Stover H. Snook

The design of manual handling tasks: revised tables of maximum acceptable weights and forces

Four new manual handling experiments are reviewed. The experiment used male and female subjects to study lifting, lowering, pushing, pulling, and carrying tasks. Each experiment used a psychophysical methodology with measurements of oxygen consumption, heart rate, and anthropometric characteristics. Independent variables included task frequency, distance, height and duration; object size and handles; extended horizontal reach; and combination tasks. The results of the four experiments were integrated with the results of seven similar experiments published previously by this laboratory. The integrated data were used to revise maximum acceptable weights and forces originally published in 1978. The revised tables are presented and compared with the original tables.

The Ergonomics Society The Society's Lecture 1978. THE DESIGN OF MANUAL HANDLING TASKS

Abstract Manual materials handling is the principal source of compensatable work injuries in the United States, and 79% of the manual handling injuries affect the lower back. The prevention of low back injuries in industry has traditionally been attemped by (1) careful selection of workers, (2) good training procedures in safe lifting, and (3) designing the job to fit the worker. This paper discusses the latter approach by describing seven studies of lifting, lowering, pushing, pulling, carrying and walking. Among the variables investigated were the height, distance and frequency of the task; the size and weight of the object; the differences in worker sex. age, and physical physique; and the effects of heat stress. The psychophysical methodology used in six of the studies is discussed, and the results are used to develop a series of tables for evaluating the design ofmanual handling tasks. The tables present the maximum weights that are acceptable to 10, 25, 50, 75 and 90% of the working population. The ...

A Study of Size, Distance, Height, and Frequency Effects on Manual Handling Tasks

The effects of size, distance, height, and frequency on lifting, lowering, carrying, pushing, and pulling tasks were studied in two series of experiments using a psychophysical methodology. In the first series, 10 male and 12 female industrial workers performed tasks at many different frequencies. Results indicate that the lower-frequency tasks (especially the 5-min and 30-min tasks) have acceptable weights and forces lower than those estimated in previous studies. In the second series, 12 female industrial workers performed tasks with a variety of box sizes, distances of lift, and heights of push and pull. Results indicate that maximum acceptable weights and forces for female workers are significantly lower, but proportionally similar, to the maximum acceptable weights and forces for male workers.

The reduction of chronic nonspecific low back pain through the control of early morning lumbar flexion. A randomized controlled trial.

Study Design. Eighteen‐month, randomized controlled trial with partial crossover. Objectives. To test the hypothesis that the control of lumbar flexion in the early morning will significantly reduce chronic, nonspecific low back pain. Summary of Background Data. Previous studies have indicated an increased risk of low back pain with bending forward in the early morning, primarily because of increased fluid content in the intervertebral discs at that time. Methods. After 6 months of recording baseline data, 85 subjects with persistent or recurring low back pain were randomly assigned to treatment and control groups. The treatment group received instruction in the control of early morning lumbar flexion. The control group received a sham treatment of six exercises shown to be ineffective in reducing low back pain. Six months later, the control group received the experimental treatment. Diaries were used to record daily levels of pain intensity, disability, impairment, and medication usage. Results. Significant reductions in pain intensity (P < 0.01) were recorded for the treatment group, but not for the control group (point estimate, 33%; 95% confidence interval, 11‐55%). After receiving the experimental treatment, the control group responded with similar reductions (P < 0.05). Significant reductions also were observed in total days in pain, disability, impairment, and medication usage. Conclusions. Controlling lumbar flexion in the early morning is a form of self‐care with potential for reducing pain and costs associated with chronic, nonspecific low back pain.

The relation between pain intensity, disability, and the episodic nature of chronic and recurrent low back pain.

STUDY DESIGN An observational study on the course of chronic and recurrent low back pain and its relation to disability and medication use performed on the basis of daily diary recording. OBJECTIVES To provide a description of daily pain reporting by individuals with self-reported chronic and recurrent low back pain, to study how the intensity and episodic nature of low back pain is related to disability and medication use, and to classify subjects according to Von Korffs categories of chronic low back pain. SUMMARY OF BACKGROUND DATA The natural history of low back pain has been described, and some classification schemes have been proposed, but little has been reported on pain characteristics and their relation to self-report of disability. METHODS Daily self-reports of pain intensity, social and work disability, and medication use were collected from 94 participants with self-reported chronic or recurrent low back pain over a 6-month period. A metric for describing the episodic nature of chronic low back pain was developed. RESULTS A significant effect of pain intensity on disability was found. During an episode, participants had significantly greater disability and medication use. Work-related disability and medication use was significantly greater in the latter half of an episode. CONCLUSIONS Pain intensity can affect disability, but the episodic nature of low back pain also affects the ability to function in both work and personal life. Intermittent increases in pain can markedly alter disability. Chronic low back pain should not be treated as a static phenomenon.

