Effects of environmental illumination and screen brightness settings on upper limb and axial skeleton parameters: how do users adapt postures?

79% of smartphone users carry their phone 22 hours a day. In this context, the main task worldwide, texting, is performed under a wide range of light and position conditions. The aim of this study was to test the effects of environmental illumination and screen brightness settings on upper limb and axial skeleton parameters. Twelve subjects performed three trials of texting under three experimental positions, two screen luminance settings and three environmental light conditions. 3 D axial skeleton and upper limb angles, smartphone orientation and face-to-smartphone distance were used as dependent variables. High environmental illumination and/or low screen brightness resulted in an increase in interaction time and a reduction in the face-to-smartphone distance by approximately 10%. Subjects attempted to compensate for the unfavourable effects of such light conditions by adopting postures rated 5 in the Rapid Upper Limb Assessment, indicating an increased risk of developing musculoskeletal disorders. Practitioner's summary: The purpose of the study was to quantify the joint angles of the upper body in experimental conditions that represent daily life. Postures were influenced by ambient illumination and display brightness. The most harmful postures were observed when the display brightness was minimum, and the ambient light was similar to a sunny day. Abbreviations: ST: seated with table; SWT: seated without any support; STA: standing; 0L: in the dark, no light; AL: ambient light; SL: strong light; BrightMin: minimum display brightness; BrightMax: maximum display brightness (BrightMax); ISB: International Society of Biomechanics; RULA: rapid upper limb assessment; MSDs: musculoskeletal disorders [ABSTRACT FROM AUTHOR]