Psychophysical study of six hand movements.

This study represents a continuation of a series of psychophysical studies on repetitive motions of the wrist and hand conducted at the Liberty Mutual Research Center for Safety and Health. The purpose of the study was to quantify maximum acceptable forces of six motions performed on separate days but within the context of the same experiment. The six motions were wrist flexion with a power grip, wrist extension with a power grip, wrist flexion with a pinch grip, wrist extension with a pinch grip, ulnar deviation with a power grip, and a handgrip task (with a power grip). A psychophysical methodology was used in which the subject adjusted the resistance on the handle and the experimenter manipulated or controlled all other variables. Thirty-one subjects performed the six tasks at repetition rates of 15, 20 and 25 motions/min. Subjects performed the tasks for 7 h per day, 5 days per week, for 4 weeks. The subjects were instructed to work as if they were on an incentive basis, getting paid for the amount of work performed. Symptoms were recorded by the subjects during the last 5 min of each hour. The results revealed that maximum acceptable torques ranged from 11 to 19% of maximum isometric torque depending on frequency and motion. Maximum acceptable torques for the tasks that could be compared with previous studies showed the same patterns of response. However, the selected forces were substantially lower using the mixed protocol. A table of maximum acceptable torques and forces is presented for application in the field.

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 Korff's 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.

Future directions of psychophysical studies.

Psychophysics is a very old branch of psychology that is concerned with the relationship between physical stimuli that occur in the "outside world", and the sensations they produce in the body's "inside world". According to modern psychophysical theory, the strength of a sensation is directly related to the intensity of its physical stimulus by means of a power function. The history and development of psychophysics is briefly reviewed, and the application of psychophysics to manual handling tasks and repetitive hand and wrist motion is described. The advantages and disadvantages of psychophysics are discussed. The following 4 directions for future research are suggested: further validation of the data, expansion into other areas of concern (eg, other body parts, jobs, and postures), broadening of the subject base by including symptomatic subjects, and greater inclusion of psychophysical measures (eg, pain, fatigue, and discomfort) in studies of musculoskeletal disorders.

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.

Meteorological conditions and self-report of low back pain.

STUDY DESIGN: Six months of daily low back pain ratings for 94 individuals were tested for the influence of prevailing weather conditions during the spring, summer, and fall seasons. Intergroup differences were tested for study participants who reported weather sensitivity and for those who did not. OBJECTIVES: To investigate the relation between pain ratings and prevailing weather conditions in a population with chronic or recurrent low back pain. SUMMARY OF BACKGROUND DATA: Weather conditions have been reported to influence pain perception in some disease states, including low back pain. Investigations of this relation in chronic or recurrent low back pain have involved varied methodologies, and conflicting results have been reported. METHODS: The effects of eight weather variables reported to influence musculoskeletal pain were tested on daily pain ratings. A post hoc weather sensitivity questionnaire was used to disperse 73 individuals into groups based on perceived weather sensitivity, and group differences were tested. RESULTS: Significant effects on pain scores were found, most notably for temperature and vapor pressure. The magnitude of the effects were small compared with autocorrelation of an individual's own pain scores. Significant differences were found between the group of individuals who were insensitive to weather conditions and that of individuals with perceived sensitivity to cold temperatures. No significant intergroup differences were found for damp, rainy conditions or changes in barometric pressure. CONCLUSIONS: Weather conditions may influence subjective reporting of low back pain significantly. Although the effects are small in magnitude, they should be considered in clinical treatment of the patient with chronic, nonspecific low back pain. Pain scores may demonstrate greater interaction with certain weather conditions in individuals perceiving sensitivity to those conditions.

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 worker's capacity to adopt particular techniques. Risk factors are also adjusted in relation to the worker's 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.

Maximum acceptable forces for repetitive ulnar deviation of the wrist.

The purpose of this experiment was to quantify maximum acceptable forces for ulnar deviation motions of the wrist at various repetition rates. Subjects grasped a handle with a power grip and moved it through a 1.40 rad (80 degrees) ulnar deviation wrist motion (similar to a knife cutting task). A psychophysical methodology was used in which the subject adjusted the resistance on the handle and the experiment manipulated or controlled all other variables. Two series of experiments were conducted. Thirteen subjects completed the first series, which investigated repetition rates of 15 and 20 motions per minute. Eleven subjects completed the second series, which investigated 15, 20, and 25 motions per minute. Subjects performed for 7 hours per day, 5 days per week, for 4 weeks in the first series and 5 weeks in the second series. The subjects were instructed to work as if they were on an incentive basis, getting paid for the amount of work they performed. Symptoms were recorded by the subjects during the last 5 minutes of each hour. The results are presented and compared with maximum acceptable forces for wrist flexion and extension.

