 965–971 Contents lists available at ScienceDirect Journal of Electromyography and Kinesiology journal homepage: www.elsevier.com/locate/jelekin