Don B. Chaffin

A Longitudinal Study of Low-Back Pain as Associated with Occupational Weight Lifting Factors

A review of the biomechanics of weight lifting as it relates to low-back stresses is presented first. This serves as the basis for the development of a Lifting Strength Rating (LSR) methodology. Then a study is reported wherein the LSR methodology is used to evaluate 103 jobs having various amounts of required two-handed load lifting. The 411 people populating these jobs were also evaluated. For a period of one year following their evaluation, any low-back pain problems in the group were noted. The primary result of the field study is that the incidence rate of low-back pain is correlated with higher lifting strength requirements as determined by assessment of both the location and magnitude of the load lifted. It is, therefore, recommended that load lifting be considered potentially hazardous, and that the LSR methodology be used to guide corrective actions.

A dynamic biomechanical evaluation of lifting maximum acceptable loads

A biomechanical evaluation of the job-related stresses imposed upon a worker is a potential means of reducing the high incidence rates of manual material handling injuries in industry. A biomechanical model consisting of seven rigid links joined at six articulations has been developed for this purpose. Using data from cinematographic analysis of lifting motions the model calculates: (1) body position from articulation angles, (2) angular velocities and accelerations, (3) inertial moments and forces, and (4) reactive moments and forces at each articulation, including the L5/S1 joint. Results indicated effects of the common task variables. Larger load and box sizes increased the rise times and peak values of both vertical ground reaction forces and predicted L5/S1 compressive forces. However, boxes with handles resulted in higher L5/S1 compressive forces than for boxes without handles. Also, in lifting the larger boxes the subjects did not sufficiently compensate with reduced box weights in order to maintain uniform L5/S1 compressive forces. Smoothed and rectified EMG of erector spinae muscles correlated significantly with L5/S1 compressive forces, while predicted and measured vertical ground reaction forces also correlated significantly, indicating the validity of the model as a tool for predicting job physical stresses.

A computerized biomechanical model-development of and use in studying gross body actions.

Gross body actions involved in heavy industry. e.g. lifting and carrying materials. are often the cause of injury to the musculoskeletal system. A computer model is developed which treats the human body as a series of seven links from which reactive forces and torques are computed at each articulation during various simulated materials handling tasks. In addition, an analysis of shearing and compressing forces at the lower lumbar spine is included. The assumptions of the present model are presented, along with a discussion of future models. IT HAS been reported by Troup (1965) that a relatively large portion of industrial injuries (as great as 12 per cent) are back disorders resulting from a lifting task. It is also evident from papers by Tichauer (1965), Raof et al. (1960) and Davis et al. (1965) that the estimation of stresses on various parts of the musculoskeletal system during lifting activities will require a complex methodology which takes account of such factors as (1) instan- taneous positions of the extremities and trunk, (2) curvature changes in the spine, (3) strength variations within different muscle groups and people, and (4) abdominal pressure effects.

BIOMECHANICAL MODEL CALCULATION OF MUSCLE CONTRACTION FORCES: A DOUBLE LINEAR PROGRAMMING METHOD

This paper presents a novel scheme for the use of linear programming to calculate muscle contraction forces in models describing musculoskeletal system biomechanics. Models of this kind are frequently found in the biomechanics literature. In most cases they involve muscle contraction force calculations that are statically indeterminate, and hence use optimization techniques to make those calculations. We present a linear programming optimization technique that solves a two-objective problem with two sequential linear programs. We use the technique here to minimize muscle intensity and joint compression force, since those are commonly used objectives. The two linear program model has the advantages of low computation cost, ready implementation on a micro-computer, and stable solutions. We show how to solve the model analytically in simple cases. We also discuss the use of the dual problem of linear programming to gain understanding of the solution it provides.

Prediction of metabolic rates for manual materials handling jobs

A mew approach for estimating metabolic rates for manual materials handling jobs is presented. This approach was applied to 48 different jobs. The model validation showed a correlation coefficient of 0.95 between the measured and predicted metabolic rates. The coefficient of variation (standard error/sample mean) was 10.2 percent.