Psychophysical studies of repetitive wrist flexion and extension

The purpose of this experiment was to investigate the feasibility of using psychophysical methods to determine maximum acceptable forces for various types and frequencies of repetitive wrist motion. Four adjustable work stations were built to simulate repetitive wrist flexion with a power grip, wrist flexion with a pinch grip, and wrist extension with a power grip. The study consisted of two separate experiments. Subjects worked for two days per week during the first experiment, and five days per week during the second experiment. Fifteen women completed the first experiment, working seven hours each day, two days per week, for 20 days. Repetition rates of 2, 5, 10, 15 and 20 motions per minute were used with each flexion and extension task. Maximum acceptable torques were determined for the various motions, grips, and repetition rates without dramatic changes in wrist strength, tactile sensitivity, or number of symptoms. Fourteen different women completed the second experiment, performing a wrist flexion motion (power grip) fifteen times per minute, seven hours per day, five days per week, for 23 days. There were no significant differences in maximum acceptable torque from day to day. However, the average maximum acceptable torque for a five days per week exposure was 36.3% lower than for the same task performed two days per week. Assuming that maximum acceptable torques decrease 36.3% for other repetition rates and motions, tables of maximum acceptable force were developed for female wrist flexion (power grip), female wrist flexion (pinch grip), and female wrist extension (power grip).

Psychophysical considerations in permissible loads

Psychophysics is identified as one of the criteria that has been used with increasing frequency in developing recommendations for permissible loads. A critique of psychophysics is offered, citing both advantages and disadvantages of the method. The relationship between permissible workloads and low-back pain is reviewed. A distinction is made between low-back pain, low-back impairment, low-back disability and low-back compensation. It is suggested that the setting of permissible workloads in industry, regardless of the criteria used, will have little impact on the incidence of low-back pain, but a significant effect upon low-back disability and low-back compensation.

Further studies of psychophysically determined maximum acceptable weights and forces

The effects of lifting boxes without handles, pulling long distances, carrying different size boxes, combining three handling tasks, and lifting with extended horizontal reach were studied using a psychophysical methodology. Six male industrial workers performed 42 variations of lifting, lowering, pushing, pulling, and carrying tasks. Lifting boxes without handles produced consistent decreases (median, 16%) in maximum acceptable weights when compared with lifting boxes with handles. Lifting with extended horizontal reach (approximately 48 cm) produced consistent decreases (median, 48%) in maximum acceptable weights when compared with lifting close to the body (approximately 17 cm). No significant heart rate or oxygen consumption differences occurred in either of these variables. No significant differences in maximum acceptable weight of carry were observed among box sizes; however, the maximum acceptable force of pulling was lower for longer (15.2 m) pulling tasks. The maximum acceptable weight for combination tasks was similar to that of the limiting component.

Distributions of manual materials handling task parameters

Abstract This study is an analysis on 25,291 manual materials handling (MMH) tasks obtained from 2442 reports from industrial locations throughout the US. The tasks consisted of 10,101 lifts, 7461 lowers, 1879 pushes, 1866 pulls and 3984 carries. The purpose of the study was to determine the percent distribution of each of the task parameters, i.e., weight, height, distance, and frequency. Secondly, the study compared the percent of reports from key industrial classifications to percent of compensation costs associated with MMH in those classifications. Analyses of the percent distributions revealed that many basic ergonomic considerations for MMH have been ignored. Redesign strategies should be focused toward minimizing hand distances, decreasing loads of lifts, lowers, and carries, decreasing frequencies of tasks, increasing heights of start of lifts, and decreasing distances of carries, pushes, and pulls. There should also be a continued effort to decrease the number of lifts and lowers which comprise 69% of the tasks surveyed. Ratios of percent of reports in key industrial classifications to the percent of compensation costs associated with MMH in those classifications indicate higher ratios in manufacturing, durable, and non-durable categories and low ratios in service, trucking and construction categories. It is concluded that continued effort should be extended toward redesign of MMH tasks and components of tasks that violate good ergonomic principles. Relevance to industry Since MMH is a significant compensable loss source, detailed knowledge of the parameters of MMH tasks should be helpful in prioritizing research efforts. Industry may also find this study a point of comparison for their own operations.

Challenges in assessing risk factors in epidemiologic studies on back disorders

In epidemiologic studies on musculoskeletal disorders, some risk factors, especially physical load, cannot be determined independently from the worker. Posture, movement and external load are the result both of physical work requirements forced on the worker and of the workers capacity to adopt particular techniques. Risk factors are also adjusted in relation to the workers health. This paper presents a dynamic model that links exposure to risk factors for back pain and disability. Its aim is to help identify core elements in exposure assessment strategies for epidemiologic studies on back disorders. In this dynamic model, risk factors are determined relative to health status in order to distinguish between etiological and prognostic factors. Measurement techniques for various risk factors are classified into self-reports, observations, and direct instrumentation. Features of commonly used techniques are discussed with respect to feasibility, accuracy, and precision. In addition, consideration is given to the optimum allocation of measurements taking into account the effects of random and systematic variation in exposure due to tasks, workplaces, and workers.