L.A. Smith and R.E. Thomas, Jr
Leo A. Smith and Robert E. Thomas, Jr are with the Department of Industrial Engineering, Auburn University, Alabama, USA.
This article discusses ergonomics research related to forest harvesting in the southeastern United States. The activities were sponsored by the United States Forest Service's Southern Forest Experiment Station between 1980 and 1992. Parallels in prevailing environmental conditions between the southern United States and areas of the humid tropics make many of the results of significant interest on a wider scale.
Placing a heart rate recording device as port of a work physiology study
The forest products industry is one of the largest employers in the southern portion of the United States. Forest harvesting activities employ numerous individuals in manual, semi-mechanized and fully mechanized tasks. These tasks have historically been physically stressful and associated with high accident and injury frequency as well as severity rates. Harvesting in the region is done primarily by numerous small-scale independent contractors. In addition, several large forest product companies operate a few "company" harvesting crews. Although still requiring the performance of strenuous manual activities, harvesting is rapidly becoming highly mechanized, utilizing expensive, short-lived equipment. It is critical that harvesting be accomplished in a cost - effective, productive manner. It is equally important that harvesting be accomplished without subjecting the work - force to undue physical stress, bodily injury or health risks.
Over the past decade, the Auburn University's Department of Industrial Engineering, in cooperation with the university's Engineering Experiment Station and the Southern Forest Experiment Station of the United States Department of Agriculture's Forest Service, has carried out a programme aimed at enhancing forest harvesting productivity through ergonomics research. The research programme has included an evaluation of the physiological demands of six common southern forest harvesting tasks; an evaluation of the effects of summertime heat stress on the productivity of both manual and mechanized harvesting; the development of a heat stress prevention guide for use by harvesting managers; an evaluation of the noise and vibration stress experienced by the users of a newly designed two-cylinder chainsaw; an evaluation of the whole body vibration exposure of skidder operators; development of a protocol for the ergonomic evaluation of forest machines and its application to mobile chippers; and an evaluation of factors affecting the selection and use of leg protection devices by chainsaw operators. The most recent study included a comparison of the anthropometric characteristics of: southern United States forest workers with existing standardized data, produced; by the International Labour Organisation (ILO) and the Society of Automotive Engineers (SAE); documentation of the work postures assumed by grapple skidder operators; and an evaluation of the workplace designs of three grapple skidders.
To provide basic data relative to the physical demands of southern harvesting tasks, a study (Smith, Wilson and Sirois, 1982; 1983) examined the physiological stress, as measured by working heart rate levels, imposed on typical employees by their work activities and work environments. Particular attention was given to the stress imposed by summer thermal conditions. Data were collected during the summer of 1981 from four different harvesting crew and harvesting site combinations in east-central Alabama. The workers surveyed were involved in common tasks of chainsaw felling, cable skidding, bucking and trimming prior to loading (a position referred to as the "landing saw person") and operating knuckle-boom loaders. Data were also obtained at one site on feller-buncher and grapple skidding activities.
The data indicated that southeastern forest harvesting tasks performed in summertime environmental conditions place physiological demands on the worker that often equal or exceed the maximum levels typically considered acceptable in occupational activities. Based on the data, the tasks considered may be ranked in decreasing order of physiological demands as follows: landing saw operator (heavy work); chainsaw felling (moderate to heavy work); cable skidding (light work); feller-buncher operation (light work); and knuckle-boom loading (light work).
The results also included an important observation that led to later studies on heat stress: the amount of increase in the average heart rate when work was performed under hot conditions compared with more moderate conditions was not as great as might be reasonably expected. This suggests that harvesting employees may perform their work so as not to experience what they perceive to be unacceptable stress by reducing the amount of muscular work performed. The manner in which crew members reduce their muscular work per unit of time may not be obvious to the casual observer; indeed, it may not even be a conscious action on the part of the crew member. For instance, it was observed that, when temperatures were hotter, there were more "hidden rest periods", such as "hung-up saws". These events lowered the physiological stress experienced but also reduced productivity. As a result of this observation, later research was directed towards determining the relationship between heat stress and worker productivity in southeastern forest harvesting tasks.
