Trapezius muscle activity increases during near work activity regardless of accommodation/vergence demand level

European Journal of Applied Physiology, Feb 2015

Aim To investigate if trapezius muscle activity increases over time during visually demanding near work. Methods The vision task consisted of sustained focusing on a contrast-varying black and white Gabor grating. Sixty-six participants with a median age of 38 (range 19–47) fixated the grating from a distance of 65 cm (1.5 D) during four counterbalanced 7-min periods: binocularly through −3.5 D lenses, and monocularly through −3.5 D, 0 D and +3.5 D. Accommodation, heart rate variability and trapezius muscle activity were recorded in parallel. Results General estimating equation analyses showed that trapezius muscle activity increased significantly over time in all four lens conditions. A concurrent effect of accommodation response on trapezius muscle activity was observed with the minus lenses irrespective of whether incongruence between accommodation and convergence was present or not. Conclusions Trapezius muscle activity increased significantly over time during the near work task. The increase in muscle activity over time may be caused by an increased need of mental effort and visual attention to maintain performance during the visual tasks to counteract mental fatigue.

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Trapezius muscle activity increases during near work activity regardless of accommodation/vergence demand level

Trapezius muscle activity increases during near work activity regardless of accommodation/vergence demand level H. O. Richter 0 1 3 C. Zetterberg 0 1 3 M. Forsman 0 1 3 0 C. Zetterberg Section of Occupational and Environmental Medicine, Department of Medical Sciences, Uppsala University , Uppsala , Sweden 1 Communicated by Fausto Baldissera 2 ) C. Zetterberg M. Forsman Department of Occupational and Public Health Sciences, Faculty of Health and Occupational Studies, Centre for Musculoskeletal Research, University of Gavle , 801 76 Gavle , Sweden 3 M. Forsman Institute of Environmental Medicine, Karolinska Institutet , Stockholm , Sweden Aim To investigate if trapezius muscle activity increases over time during visually demanding near work. Methods The vision task consisted of sustained focusing on a contrast-varying black and white Gabor grating. Sixtysix participants with a median age of 38 (range 19-47) fixated the grating from a distance of 65 cm (1.5 D) during four counterbalanced 7-min periods: binocularly through 3.5 D lenses, and monocularly through 3.5 D, 0 D and +3.5 D. Accommodation, heart rate variability and trapezius muscle activity were recorded in parallel. Results General estimating equation analyses showed that trapezius muscle activity increased significantly over time in all four lens conditions. A concurrent effect of accommodation response on trapezius muscle activity was observed with the minus lenses irrespective of whether incongruence between accommodation and convergence was present or not. Conclusions Trapezius muscle activity increased significantly over time during the near work task. The increase in muscle activity over time may be caused by an increased need of mental effort and visual attention to maintain performance during the visual tasks to counteract mental fatigue. Attention fatigue; Accommodation; Bksy test; Compensatory effort; Contrast threshold tracking; Electromyography; Visual ergonomics - Abbreviations BM Binocular minus 3.50 D blurring of grating and compensatory reduction of optical blur by binocular plus accommodation/vergence D Dioptre: the refractive power of a lens or an optical system. It is the reciprocal of distance in meters so that 1 dioptre corresponds to 1 m, 2 dioptres corresponds to 0.50 m, etc ECG Electrocardiography EMG Electromyography GEE General estimating equation MM Monocular minus 3.50 D blurring of grating and compensatory reduction of optical blur by monocular plus accommodation/vergence MN A no-blur monocular reference viewing condition MP Attempts at reduction of optical blur by monocular negative accommodation/vergence in response to +3.50 D blurring of grating RMS Root mean square RVE Reference voluntary electrical activity List of symbols Accommodation refers to the lenticular-based change made in the refractive state of the eye to attain and maintain a maximally high contrast foveal retinal image. There are four components that contribute to the overall accommodative response, namely, blur, disparity, and proximal and tonic accommodation (Ciuffreda 1998). The accommodation/vergence response consists of a tightly coupled triad of eye movements: dioptric (D) adjustment of the crystalline eye lens, convergence/divergence of both eyes toward the locus of fixation, and pupillary constriction/dilatation. The ciliary muscle adjusts the curvature of the lens, thereby changing its refractive power, allowing the formation of clear retinal images of objects located at a different distances (Ciuffreda 1998; Franzn et al. 2000). The accommodative response degrades rapidly if the stimulus target (e.g., an alphanumerical character) is not projected directly into the fovea, the central portion of the retina where sensory photoreceptors responsible for highresolution processing are located (Campbell 1954; Gu and Legge 1987). Campbell (1954) concluded that the photoreceptors involved in the accommodation reflex are the foveal cones and that in the absence of a foveal stimulus the accommodative reflex is not fully elicited. To keep the stimulus target projected into fovea, there is a need for compensatory eye-movements and eye-neck (head) stabilization. A neural command ought to impact on neck/shoulder muscle function through increased static muscle activity. Comparatively little is known about this sort of stabilizing eye-neck muscle synergy (Pelz et al. 2001; Richter et al. 2010). Neck/shoulder muscle activation, measured with surface electromyography (EMG), during simulated near work, has shown that large accommodation response, when the ciliary muscle is highly contracted, is coupled to an increase in trapezius muscle activity level (Richter et al. 2010, 2011). A recent study with relatively low demands on accommodation and convergence, comparable to visual demands needed when working with, e.g., a smart phone or a tablet, showed that incongruence between accommodation and convergence may be an important factor in the relation between (...truncated)


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H. O. Richter, C. Zetterberg, M. Forsman. Trapezius muscle activity increases during near work activity regardless of accommodation/vergence demand level, European Journal of Applied Physiology, 2015, pp. 1501-1512, Volume 115, Issue 7, DOI: 10.1007/s00421-015-3125-9