Recommended maximum holding times for prevention of discomfort of static standing postures

The aim of the present study was threefold; (1) to analyze the influence of posture on the maximum holding time (MHT), (2) to study the possibility of classifying postures on the basis of MHT, and (3) to develop ergonomic recommendations for the MHT of categories of postures. For these purposes data concerning the MHT of 19 symmetric standing postures from 7 experimental studies from the literature were analyzed. All postures were defined by the position of the hands with respect to the feet. For each posture the mean MHT over all available data was calculated. The results show that for the 19 postures this mean MHT ranges from 2 to 35 minutes. For a given posture the variation in MHT between different studies is large. It seems that in particular the type of task (boring versus interesting) performed while maintaining the posture has a great influence on the MHT. On the basis of the mean MHT postures were classified into three classes. Comfortable postures are defined as postures that have a MHT of more than 10 minutes and are recommended not to be maintained more than 2 minutes. Moderate postures have a MHT of 5-10 minutes and are recommended to be maintained for less than 1 minute. Uncomfortable postures, having a MHT up to 5 minutes, are not acceptable. It is estimated that with theses recommendations a discomfort of more than 2 (weak discomfort) on the Borg 10-point rating scale (up to maximum discomfort) is prevented for at least 50% of the population, and a discomfort of more than 5 (strong discomfort) is prevented for at least 95% of the population. The recommended holding time valid for each class corresponds with the lowest recommended holding time of that class. Therefore the recommendations are safe for all postures. Our classification of postures corresponds well with classifications based on biomechanical and anthropometric data and is more strict than the OWAS-classification. The maximum holding time (MHT) was categorically studied to analyze its influence on postures, classify postures based on MHT and to develop ergonomic recommendations for MHT. Postures were based on the mean MHT. Comfortable postures have more than 10 minutes MHT and recommended not to be maintained for more than 2 minutes. Moderate postures have 5-10 minutes MHT and were recommended to be maintained for less than 1 minute. Uncomfortable postures with a 5-minute MHT were not accepted. These recommendations have prevented discomfort of more than 2 (weak discomfort) on the Borg 10-point rating scale for at least 50% of the population, and a discomfort of more than 5 (strong discomfort) was prevented for at least 95% of the population.