A single metric for quantifying biomechanical stress in repetitive motions and exertions.

The relative effects of repetition, force and posture were studied in order to investigate how continuous biomechanical measurements can be combined into a single metric corresponding to subjective discomfort. A full factorial experiment was conducted involving repetitive wrist flexion from a neutral posture to a given angle against a controlled force. Seven subjects performed the task using two paces (20 and 4 motions/min), two force levels (15 and 45 N) and two angles (15 and 45 degrees) for 1 h each. Discomfort was reported on a 10 cm visual analogue scale anchored between 'no discomfort' and 'very high discomfort'. Repeated measures analysis of variance showed that all main effects were statistically significant (p < 0.05) and no significant interactions were observed. A linear regression model was fitted to the data and used for generating frequency weighted digital filters that shape continuous recordings of repetitive motions and exertions into an output proportional to relative discomfort. The resulting high-pass digital filter had a 22 dB/decade attenuation slope. A simulated industrial task used for validating the model involved repetitively transferring pegs across a horizontal bar and inserting them into holes against a controlled resistance. Angular wrist data were recorded using an electrogoniometer and filtered. Six subjects performed the task of the three conditions consisting of (1) 15 wrist flexion, 15 N resistance and 6 motions min, (2) 15 wrist flexion. 45 N resistance and 12 motions/min, and (3) 45 degrees wrist flexion, 45 N resistance and 15 motions/min. Subjective discomfort was reported after performing the task for 1 h. Pearson correlations between subjective discomfort ratings and the integrated filtered biomechanical data for individual subjects ranged from 0.90 to 1.00. The pooled correlation across subjects was 0.67. This approach may be useful for physical stress exposure assessment and for design of tasks involving repetitive motions and exertions.

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).

The effect of back belts on lumbar muscle fatigue.

STUDY DESIGN: An experiment was conducted to investigate the effect of wearing a weight lifting belt on the endurance and fatigue characteristics of the extensor muscles of the spine. OBJECTIVE: To evaluate the effectiveness of back belts in preserving the endurance characteristics of the spinal musculature. SUMMARY OF BACKGROUND DATA: Previous studies have investigated the use of back belts in increasing intra-abdominal pressure and thus increasing the stabilization of the spine. Other studies have looked at the association of low back disability and wearing back belts. No study has examined the relationship between wearing back belts and measures of spinal muscle fatigue and endurance. METHODS: Thirteen male industrial workers performed a lifting and lowering task from the floor to a 76.2 cm height at 4.3 min-1 2 days a week for 4 weeks. The load was psychophysically determined and averaged 28.1 kg for a total of 28.9 metric tons moved in 4 hours. Subjects lifted for 1, 2, 3, and 4 hours, respectively, for the first 4 days of the experiment. These were considered training days. During the last 4 days, subjects lifted for 4 hours per day; 2 days with a back belt, and 2 days without a back belt. Maximum isokinetic endurance of the extensors was measured for 50 repetitions (5 sets of 10 repetitions) at the end of 4 hours of lifting. Median frequencies of the electromyographic signal from six locations over the back extensors were measured for 30 seconds at 80% of maximum isometric voluntary contraction after 4 hours of lifting. RESULTS: There were no significant differences in maximum isokinetic endurance, or in the slope of median frequency, between lifting with a back belt and lifting without a back belt. Similarly, a modified Borg scale and a postexperiment survey question were unable to distinguish between wearing a back belt and not wearing a back belt. CONCLUSION: There were no significant differences in maximum isokinetic endurance and electromyographic spectral parameters of the back extensor muscles as a result of wearing a back belt during heavy lifting and lowering tasks.

A review of machine-guarding recommendations.

