¹Institute of Occupational and Social Medicine and Health Services Research, University Hospital of Tübingen, Tübingen, Germany
²Hertie Institute of Clinical Brain Research, Tübingen, Germany

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Introduction: Wrist position and exerted torque during manual handling tasks may affect the development of work-related musculoskeletal disorders of the hand and wrist. When assessing work-related risks in the hand-arm area, the applied force and joint position are often considered. However, the risk categories of established ergonomic assessment methods are often not verified by empirical data and are therefore sometimes difficult to understand in terms of their physiological effects. Therefore, the aim of this study was to analyse the influence of wrist position and exerted torque on muscular strain of the forearm flexor muscles.

Methods: Nineteen healthy participants (eleven females and eight males) performed submaximal isometric wrist flexion contractions in six wrist positions (60°, 40° and 20° palmar flexion, 0° neutral position and -20° and -40° dorsal extension) applying four torque levels (10%, 20%, 30%, 40% of the torque generated during a maximum voluntary contraction in the neutral (0°) wrist position = Torque-Max). Muscular strain was assessed as the normalized electromyographic activity of the flexor digitorum superficialis (FDS) and flexor carpi radialis (FCR). Exerted wrist flexion angles and torques were assessed using a custom-built torque application and measurement device.

Results: Wrist position and exerted torque were related to muscular strain. The electromyographic activity increased when deviating from the neutral wrist positions and when higher torque levels were applied. Deviations from the neutral wrist position led to higher strain, especially in palmar flexed wrist positions.

Furthermore, there was a statistically significant interaction between wrist position and torque level indicating a higher influence of wrist posture for higher torque levels.

Discussion: The study confirmed previous findings that muscular strain of forearm flexors during isometric wrist flexion contractions depends on the wrist position and the exerted torque. However, the interaction between wrist position and torque level indicates that non-neutral wrist postures may be of minor importance when the torque level is lower than 10% of the muscle’s maximum capacity assessed under neutral wrist posture condition. In addition, the effect of joint angles and forces appears to be exponential rather than linear, but in many ergonomic assessment procedures a linear increase in risk is often assumed.

Conclusion: These investigations can contribute to further specify ergonomic recommendations of the hand and wrist for manual work tasks.