Interobserver repeatability and validity were assessed for a new semiquantitative, time-based observation method for the estimation of physical loads imposed on the upper extremities. Six risk factors of upper extremity disorders were included in the method: repetitive use of hand, use of hand force, pinch grip, non-neutral wrist posture, elevation of upper arm, local mechanical pressure. Two occupational health nurses were trained to use the method. They observed 127 work cycles at a food-processing plant and a paper mill. The method was validated against expert observations from the video, continuous recordings of myoelectric activity (EMG) of forearm muscles, and wrist posture measured with goniometers. Interobserver repeatability was good or moderate for repetitive use of hand, hand force, pinch grip (range ? = 0.58 - 0.71 on the right; 0.60 - 0.61 on the left side). Interobserver repeatability was moderate or poor for non-neutral wrist posture, elevation of the upper arm and local mechanical pressure. Validity ranged from moderate to good for repetitive use of hand, use of hand force, pinch grip and non-neutral wrist posture when expert observation was used as reference standard. When observations were validated against force estimations (EMG) and wrist goniometer data, validity was poor. In the absence of generally accepted reference values, arbitrarily chosen limits were used for the proportional duration of some physical load factors. Studies should be carried out to assess the limits that best differentiate between safe and hazardous jobs

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