Isometric strength testing as a means of controlling medical incidents on strenuous jobs

This investigation was performed to determine if isometric strength tests can be used to select workers for strenuous jobs and to reduce occupational injuries which are caused by a mismatch between worker strength and job strength requirements. Twenty jobs in a tire and rubber plant were studied biomechanically to identify critical strength-demanding tasks. Four strength tests were designed to simulate these tasks, and performance criteria were established for passing the tests. New applicants were administered the tests during their preplacement examinations to determine if they possessed sufficient strength to qualify for the jobs. The medical incidence rate of employees who were selected using the strength tests was approximately one-third that of employees selected using traditional medical criteria. It was concluded that isometric strength tests can be used to reduce occupational injuries and should be considered for implementation in industries with strenuous jobs.

Some biomechanical aspects of the carpal tunnel

Abstract Previously presented evidence indicates that carpal tunnel syndrome is related to compression of the median nerve inside the carpal tunnel. Biomechanical arguments in which the extrinsic finger flexor tendons inside the carpal tunnel are characterized as a frictionless pulley-belt mechanism are presented to show quantitatively how wrist size, wrist position and hand position affect forces on the tendons and their adjacent structures.

An investigation of the relationship between displacements of the finger and wrist joints and the extrinsic finger flexor tendons.

Abstract Several investigators have developed biomechanical models of finger flexor tendon displacements during pinching or gripping exertions of hands. Landsmeer has developed the most comprehensive set of models for this purpose. This paper describes experiments in which various sized cadaver hands were used to statistically evaluate the Landsmeer models. In so doing, the effects of hand and wrist anthropometry are included. The results indicate that the tendons displace in relation to joint positions as described by that Landsmeer model in which the tendon is depicted as sliding over the curved articular surface of the proximal bone of the joint. Joint thickness effects were found to modify the parameters in the model as intuitively expected. An empirical prediction model of the anthropometric effects was developed. Further, the tendon displacements for various wrist orientations were expressed empirically for the first time and were shown to be consistent with expected anatomical considerations.

High-pass filtering to remove electrocardiographic interference from torso EMG recordings

UNLABELLED Removal of electrocardiographic (ECG) contamination of electromyographic (EMG) signals from torso muscles is often attempted by high-pass filtering. This study investigated the effects of the cut-off frequency used in this high-pass filtering technique on the resulting EMG signal. Surface EMGs were recorded on five subjects from the rectus abdominis, external oblique, and erector spinae muscles. These signals were then digitally high-pass filtered at cut-off frequencies of 10, 30, and 60 Hz. Integration and power analyses of the filtered EMGs were subsequently performed. It was found that an increase in the cut-off frequency affects the integrated EMG signal by (1) reducing the ECG contamination, (2) decreasing the amplitude, and (3) smoothing the signal. It was concluded that the use of a high-pass filter is effective in reducing ECG interference in integrated EMG recordings, and a cut-off frequency of approximately 30 Hz was optimal. RELEVANCE Electromyographic recordings of torso muscles are often used in the development of low-back biomechanical models. Unfortunately, these recordings are usually contaminated by electrocardiographic interference. High-pass filtering methods are sometimes used to diminish the influence of ECG from surface EMGs; however, the effects of these filters on the recorded and processed EMG have not been reported. The findings show that high-pass filtering is effective in reducing ECG contamination and motion artefact from integrated EMGs when the appropriate cut-off frequency is used. Inappropriate cut-off frequencies lead to either incomplete ECG removal or excess filtering of the EMG signal.

Improving digital human modelling for proactive ergonomics in design

This paper presents the need to improve existing digital human models (DHMs) so they are better able to serve as effective ergonomics analysis and design tools. Existing DHMs are meant to be used by a designer early in a product development process when attempting to improve the physical design of vehicle interiors and manufacturing workplaces. The emphasis in this paper is placed on developing future DHMs that include valid posture and motion prediction models for various populations. It is argued that existing posture and motion prediction models now used in DHMs must be changed to become based on real motion data to assure validity for complex dynamic task simulations. It is further speculated that if valid human posture and motion prediction models are developed and used, these can be combined with psychophysical and biomechanical models to provide a much greater understanding of dynamic human performance and population specific limitations and that these new DHM models will ultimately provide a powerful ergonomics design tool.