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Saturday, 12 March 2011 17:38

Physical Load

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Manual Forest Work

Workload. Manual forest work generally carries a high physical workload. This in turn means a high energy expenditure for the worker. The energy output depends on the task and the pace at which it is performed. The forest worker needs a much larger food intake than the “ordinary” office worker to cope with the demands of the job.

Table 1 presents a selection of jobs typically performed in forestry, classified into categories of workload by the energy expenditure required. The figures can give only an approximation, as they depend on body size, sex, age, fitness and work pace, as well as on tools and working techniques. It does, however, give a broad indication that nursery work is generally light to moderate; planting work and harvesting with a chain-saw moderate to heavy; and manual harvesting heavy to very heavy. (For case-studies and a detailed discussion of the workload concept applied to forestry see Apud et al. 1989; Apud and Valdés 1995; and FAO 1992.)

Table 1. Energy expenditure in forestry work.


Kj/min/65 kg man    

Workload capacity





Work in forestry nursery

Cultivating tree plants




















Clearing draining ditches with spade





Tractor driving/harrowing while sitting





Planting by hand





Planting by machine





Work with axe-Horizontal and perpendicular blows

Weight of axe head

Rate (blows/min)




1.25 kg





0.65-1.25 kg





Felling, trimming, etc. with hand tools






Carrying logs





Dragging logs





Work with saw in forest

Carrying power saw





Cross-cutting by hand





Horizontal-sawing power saw





Mechanized logging





Operating harvester/forwarder





Fuelwood preparation

Sawing small logs by hand





Cleaving wood





Dragging firewood





Stacking firewood





L = Light; M = Moderate; H = Heavy; VH = Very heavy; EH = Extremely heavy

Source: Adapted from Durnin and Passmore 1967.

Musculoskeletal strain. Manual piling involves repeated heavy lifting. If the working technique is not perfect and the pace too high, the risk of musculoskeletal injuries (MSIs) is very high. Carrying heavy loads over extended periods of time, such as in pulpwood harvesting or fuelwood harvesting and transport, has a similar impact.

A specific problem is the use of maximum body force, which could lead to sudden musculoskeletal injuries in certain situations. An example is bringing down a badly hung-up tree by using a felling lever. Another is “saving” a falling log from a pile.

The work is done using only muscle force, and most often it involves dynamic and not simply repetitive use of the same muscle groups. It is not static. The risk for repetitive strain injuries (RSIs) is usually small. However, working in awkward body positions can create problems such as low-back pain. An example is using an axe to delimb trees which are lying on the ground, which requires working bent over for long periods of time. This puts great strain on the lower back and also means that the muscles in the back do static work. The problem can be reduced by felling trees across a stem that is already on the ground, thus using it as a natural workbench.

Motor-Manual Forest Work

The operation of portable machines such as chain-saws may require even greater energy expenditure than manual work, because of their considerable weight. In fact, the chain-saws used are often too big for the task at hand. Instead, the lightest model and the smallest guide bar possible should be used.

Whenever a forest worker who uses machines also does the piling manually, he or she is exposed to the problems described above. Workers have to be instructed to keep the back straight and to rely on the big muscles in the legs to lift loads.

The work is done using machine power and is more static than manual work. The operator’s work consists of choosing, moving and holding the machine in the right position.

Many of the problems created originate from working at a low height. Delimbing a tree that is lying flat on the ground means working bent over. This is a similar problem to that described in manual forest work. The problem is compounded when carrying a heavy chain-saw. Work should be planned and organized so the working height is close to the hip of the forest worker (e.g., using other trees as “workbenches” for delimbing, as described above). The saw should be supported by the stem as much as possible.

Highly specialized motor-manual work tasks create very high risk for musculoskeletal injuries since the work cycles are short and the specific movements are repeated many times. An example is the fellers working with chain-saws ahead of a processor (delimbing and cutting). Most of these forest workers that were studied in Sweden had neck and shoulder problems. Doing the whole logging operation (felling, delimbing, crosscutting and certain not-too-heavy piling) means the job is more varied and the exposure to specific unfavourable static, repetitive work is reduced. Even with the appropriate saw and a good working technique, chain-saw operators should not work more than 5 hours a day with the saw running.

Machine Work

The physical workloads in most forest machines are very low compared to manual or motor-manual work. The machine operator or the mechanic is still sometimes exposed to heavy lifting during maintenance and repairs. The operator’s work consists of guiding the movements of the machine. He or she controls the force to be exerted by handles, levers, buttons and so on. The work cycles are very short. The work for the most part is repetitive and static, which can lead to a high risk for RSIs in the neck, shoulder, arm, hand or finger regions.

In machinery from the Nordic countries the operator works only with very small tensions in the muscles, using mini–joy sticks, sitting in an ergonomic seat with armrests. But still RSIs are a major problem. Studies show that between 50 and 80% of machine operators have neck or shoulder complaints. These figures are often difficult to compare since the injuries develop gradually over a long period of time. The results depend on the definition of injury or complaints.

Repetitive strain injuries depend on many things in the work situation:

Degree of tension in the muscle. A high static or repeated, monotonous muscle tension can be caused, for example, by using heavy controls, by awkward working positions or whole-body vibrations and shocks, but also by high mental stress. Stress can be generated by high concentration, complicated decisions or by the psychosocial situation, such as lack of control over the work situation and relations with supervisors and workmates.

Time of exposure to static work. Continuous static muscle tensions can be broken only by taking frequent pauses and micropauses, by changing work tasks, by job rotation and so on. A long total exposure to monotonous, repetitive work movements over the years increases the risk of RSIs. The injuries appear gradually and may be irreversible when manifested.

Individual status (“resistance”). The “resistance” of the individual changes over time and depends on his or her inherited predisposition and physical, psychological and social status.

Research in Sweden has shown that the only way to reduce these problems is by working with all these factors, especially through job rotation and job enlargement. These measures decrease the time of exposure and improve the well-being and psychosocial situation of the worker.

The same principles can be applied to all forest work—manual, motor-manual or machine work.

Combinations of Manual, Motor-Manual and Machine Work

Combinations of manual and machine work without job rotation always mean that the work tasks become more specialized. An example is the motor-manual fellers working ahead of a processor which is delimbing and cutting. The work cycles for the fellers are short and monotonous. The risk of MSIs and RSIs is very high.

A comparison between chain-saw and machine operators was made in Sweden. It showed that the chain-saw operators had higher risks of MSIs in the low back, knees and hip as well as high risks of hearing impairment. The machine operators on the other hand had higher risks of RSIs in the neck and shoulders. The two types of work were subject to very different hazards. A comparison with manual work would probably show still another risk pattern. Combinations of different types of work tasks using job rotation and job enlargement give possibilities to reduce the time of exposure for many specific hazards.



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