Work-related upper extremity muscle skeletal disorders (WRUEMSDs) in Norway constitute a major cause and are the largest subscriber for several work disabilities for both genders. This is associated with absenteeism, long sick leave periods and sick attendance in combination with early retirement. In order to significantly contribute to reduce this problem it is of great importance to uncover the mechanisms behind the high prevalence of MSD. Despite many decades of EMG recording and analysing of muscle work pattern, no firm evidence has been found between EMG readings and muscle pain. The mechanistic understanding of muscle pain development is far from a complete understanding. However, over the last years a new promising method for EMG data treatment has been reported, showing association between sustained low-level muscle activity (SULMA) and pain. In this project we want to increase the data material and the analyses of the SULMA method in order to establish a firmer relationship between muscle work pattern and muscle pain development. The overall hypotheses is based on the well-established ´Cinderella theory´ of pain development, claiming that only a few motor units are involved in muscle activity at low static levels, leaving those active units susceptible to fatigue. To improve the scientific base for the SULMA method we want to use forest machine operators as an occupational model group, and perform whole day field EMG measurements of high quality. This research topic is in agreement in the contract between the Ministry of Agriculture and Forestry and the Natural Resources Institute Finland as stated in the Ministry’s Diario 175/031/2014.