A series of machine-guarding drawings were developed during the 1940s, which represent the relationship between gap size and safe distance. Since the development of these drawings, larger and more comprehensive anthropometric surveys have been conducted, but they have never been compared with the drawings. The purpose of this investigation was to determine whether these original recommendations presented in the drawings are still consistent with data in the larger anthropometric surveys, especially relating to women and minorities, who are now much more prevalent in the American workplace. In this study, data from six anthropometric surveys were compared with the original machine-guarding recommendations. To minimize the percentage of the population at risk, the length dimensions were chosen from the 99th percentile male population, and thickness dimensions were chosen from the 1st percentile female population. Many of these recommendations were found to be valid, although several important modifications were suggested. Several currently recommended dimensions show no correlation to the incompressible joints utilized in this study, and it is recommended that they be eliminated. The recommendations are presented in tables of maximum gap size and minimum distance.

The cost of compensable upper extremity cumulative trauma disorders.

There is little information available of the costs of upper extremity cumulative trauma disorders. Cost data were collected from computerized records of the Liberty Mutual Insurance Company for upper extremity cumulative trauma disorder workers' compensation claims (N = 6,067) and for all claims (N = 731,087) initiated from 45 states during 1989. The data were not analyzed until July 1992, allowing more accurate "closing cost" data to be used in the analysis. Upper extremity cumulative trauma disorder cases represented 0.83% of all claims and 1.64% of all claims costs. The mean cost per case for upper extremity cumulative trauma disorders was $8070; median cost per case was $824. Medical costs represented 32.9% of the total costs; indemnity costs were 65.1%. The total compensable cost for upper extremity cumulative trauma disorders in the United States was estimated to be $563 million.

The cost of 1989 workers' compensation low back pain claims.

STUDY DESIGN: Cost data were gathered from computerized records of the Liberty Mutual Insurance Company for low back pain workers' compensation claims (N = 119,107) and for all claims (N = 731,087) initiated from 45 jurisdictions (states) during 1989. OBJECTIVE: This study provided more current, accurate, and additional information to estimate the costs and incidence associated with compensable low back pain compared with all compensation claims. METHODS: The first group of data included all compensable low back claims selected by specific codes: body part codes consisted of low back area, sacrum and coccyx, disc, and multiple trunk; injury codes consisted of strain, sprain, inflammation, rupture, hernia, fracture, and contusion. The second sample included all compensable claims, including both occupational injuries and illnesses. RESULTS: Low back pain cases represented 16% of all claims but 33% of all claims costs; 55.4% of the low back pain cases received medical payments only (i.e., did not receive indemnity payments for lost time). The mean cost per case for low back pain was $8321; median cost per case was $396. Medical costs represented 32.4% of the total costs; indemnity costs (i.e., payment for lost time) represented 65.8%. CONCLUSIONS: Since indemnity costs represent the greatest percentage of workers' compensation expenditure, the primary goal of low back pain management should be the prevention or reduction of prolonged disability.

Perceived exertion and discomfort associated with driving screws at various work locations and at different work frequencies.

Eighteen subjects drove screws with air-powered tools into perforated sheet metal at three vertical and two horizontal work locations using three different work paces (8, 10, and 12 screws/min). Subjects drove screws with a pistol-shaped tool on the vertical orientation at knee, elbow, and shoulder height. They used an in-line tool to drive screws on the horizontal surface. A horizontal beam was placed just below each subject's elbow height and they drove screws into it with the lower arm perpendicular to the torso and with the arms fully extended. Subjects drove screws for 10 min at each work location and frequency combination before they assessed the condition using the Borg ten-point ratio rating scale. Subjects also ranked seven body areas according to discomfort for each work location. A two-factor ANOVA (and comparable non-parametric statistics) showed that both work location and frequency were significant factors in determining the Borg ratings. As work pace increased, so did the Borg ratings of perceived exertion for each work location. For each incremental increase in work pace, the Borg ratings of perceived exertion increased 12% to 25%, depending on the work location. Driving screws at elbow height on the vertical surface and with the lower arm close to the body on the horizontal surface were the work locations with the smallest ratings of perceived exertion. The ratings of perceived exertion for driving screws at elbow height on the vertical surface were 18% to 50% lower than the ratings for driving screws at knee or shoulder height and the ratings of perceived exertion for driving screws with the lower arm close to the body on the horizontal surface were 21% to 24% lower than driving screws with the arms fully extended. No significant difference was found among the discomfort ranks given to the various body parts for the two horizontal work locations. Differences were found among the body part discomfort rankings for the vertical work locations. While driving screws at knee height, the torso was most stressed; the wrist and hand were most stressed while driving screws at elbow height, and the shoulder and upper arm were the body parts that were stressed the most while driving screws at shoulder height.

