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Objective:The aim of this study was to examine the logic behind the knowledge of low-back problem causal pathways.Background:Low-back pain and low-back disorders (LBDs) continue to represent the major musculoskeletal risk problem in the workplace, with the prevalence and costs of such disorders increasing over time. In recent years, there has been much criticism of the ability of ergonomics methods to control the risk of LBDs.Method:Logical assessment of the systems logic associated with our understanding and prevention of LBDs.Results:Current spine loading as well as spine tolerance research efforts are bringing the field to the point where there is a better systems understanding of the inextricable link between the musculoskeletal system and the cognitive system. Loading is influenced by both the physical environment factors as well as mental demands, whereas tolerances are defined by both physical tissue tolerance and biochemically based tissue sensitivities to pain. However, the logic used in many low-back risk assessment tools may be overly simplistic, given what is understood about causal pathways. Current tools typically assess only load or position in a very cursory manner.Conclusion:Efforts must work toward satisfying both the physical environment and the cognitive environment for the worker if one is to reliably lower the risk of low-back problems.Application:This systems representation of LBD development may serve as a guide to identify gaps in our understanding of LBDs.

Objective: This review evaluates (in retrospect) the contribution of Liles and associates (1984) to the causality debate of the work relatedness of low back pain. Background: Often it takes years to appreciate the role of a paper with respect to the body of literature as a whole. Method: Although many papers appear remarkable when they are first published, the real value of a contribution often can be appreciated by considering how the paper "fills in the pieces of the puzzle" over time. This paper examines how the Liles paper influenced low back pain causality efforts after its introduction. Results: This analysis indicates that Liles and associates contributed to the science of low back disorder causality by (a) advancing the idea of quantitative measures used for field studies, (b) identifying a dose-response relationship for low back pain, and (c) recognizing the influence of a system of work and nonwork influences related to low back pain development. Conclusion: The Liles contribution to Human Factors has proven to play a pivotal role in our understanding of how low back pain is influenced by work exposure. Application: The concepts introduced here can help future efforts associated with understanding musculoskeletal disorder causality and work.

An experiment was performed to evaluate the motions of the lumbar spine during a constant load velocity lift. For the purposes of this study, a constant load velocity refers to the linear vertical velocity of the load. This vertical load velocity was controlled using a modified angular isokinetic dynamometer, which produced linear isokinetic motion during a lift. A lumbar monitor was used to observe the position, velocity, and acceleration changes that occurred in the lumbar spine during the lifting task. The results indicate that under constant load velocity conditions, significant angular accelerations occur at the lumbar level. The nature of these accelerations was found to depend on several variables associated with a lifting task, such as the load velocity and the asymmetry of the lift. The physical significance of these results would be increased spinal loading above that which would be predicted using a static model.

It is important to understand how trunk strength varies as a function of workplace factors so that the work environment can be designed to minimize the risk of low back injury. In this study maximal trunk torque production around the lumbosacral junction was measured in 44 subjects as trunk concentric and eccentric isokinetic velocity and trunk asymmetric line of action were varied. Trunk torque decreased by approximately 8.5% of maximum for every 15 deg of asymmetric trunk angle. Increases in concentric velocity decreased trunk strength, whereas increases in eccentric trunk velocity increased strength. Significant interactions were also found, and it was determined that the common finding that eccentric strength exceeds concentric strength is true only for forward trunk angles at all asymmetric angles. These results should have significant implications for the design of manual materials handling tasks.

This study investigated differences in railroad spike maul methods between experienced track workers and novice subjects. Differences were analyzed by developing a quantitative measure of the orientation of the spike maul path during spiking. The relationship between spiking motion pattern and spiking performance in both novice and experienced subjects was also investigated. Subjects were videotaped while using a spike maul on an instrumented spike force measuring system. Analysis of videotapes and performance measures revealed significant differences in motion patterns and performance between novice and experienced subjects. It was concluded from analysis of spike maul motion patterns that experienced track workers had developed a more efficient ballistic technique than had novices. This technique was evidenced by positioning of the maul close to the body's center of gravity, flexing the elbows, and using wrist motion on the downswing.

Typically, the simplest and most cost-efficient ergonomic solution to offset the rising costs of low back injuries is to reduce the box weight that is lifted. However, there is limited research on how a worker interacts with the box. In the present study, we quantify the utility of reducing the weight that is lifted---specifically, how changes in the box weight affect trunk kinematics, trunk moments, and ultimately, spinal loads. In the experiment, 15 participants lifted a variety of box weights (from 9.1 to 41.7 kg) from knee height, carried it a distance of 5 feet (1.5 m), and placed it on a shelf at elbow height. For the lower weights, small increases in box weight (3-9 kg) were offset by the trunk dynamics (sagittal velocity), resulting in no difference in spinal loads. At the same time, spinal loads were found to be significantly higher for weights above 25 kg. Thus, when making ergonomic changes (reduction of box weight), it is important to consider how workers will interact with the box. These results indicate that purely weight-based ergonomic controls might not sufficiently reduce the risk of low back disorders. Furthermore, this study provides additional evidence of the utility of using more complex spinal load models (dynamic, multiple muscle models) when evaluating highly dynamic and complex tasks.