A number of studies were undertaken regarding the effects of heat stress on forest harvesting activities. The first (Smith, Seay and Sirois, 1985) examined productivity in a variety of harvesting tasks and was a direct outgrowth of the physiological demands study described above. The second (Rummer and Smith, 1990) examined the productivity implications of air-conditioned harvesting machines. The third (Smith and Rummer, 1988) proposed a guide to assist harvesting managers in the prevention of heat stress events.
Typical duties of landing saw workers
The objective of the first study was to evaluate the relationship between worker productivity and the level of environmental heat load present during common southern forest harvesting activities. The tasks evaluated were chainsaw felling, cable skidding, bucking at the landing, feller-buncher operation and bucking bunched thinning. Historical production data covering a period of 806 crew days and direct observation data totaling more than 20000 observations were examined.
The measure of productivity used in the historical data evaluation was "pounds harvested per worker-hour". The production behaviour variables evaluated by the direct observation study were percentage productive activities, percentage unproductive (operator controlled) activities; worker effort level, worker efficiency, percentage unsafe behaviour and percentage non-use of personnel protection equipment.
The results indicate that logging crew productivity as a whole and the productivity of chainsaw fellers and cable skidder operators, in particular, experience productivity decreases of 5 to 15 percent when temperatures (as measured by the Oxford index and the wet-bulb globe temperature test) exceeded 25 and 26°C, respectively. Neither type of bucking performance was significantly affected by the hotter environmental conditions, perhaps because the workers concerned were underutilized in the harvesting crews observed. The feller-buncher operators experienced a 2 to 3 percent decrease in percentage productive activities under the hotter conditions. Both of the companies cooperating in the study had active occupational safety programmes which strongly encouraged appropriate work behaviour. None of the observed crew members had unsafe behaviour rates of more than 2 percent. Although there was a tendency for about one-half of the workers to become more engaged in unsafe behaviour under higher temperature levels, they did not radically modify their behaviour. It must be noted, however, that the workers who were exposed to the most direct sun, i.e. the buckers at the landing, tended to neglect their personal protective equipment under the hotter conditions. Harvesting management should be alert to this possibility and apply appropriate supervisory controls.
Cable skidder operator pulling the cable to secure a load
Measuring heat load in the forest
Heat stress prevention guide
Another heat-related study (Smith and Rummer, 1988) drew on data collected in the previous work physiology study and heat stress studies, which examined the productivity implications of air conditioned harvesting machines, to develop a work practices guide, entitled Guide to the prevention of heat stress in forest harvesting tasks and intended for use by harvesting managers. The guide permits the assessment of the risk of heat stress occurrence by consulting conveniently designed tables. Manual, semi-mechanized and mechanized harvesting tasks are covered. The guide includes discussions of: suggested work practices that may be followed so as to reduce the risk of heat stress; certain "special situations" in which the risk may be higher than usually indicated by the meteorological conditions; heat illnesses as well as actions that should be taken if symptoms are observed; and the effects of hot working conditions on safe work behaviour. The guide now needs to be actively tested in the field by harvesting managers.
Occupational vibration exposure is typically considered in two categories: segmental and whole-body. The greatest health concern in the area of segmental vibration is the occurrence of hand-arm vibration syndrome (HAYS) among users of powered hand-tools, including chainsaws. Whole-body vibration is typically transmitted to workers by the body support structure of buildings that house vibrating equipment and by vehicles of all types, including forest harvesting machines. Whole-body vibration can lead to discomfort, reduced work proficiency and a variety of physical symptoms - nausea and microfractures of bones, for instance.
Rummer, Smith and Stokes (1985) compared the vibration and noise exposures associated with the use of two single-cylinder chainsaw models and a then newly designed twin-cylinder model. Both static bench and dynamic bucking tests were performed and the results indicated that the new two-cylinder design had the lowest vibration level. The latency period before there was a 50 percent chance of developing HAVS as a result of using the saws for four hours per day was estimated to be more than 26 years for the two-cylinder design, compared with less than 12 years for the one-cylinder models. Noise levels for all of the saws exceeded recommended limits. Under constant speed conditions, the two-cylinder saw had a lower level of noise; under operator - selected speed conditions, the noise levels were equivalent. In fact, the operators appeared to use the noise level produced to establish the speed at which they ran the saws. If it is possible to generalize from this phenomenon, it may have significant training implications.