Effect of tool shape and work location on perceived exertion for work on horizontal surfaces.

Thirty subjects drove screws into perforated sheet metal mounted on a horizontal surface using three air-powered tools that varied in shape (right-angle, in-line, and pistol-shaped). The four horizontal work locations ranged from 13-88 cm in front of the body and were placed at 25 cm intervals. The vertical placement of the horizontal beam was at midthigh, elbow, and midchest height. Subjects drove 25 screws at each tool/work location combination before rating that condition using the Borg 10-point ratio rating scale. The ratings of perceived exertion increased with increasing horizontal distance from the body. When tool shape was not considered, the perceived exertion was virtually equal for driving screws at midthigh or elbow height. The ratings at midchest height were significantly higher than elbow and midthigh height. When tool shape was taken into account, subjects perceived less exertion driving screws with the pistol-shaped tool at midthigh height. The in-line and right-angle tools had the lowest ratings of perceived exertion for driving screws at elbow and midchest height.

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.

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.

Perceived exertion and discomfort versus work height with a pistol-shaped screwdriver.

Seven vertical locations were rated after 36 subjects drove 25 No. 6 sheet metal screws into 18-gauge perforated sheet metal (hole size of 0.28 cm) with an air-powered pistol-shaped screwdriver at each location. The seven vertical heights ranged from 38 to 191 cm. Subjects rated each level using three psychophysical scales and were asked to imagine that they were assembly line workers required to drive screws at that work location using that particular tool for 8 hr. The three scales were the Borg 10-point ratio rating scale and two visual analogue scales. All subjects, regardless of anthropometry (5th percentile females to 95th percentile males), preferred driving screws between 114 and 139 cm. At the highest level of 191 cm, the shortest subjects disliked the level twice as much as the taller subjects. The three psychophysical scales were found to be comparable in sensitivity and use, although most subjects preferred the Borg rating scale. Future research and limitations of the current study are discussed.

Stairway risers and treads: acceptable and preferred dimensions.

Stairway preference and acceptability were investigated with psychophysical techniques. A series of six experiments was conducted where subjects ascended and descended 19 sets of stairways with different riser and tread (run) dimensions. Subjects were instructed to identify stairways that they considered acceptable, and the one stairway they most preferred. The optimum riser was 7.2 in (183 mm), and the optimum tread (run) was 11 or 12 in (279 or 300 mm). These dimensions were acceptable to both males and females, young and old, and subjects of greater or lesser stature. Larger dimensions were not as acceptable to shorter subjects, and smaller dimensions were not as acceptable to taller subjects. The 4-in (102 mm) riser was almost totally unacceptable, and never preferred. The 5.14 and 9-in (131 and 229 mm) risers were acceptable to less than one-third of the subjects, and rarely or never preferred. These results are compared with existing practices and recommendations.

The effects of task duration on psychophysically-determined maximum acceptable weights and forces.

The purpose of this experiment was to investigate maximum acceptable weights and forces when performing manual handling tasks continuously for four hours at frequencies of 4.3 min-1 or slower. Twelve female and ten male second shift industrial workers performed 18 varieties of lifting, lowering, pushing, pulling, and carrying. A psychophysical methodology was employed, whereby the subjects were asked to select a workload they could sustain for 8 h 'without straining themselves or without becoming unusually tired, weakened, overheated or out of breath'. Measurements of heart rate, oxygen consumption, dynamic and static strengths were also taken. The weights selected after 40 min were not significantly different from the weights selected after four hours. The average oxygen consumption for the fast tasks was 28% VO2 max, within physiological guidelines for eight hours. The results also revealed that the maximum acceptable weights for the combination task of lifting, carrying, and lowering were limited by the lifting and lowering components. It is concluded from the results of this study that the psychophysical methodology is appropriate for determining maximum acceptable weights for task frequencies of 4.3 min-1 or slower. It is also concluded that the maximum acceptable weight for a combination task is limited by the lowest acceptable weight of any of the components.