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

Abstract

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.

Fig. 1

Schematic illustration of the accommodative and convergence stimulus (dioptric consequences) for the different lenses used in the experiment (dotted line = accommodative stimulus diopters; full line = convergence stimulus diopters). BM binocular −3.5 D, MM monocular −3.5 D, MN monocular 0 D, and MP monocular +3.5 D

Fig. 2

Frequency counts of individual mean accommodation response for the 7-min vision task for the four lens conditions (accommodation response_{7-min average}). BM binocular −3.5 D, MM monocular −3.5 D, MN monocular 0 D, and MP monocular +3.5 D

Fig. 3

Mean accommodation response for minutes 1 through 7 for the vision task in the four lens conditions (accommodation response_{1-min averages}). Horizontal stippled lines denote stimulus diopters. Error bars represent 95 % CI. BM binocular −3.5 D, MM monocular −3.5 D, MN monocular 0 D, and MP monocular +3.5 D

Fig. 4

Mean of contrast thresholds for minutes 1 through 7 for the vision task for the four lens conditions (contrast thresholds_{1-min averages}). Error bars represent 95 % CI. BM binocular −3.5 D, MM monocular −3.5 D, MN monocular 0 D, and MP monocular +3.5 D

Fig. 5

Mean trapezius muscle activity (% RVE) during rest (white circles) and during the 7-min vision task (black circles) as a function of time for the four lens conditions. Error bars represent 95 % CI. BM binocular −3.5 D, MM monocular −3.5 D, MN monocular 0 D, and MP monocular +3.5 D

Fig. 6

Predicted mean values from the GEE models (Table 2) for the trapezius muscle activity during the 7-min vision task. Error bars represent 95 % CI. BM binocular −3.5 D. MM monocular −3.5 D, MN monocular 0 D, and MP monocular +3.5 D