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
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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)