Most of the reported studies on whole body vibration from forest equipment operation have concentrated on measuring vibration exposure levels and comparing them with various exposure guidelines. Rummer (1988) examined forest workers' attitudes towards whole - body vibration and developed a computer ride simulation model that can be used to improve the design of forest machines. The study surveyed a representative sample of 26 forest equipment operators and rated their subjective assessment of whole-body vibration as a source of environmental stress.
Measuring machine properties for whole body vibration analysis
Although the survey did not reveal evidence of physical injury resulting from long-term whole-body vibration exposure, the responses indicated that forest workers perceived whole-body vibration to be one of the most discomforting aspects of their work. Significantly, it was found that the typical working day includes substantial (1.5 hours) travel time to the harvesting site; thus, vibration exposure in the crew truck is probably a significant component of the total daily whole-body vibration exposure.
Historically, ergonomics has ranked low on the priorities list of design parameters used by most engineers. Higher priority is given to items that appear to have a more direct relationship with profits, such as performance capability, reliability and cost. The Southern Forest Experiment Station has sponsored studies related to equipment design per se, with particular attention paid to personal protection (Smith, Helm and Rummer, 1988); development of an ergonomic evaluation protocol (Smith, Adsit and Rummer, 1990); and the evaluation of the work station in grapple skidders (Thomas and Smith, 1991).
A large percentage of logging accidents involve the use of chainsaws. Leg protection for chainsaw operators is available in several varieties and, when used properly, is effective in preventing or mitigating many injuries. Unfortunately, leg protection has not been widely used by loggers in the southern United States. Smith, Helm and Rummer (1988) sought to encourage the use of leg protection by evaluating several factors influencing its selection and use; for example, protective ability, appearance, cost, thermal comfort, mobility, durability and style.
Leggings, protective pants, work pants with safety insert pads and chaps, with regular work pants as a control, were evaluated in both laboratory and work site studies by five subjects at each test location. A paired comparison test and a ranking test were used to obtain subjective opinions on the leg protection. In the laboratory, a simulated forest harvesting obstacle course was negotiated by the subjects wearing each of the leg protection types at two different temperature conditions: 23 and 35°C. Motion restrictions imposed by the leg protection and leg skin temperature responses were measured during each trial and subjective opinions were obtained from a questionnaire following each trial. The work site subjects were encouraged to wear the protection equipment for a week and report their subjective opinions by responding to a questionnaire.
The results suggest that protective ability and thermal comfort are considered the two most important factors, followed closely by mobility, in leg protection selection Interestingly, although subjects rated appearance of the equipment as being the least important factor, all of their informal comments focused on the leg protection's appearance, thus suggesting that it is an important factor. The effect of the leg protection on worker mobility was not consistent, suggesting that different types of leg protection affect mobility in different ways for different people. Based on this study it would appear that anyone promoting the use of chainsaw leg protection devices should provide a choice of fabric colours, demonstrate that it is actually protective and attest that it is neither too hot nor too restrictive of mobility.
A video camera mounted to record operators' work postures
Ergonomic analysis of forestry machines
Smith, Adsit and Rummer (1990) sought to compile an evaluation protocol to facilitate the ergonomic analysis of forestry machines, based on information available from current ergonomic design references, literature reports of related equipment evaluation studies and government and industry design standards. Once developed, the protocol was applied in a study on mobile chippers.
The evaluation protocol was developed in the form of a checklist. The specific areas of ergonomic concern treated included cab access, cab interior dimensions, operator seat dimensions, control location, control actuating forces, instrumentation, visibility, cab climate, air quality, noise, vibration and maintenance. In addition to the checklist itself, a questionnaire was developed to obtain subjective information from the operator.
Six mobile chippers were evaluated in the case-study and the results indicated that there are several areas of ergonomic deficiency associated with the models evaluated. Most notable were cab access system dimensions and operator seat characteristics. In some models, the visibility from the cab, the cab thermal climate and the cab noise level were also found to be inadequate. The responses to the subjective questionnaire reinforced these findings.
Thomas and Smith (1991) found that grapple skidder operators spend from 27 to 57 percent (an average of 41 percent) of their operating shifts in twisted postures that are suspected of being associated with lower back injuries. Location of the grapple controls was found to be a potential major contributor to the magnitude of twisted postures assumed by the operators while vibration levels around the operator's seat decreased proficiency performance levels in all of the machines studied. The anthropometric evaluation found that skidder drivers in the southern United States were generally taller than the populations measured to determine the SAE and ILO standards. Skidder operators were also found to be substantially heavier than the norms in the SAE standard.
