Highly repetitive work operations in a modern milking system. A case study of wrist positions and movements in a rotary system

With the use of electrogoniometers wrist positions and movements were measured in 13 milkers while working in a modern rotary milking system. The rotary system put considerable demands on the wrists and hands regarding both velocities and repetitiveness. Values were found close to those described in other repetitive industrial work with high risk of wrist and hand disorders. For the right hand the repetitiveness was 0.57 Hz and 0.46 Hz for the left hand. In addition, the median value (50th percentile) of the angular velocity distribution was also high, being 36°/s for the right hand and 26°/s for the left, and with respect to the peak value (90th percentile) the corresponding values were 155°/s and 135°/s, respectively. Furthermore, when milking in the rotary system, there was less possibility to hold the hands still than in the other milking systems. The right hand rested only 1.4% of the milking time and the left only 1.0%. The hands were therefore moving throughout almost the entire milking procedure. High velocity, repetitiveness and fewer opportunities for rest are risk factors that might lead to disorders in the wrists and hands. Regarding wrist positions, the left wrist was held in a more dorsiflexed position than the right, 37° and 29°, respectively. Compared with tethering and loose-housing parlour milking, the wrist positions were, however, improved in the rotary system. When introducing new milking systems these negative effects on wrist and hand movements must be borne in mind in order to minimize the prevalence of wrist and hand disorders.