This study examined the effects of trunk speed and exertion level on temporal aspects of trunk muscle activity patterns during dynamic, asymmetric lifting. Electromyographic (EMG) data from eight trunk muscles were collected along with trunk torque output, position, and velocity data during several repetitions of four speed/loading combination conditions. During analysis, each muscle's EMG record was reduced to three key events: a start, a peak, and an end point. For each subject, temporally ordered event lists were constructed for each test condition. Networks of events that consistently occurred regardless of loading or speed levels were constructed for each subject. Two event pairs occurred consistently for all subjects under all conditions, whereas some pairs occurred in association with specific speed or resistance levels. Temporal information related to muscle activity could be used in biomechanical models in order to predict changes in spinal loading during the course of workplace tasks.

A three-dimensional motion model has been developed that estimates loads on the lumbar spine under laboratory conditions that simulate manual materials handling conditions. Eleven subjects experienced spinal loading during an experiment in which conditions of trunk velocity, trunk torque output, and trunk asymmetric posture were varied in a series of isokinetic velocity trunk extensions. The electromyographic activity of 10 trunk muscles, subject anthropometry, and trunk kinetics were used as input to a biomechanical simulation model described in Part I of this study. The model calculated estimates of compression, shear, and torsion loading in the lumbar spine, as well as the torque production of the trunk, continuously throughout the exertion. Trunk torque estimates derived from this model were compared with measured trunk torque. The effects of trunk motion, posture, and torque level on spine loading as estimated by the model are discussed. It was concluded that this approach provides a straightforward means of assessing loading of the spine attributable to laboratory simulations of workplace conditions.

This research investigated how changes in hammer handle angle and hammering orientation affected muscle fatigue in the forearm and subjective ratings of body discomfort. Forearm muscle fatigue and discomfort ratings were not significantly affected by handle angle, but they were significantly higher in the wall hammering orientation than in the bench orientation. The research in this article and in the companion article (Part I) reveal that for novices, hammers with handles angled in the range of 20-40 deg are advantageous because (1) they reduce ulnar deviation and may possibly decrease the incidence of hand/wrist disorders, and (2) they do not significantly affect hammering performance in the bench conditions, forearm muscle fatigue, or subjective ratings of body discomfort.

The effects of railroad claw bar design factors and method of use upon performance and back stress were investigated. Novice and experienced subjects were tested for their ability to use the claw bar to remove track spikes. Two standard claw bar designs were evaluated. The method of tool use was described by body position relative to the claw bar and by body action. Tool design had no significant effect on either tool-lifting performance or back-stress measures. Body action patterns, but not body position, were found to have significant effects on both performance and back stress. Results support the use of a body action involving the use of the arms only. This method provides the required impulse forces with minimum back stress and also provides proper balance to avoid falls from sudden spike removal. Proper technique, as opposed to total force application, was found to be the key factor in tool use. Novices attempt to apply a large, sustained force using the whole body, whereas experienced trackmen use a snapping action to generate the required forces. Trackmen generated 50% more lifting force than did the novices.

An experiment was performed to study the effects of hand-tool design factors and methods of tool use upon tool force generation and loading of the back. Forty novice and experienced subjects were tested on their ability to drive a spike with a railroad spike maul. Method of tool use affected the spike-driving performance of novice subjects, whereas tool striking surface area influenced the ability of experienced trackmen to drive spikes. Method of tool use also affected the components of spine loading. Tool force generation was considered as a function of spine loading indices to create efficiency measures. Efficiency was used to evaluate the cumulative trauma effects of the hand tool.

Most biomechanical assessments of spinal loading during industrial work have focused on estimating peak spinal compressive forces under static and sagittally symmetric conditions. The main objective of this study was to explore the potential of feasibly predicting three-dimensional (3D) spinal loading in industry from various combinations of trunk kinematics, kinetics, and subject-load characteristics. The study used spinal loading, predicted by a validated electromyographyassisted model, from 11 male participants who performed a series of symmetric and asymmetric lifts. Three classes of models were developed: (a) models using workplace, subject, and trunk motion parameters as independent variables (kinematic models); (b) models using workplace, subject, and measured moments variables (kinetic models); and (c) models incorporating workplace, subject, trunk motion, and measured moments variables (combined models). The results showed that peak 3D spinal loading during symmetric and asymmetric lifting were predicted equally well using all three types of regression models. Continuous 3D loading was predicted best using the combined models. When the use of such models is infeasible, the kinematic models can provide adequate predictions. Finally, lateral shear forces (peak and continuous) were consistently underestimated using all three types of models. The study demonstrated the feasibility of predicting 3D loads on the spine under specific symmetric and asymmetric lifting tasks without the need for collecting EMG information. However, further validation and development of the models should be conducted to assess and extend their applicability to lifting conditions other than those presented in this study. Actual or potential applications of this research include exposure assessment in epidemioligical studies, ergonomic intervention, and laboratory task assessment.

A model of the regulation of muscle strength exertion is proposed. Based on the model, methods are discussed to assess whether a subject exerts maximal or submaximal efforts. Results with 40 subjects indicate that a simple technique may be developed to judge if a subject is following instructions to exert a maximal contraction in a routine test of voluntary muscle strength.