The ergonomics research sponsored by the Southern Forest Experiment Station to date has focused primarily on harvesting systems with a significant manual labour component. Employees use very little, if any, personal protective equipment and work in environmental conditions characterized by wet-bulb globe temperatures of between 20 and 35°C. The results of this research provide useful insights for other regions with similar harvesting systems and/or environmental characteristics.
During the next decade, forest harvesting in the southern United States will become increasingly mechanized as new equipment is introduced to per form traditional activities and as demand grows for activities (large-scale thinning) and products (chips for fuel) that can only be accomplished efficiently with mechanized systems. Additionally, new safety regulations will mandate the use of appropriate personal protection equipment. Accordingly, the next phase of the United States Forest Service/ Auburn University forest harvesting ergonomics research programme should focus on mechanized harvesting systems and on enhancing the design and use of personal protective equipment. Included should be an expansion of the previous heat stress and vibration studies to further documentation on the exposure of mechanized equipment operators to beat, vibration, noise and dust hazards and to develop methods to minimize the negative effects of this exposure on operator safety, health and productivity. Also included should be an expansion and refinement of the forest worker anthropometric data base for use by equipment designers as well as an additional ergonomic evaluation of existing machines to suggest improvements in workplace design and design for ease of maintenance. Research relative to personal protection equipment should lead to ensembles that provide: protection while having a minimum negative impact on worker mobility and heat load. The efficacy of currently popular (but unproven) devices, such as back support belts and wrist/forearm supports, must also be examined.
Axelson, O. 1974. Heat stress in forest work. Rome, FAO.
Rummer, R.B. 1988. Whole-body vibration exposure of forest equipment operators in the southeastern United States. Doctoral dissertation. Auburn, Alabama, Department of Industrial Engineering, Auburn University.
Rummer, R.B. & Smith, L.A. 1990. Utilization of air-conditioned feller/bunchers. Int. J. Ind. Ergon., 5: 93-302.
Rummer, R.B., Smith, L.A. & Stokes, B.J. 1985. Two-cylinder chainsaw vibration and noise test. Technical release 85-R-13. American Pulpwood Association, Inc.
Smith, L.A. 1980. Final Report No. 19-353. New Orleans, USDA, Forest Service.
Smith, L.A. & Rummer, R.B. 1988. Development of a heat stress prevention activity guide for use by southern forest harvesting managers. Final Report No. 1987-065. New Orleans USDA, Forest Service.
Smith, L.A. & Sirois, D.L. 1982. Ergonomic research: review and needs in southern forest harvesting. Forest Prod. J., 32(4): 44-49.
Smith, L.A. & Wilson, G.D. 1982. Equipment note: an inexpensive system for remote monitoring of work heart rate. Ergonomics, 26(7): 719-722.
Smith, L.A. Adsit, D.A. & Rummer, R.B. 1990. An ergonomic evaluation of forest machines: development of an evaluation protocol and case-study of mobile chippers. Final Report No. 19-88-074. New Orleans, USDA, Forest Service.
Smith, L.A., Helm, A.M. & Rummer, R.B. 1988. Evaluation of factors affecting the selection and use of chainsaw leg protection. Final Report No. 19-86-048. New Orleans, USDA, Forest Service.
Smith, L A., Seay, M.S. & Sirois, D.L. 1985. Evaluation of the effect of heat stress on worker productivity for selected southern forest harvesting tasks. Final Report No. 19-82-082. New Orleans, USDA, Forest Service.
Smith, L.A. Wilson, G.D. & Sirois, D.L 1982. Assessment of the physiological stress of selected forest harvesting activities in the southeastern United States. Final Report No. 19-80-407. New Orleans, USDA, Forest Service.
Smith, L.A., Wilson, G.D. & Sirois, D.L. 1983. Work physiology evaluation of southern forest harvesting tasks. Forest Prod. J., 33(9): 38-44.
Thomas, R.E. & Smith, L.A. 1991. Reducing back injuries in grapple skidder operators through ergonomic design. Project No. 19-91-033. New Orleans, USDA, Forest Service.
