Does the human factor limit the productivity in mechanized spacing and cleaning of young stand? Is the limitation continued to be associated with the design of the work place in the tractor, or is it caused by shortage in machine functions, lack of information from the work site or is it the operator's mental capacity. One limitation in the forestry man/machine system has for a long time been the operators' problems with neck and shoulders.
An analysis of mechanized spacing and cleaning was carried out in order to understand why, how and when the operator gather information; to understand which mental processes the operator use, as well as how he execute the physical work and how he gets feed-back from the work. Description of the work and its influence on the operator's reactions is based on interviews, time-studies, the analysis of video films as well as on observations. The operator's heart rate variability was measured as an indicator of mental effort. The electrical activity in the trapezius muscle was measured and analysed with consideration to breaks.
There are many work elements which call for the operator's attention. The managing of the machine occupy the attention while biological assessments are needed. The limited sight hampers the operator possibility to quickly grasp the necessary information about the young tree stand and the terrain. The intense operation of the crane caused few pauses (emg-gaps) in the trapezius muscle. The low heart rate variability indicated a high mental workload, probably explained by high concentration.
Proposals for technical development are given. A better sight, more "sensibility" of the cleaning head and aids for orientation would give the operator more efficient information. A partly robotized crane, a turnable cabin which adjusts its horizontal position automatically and better absorbs the vibrations, would give a more relaxed working environment.
Man's unique advantage to handle this complex forest-man-machine system must in the future be taken care of more efficiently. To keep a profitable forestry, the machine, the crane and the cleaning head must be easier to handle. With such a development, the operator can make better and faster judgements on the forest and its biological conditions.
Key words: forestry machine, information ergonomics, mental and physical workload.
Correspondence: Sten Gellerstedt, Dept. of Operational Efficiency, Swedish University of Agricultural Sciences, S-776 98 Garpenberg, Sweden
Telefax: +46-225-26100
E-mail: sten.gellerstedt@stek.slu.se