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To study the orbicularis oculi muscle response to asthenopia-inducing conditions.Twenty subjects (18–36 years) screened for 20/20 vision in each eye participated in the study. Subjects read passages under different asthenopia-inducing conditions. The inducing conditions were glare, low contrast, small font size, refractive error, up gaze, accommodative stress and convergence stress. Surface electromyography (EMG) was used to study the orbicularis oculi response from the right eye. Palpebral fissure height was measured from recorded video images of the right eye. At the end of each condition subjects were asked to rate the severity and type of visual discomfort experienced.Outcome measures for the asthenopia-inducing conditions were compared with their respective nonstress controls. Repeated measures analysis of variance was used to analyze the data. Refractive error (p = 0.0001), glare (p = 0.0001), low contrast (p = 0.007), small font (p = 0.034), and up gaze (p = 0.001) resulted in a significant increase in EMG power. Refractive error (p = 0.0001) and glare (p = 0.0001) also caused significant reduction in aperture size. Reading a low contrast text caused a reduction in blink rate (p = 0.035), whereas refractive error (p = 0.005) and glare (p = 0.01) caused an increase in blink rate. All conditions induced significant visual discomfort (p < 0.001).Refractive error and glare, which reduce image quality and benefit from eyelid squint, caused increased EMG power, eyelid squint response and increased blink rate. Low contrast and small font, which reduce image quality but do not benefit from eyelid squint, resulted in increased EMG power without changes in aperture size and reduced blink rate (for low contrast). Accommodative and convergence stress (in subjects with normal accommodative and vergence abilities) did not cause changes in EMG power, aperture size or blink rate. These results suggest that contraction of the orbicularis oculi is a part of the asthenopia mechanism related to compromised image quality.