Effect of muscle load tasks with maximal isometric contractions on oxygenation of the trapezius muscle and sympathetic nervous activity in females with chronic neck and shoulder pain

Shiro et al. BMC Musculoskeletal Disorders 2012, 13:146 http://www.biomedcentral.com/1471-2474/13/146 RESEARCH ARTICLE Open Access Effect of muscle load tasks with maximal isometric contractions on oxygenation of the trapezius muscle and sympathetic nervous activity in females with chronic neck and shoulder pain Yukiko Shiro1, Young-Chang P Arai2*, Takako Matsubara2,3, Shunsuke Isogai4 and Takahiro Ushida2 Abstract Background: Sympathetic nervous activity contributes to the maintenance of muscle oxygenation. However, patients with chronic pain may suffer from autonomic dysfunction. Furthermore, insufficient muscle oxygenation is observed among workers with chronic neck and shoulder pain. The aim of our study was to investigate how muscle load tasks affect sympathetic nervous activity and changes in oxygenation of the trapezius muscles in subjects with chronic neck and shoulder pain. Methods: Thirty females were assigned to two groups: a pain group consisting of subjects with chronic neck and shoulder pain and a control group consisting of asymptomatic subjects. The participants performed three sets of isometric exercise in an upright position; they contracted their trapezius muscles with maximum effort and let the muscles relax (Relax). Autonomic nervous activity and oxygenation of the trapezius muscles were measured by heart rate variability (HRV) and Near-Infrared Spectroscopy. Results: Oxyhemoglobin and total hemoglobin of the trapezius muscles in the pain group were lower during the Relax period compared with the control group. In addition, the low frequency / high frequency (LF/HF) ratio of HRV significantly increased during isometric exercise in the control group, whereas there were no significant changes in the pain group. Conclusions: Subjects with neck and shoulder pain showed lower oxygenation and blood flow of the trapezius muscles responding to isometric exercise, compared with asymptomatic subjects. Subjects with neck and shoulder pain also showed no significant changes in the LF/HF ratio of HRV responding to isometric exercise, which would imply a reduction in sympathetic nervous activity. Keywords: Chronic neck and shoulder pain, Sympathetic nervous activity, Muscle oxygenation * Correspondence: 2 Multidisciplinary Pain Centre, Aichi Medical University, School of Medicine, Nagakute, Japan Full list of author information is available at the end of the article © 2012 Shiro et al.; licensee BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Shiro et al. BMC Musculoskeletal Disorders 2012, 13:146 http://www.biomedcentral.com/1471-2474/13/146 Background Chronic neck and shoulder pain is very common symptom especially in females. A systematic review reports that arm force, arm posture, duration of sitting, work place design [1], repetitive hand and finger movements and monotonous work task [2] cause neck and shoulder disorders. In particular, a correlation has been identified between neck and shoulder pain and the trapezius muscle [3,4]. The trapezius muscle is well capillarized [4,5]. Most hypotheses for the development and maintenance of work-related muscle pain propose that muscle-cell activity produces energy demands that are not met by circulation, thereby resulting in hypoxia, energy crisis, and accumulation of metabolites in the muscle [6]. Several studies showed that metabolic insufficiencies are related to pain perception of workers with trapezius myalgia [7,8]. Furthermore, impaired regulation of microcirculation occurs in the trapezius muscle in cases of chronic neck pain [4,5] and insufficient muscle blood flow and oxygenation have been observed among workers with chronic neck and shoulder complaints [7,8]. In contrast, activation of skeletal muscle fibers by somatic nerves leads to vasodilation. A previous study showed that low-level static contraction did increase trapezius muscle blood flux [9]. A lot of the results in an association between muscle activity and pain are not consistent. The pathogenic mechanisms of chronic neck and shoulder pain development are likely to be multifactorial. Sympathetic nerve activity contributes to vasoconstriction and the maintenance of arterial blood pressure [10]. However, several studies have demonstrated autonomic dysfunction in chronic pain syndromes. Impaired function of sympathetic nerves was observed in patients with complex regional pain syndrome (CRPS) [11]. Furthermore, patients with fibromyalgia showed autonomic dysfunction characterized by persistent autonomic nervous system hyperactivity at rest and hypo-reactivity during stress [12]. Thus, we speculate that there may be autonomic dysfunction in subjects with chronic neck and shoulder pain. Heart rate variability (HRV) has been used as a biomarker of autonomic nervous system function. HRV is a reliable method to obtain information on sympathetic and parasympathetic contributions to heart rate, and several studies have shown that pain increases sympathetic activity [13]. Frequency fluctuations of HRV in the range of 0.04-0.15 Hz (low frequency, LF) are considered to be markers of sympathetic and parasympathetic nerve activity, and high frequency (HF) fluctuations in the range of 0.15-0.4 Hz are considered markers of parasympathetic nerve activity. Thus, the LF/HF ratio is considered to be an index of sympathetic nerve activity [13,14]. We hypothesized that subjects with chronic neck and shoulder pain would have autonomic dysfunction to Page 2 of 7 muscle load, thereby leading to insufficient muscle blood flow and oxygenation of the trapezius muscle. The aim of the present study was to see how heart rate variability and oxygenation of the trapezius muscle respond to muscle load tasks with maximal isometric contraction in subjects with chronic neck and shoulder pain. Methods After receiving approval from the Nagoya Gakuin University Board of Ethics and obtaining written informed consent, 30 female participants were recruited for the present study. Exclusion criteria were serious conditions such as previous trauma to the neck or shoulder, cardiovascular or neurological disease, diabetes, menstruation, or administration of sedatives, analgesics, or other medication. Participants were assessed on pain intensity using a verbal rating scale (VRS) and visual analogue scale (VAS), pain-related disability using Neck Disability Index (NDI). For the VRS, the intensity of neck pain was rated on a numerical scale from 0 to 3 (0 = no pain, 1 = mild pain, 2 = moderate pain, and 3 = severe pain). For the VAS, the pain intensity was assessed using a horizontal 100-mm line with the words “no pain” at one end and “worst pain imaginable” at the other. The participants were assigned into two groups; a pain group consistin (...truncated)