Shoulder-arm muscle load and performance during control operation in forestry machines: Effects of changing to a new arm rest, lever and boom control system

Abstract

Three work station design studies in forestry machines were carried out with regard to shoulder/ arm muscle load. (1) In an arm rest study, a movable (in the sagittal plane) arm rest was compared with a fixed one; (2) in a lever study, a mini lever with a small deflection was compared with an ordinary hand lever; (3) in a boom control study, a computer controlled boom system was compared with an ordinary boom system. In the arm rest study eight male subjects carried out lever operations in the laboratory. In the lever and boom studies, 15 male forestry students operated forwarders. Muscle load, cycle time, skill, perceived exertion, heart rate and preferred set up were recorded. The muscle loads on the upper trapezius, infraspinatus, extensor carpi ulnaris and flexor carpi radialis were recorded by electromyography. With mini lever and movable arm rest the upper trapezius load was reduced with preserved or decreased cycle time, although the grand mean of the ‘static’ load level was only about 2% of maximal voluntary contraction (MVC) and the effects of the new designs were small (about 0.7% MVC). The subjects preferred the lever/arm rest also giving the lowest muscle load. No differences between levers or arm rests were found in skill, perceived exertion or heart rate. It was not possible to draw any definitive conclusions regarding effects of the boom control system, due to technical problems. According to the literature, the duration of lever operation has increased considerably in forestry during recent years as part of a rationalization process. Partly due to this, it is suggested that the investigated work station improvements may not be sufficient to eliminate the risk for shoulder-neck disorders.