Gary A. Mirka

Accuracy of a three-dimensional lumbar motion monitor for recording dynamic trunk motion characteristics

There has been an abundance of evidence in the past decade that indicates that the asymmetric positioning as well as the dynamic action of the trunk during work greatly affects the ability of a worker to perform a lifting task. This is true because trunk strength decreases as the trunk moves more asymmetrically and more rapidly. Loading of the spine is also believed to increase under these conditions, since significantly greater trunk muscle activity has been observed under these conditions. Therefore, we must begin to document the asymmetric positions as well as the dynamic motion characteristics of the trunk when workers are exposed to various work tasks. This paper describes a lumbar motion monitor (LMM) that has been developed for this purpose. The LMM is an exoskeleton of the spine that is instrumented so that instantaneous changes in trunk position, velocity and acceleration can be obtained in three-dimensional space. The current study has assessed the accuracy and reliability of the LMM to measure such motion components. The results of this analysis indicate that the LMM is extremely reliable and very accurate. This study has shown that the LMM is about twice as accurate as a video-based motion evaluation system. The benefits and implications of using an LMM for work assessment and clinical use are discussed.

A stochastic model of trunk muscle coactivation during trunk bending

Biomechanical models of the spine have traditionally assumed that workplace lifting conditions (weight, posture, motion, etc.) precisely dictate the magnitude of individual muscle forces necessary to maintain a biomechanical balance within the trunk. However, because there are a large number of muscle groups within the trunk there is also an infinite number of possible combinations of muscle forces that can satisfy this biomechanical balance requirement for a given condition. Currently there are no methods available to predict this possible variability in muscle activity. Such variability in a multiple muscle system can result in variations in spinal loading. To quantitatively capture this trunk muscle variability during bending motions, such as those involved in lifting, a stochastic (probabilistic) model of trunk muscle activation was developed. The model was based on a simulation of experimentally derived data and predicted the possible combinations of time-dependent trunk muscle coactivations that could be expected given a set of trunk bending conditions. These simulated muscle activities were then used as input to an electromyographically assisted biomechanical model so that the magnitude and variability of the spine reaction forces could be estimated. This procedure allows one to assess the range of spinal loads that would be expected with a particular task. Significant variability in muscle activities was observed for each specific lifting condition and explained biomechanically. The results indicated that the variability in trunk muscle force had a small effect on spinal compression variability (+/- 7% of the mean compression), but greatly influenced both lateral (+/- 90% of mean) and anteroposterior shear forces (+/- 40% of mean). A validation study confirmed that the model predictions were reasonable estimates of muscle activity variability under previously untested conditions. This work could help explain how some repetitive lifting motions could increase the risk of acquiring a low back disorder and the simulation model could help drive electromyographically assisted models without the need for recording actual electromyographic activity.

The quantification of EMG normalization error

Electromyography (EMG) and normalized EMG have been accepted as methods of quantifying the activity level of a muscle. Normalized EMG, in conjunction with the EMG/force relationship and muscle cross-sectional area data, allows researchers to estimate the amount of muscle force exerted across a joint. An accurate description of this muscle force is a critical input to models designed to describe the risk of injury of a task. In order to be able to make statements about the relative intensity of an EMG signal, researchers who use normalization procedures take a given EMG activity level, at a known joint angle, and relate it to some reference activity level obtained at that particular joint angle. However, there have been studies where the EMG activity of an unrestricted dynamic task, such as walking, cycling, performing an occupational task, etc., has been normalized with respect to an EMG value taken during a single maximum voluntary contraction performed at one reference joint angle. This practice will render inaccurate results because at different joint angles there are changes in the portion of the muscle within the viewing area of the electrode, as well as changes in the length/tension relationship of the muscle which would cause changes in the maximum EMG value. The present study was an attempt to quantify the errors associated with normalization relative to a reference EMG value collected at an arbitrary joint position. Four subjects performed a series of controlled trunk extension exertions. As they performed these exertions the EMG activities were collected for eight trunk muscles. The task EMG values that resulted were then: (1) all normalized with respect to the maximum EMG at a single arbitrary trunk angle and (2) each normalized with respect to that specific trunk angles maximum EMG. The results show that for the primary trunk extensors (erector spinae) large errors (greater than 75%) resulted from normalization using a single reference point and the magnitude of these errors followed consistent patterns within subjects as a function of trunk angle.

A comprehensive evaluation of trunk response to asymmetric trunk motion.

An experiment was performed to determine the reaction of the trunk muscles, using electromyography, and intra-abdominal pressure to components of trunk loading commonly seen in the workplace during manual materials handling. These components included angular trunk velocity, trunk position in three-dimensional space and trunk torque exertion level. The experiment was performed using 44 subjects. Subjects produced constant trunk extension torque about the lumbosacral junction while moving the trunk under constant angular velocity (isokinetic) conditions. Significant reactions to trunk angular velocity, trunk torque level, and unique combinations of trunk position and velocity were seen in all muscles of the trunk. The other components affected the muscles selectively according to function. Intra-abdominal pressure only reacted significantly to trunk angle and some unique trunk angle-asymmetry positions. The biomechanical implications of these findings are discussed. The reactions of the muscles to the various workplace components also were described quantitatively through equations that predict muscle activity levels.

The Effects of Preview and Task Symmetry on Trunk Muscle Response to Sudden Loading

The effect of warning time (preview) and task symmetry on the trunk muscular response to sudden loading conditions was investigated. Eleven subjects were asked to catch falling weights with four levels of preview (0, 100, 200, and 400 ms) in sagittally symmetric posture and asymmetric posture. For each of the eight muscles sampled with surface electrodes, the integrated electromyographic (EMG) signal was interpreted in terms of its peak value, mean value, onset rate, and lead/lag time with reference to the weight drop. Results show linear relationships between preview times and peak EMG, preview times and mean EMG, and preview times and lead times. The results show significant change when going from symmetric to asymmetric conditions across most dependent measures. Analysis of peak changes in compression were performed across all conditions but yielded unexpected results.

A field evaluation of monitor placement effects in VDT users

Appropriate visual display terminal (VDT) location is a subject of ongoing debate. Generally, visual strain is associated with higher placement, and musculoskeletal strain is associated with lower placement. Seeking resolution of the debate, this paper provides a comparison of results from previous lab-based monitor placement studies to recommendations and outcomes from viewing preference and neutral posture studies. The paper then presents results from a field study that addressed two outstanding issues: Does monitor placement in a workplace elicit postures and discomfort responses similar to those seen in laboratory settings? Results showed placements in the workplace elicited postures similar to those in lab studies. Additionally, preferred VDT location generally corresponded to the location in which less neck discomfort was reported, though that trend requires further investigation. Overall, there seems to be consistent evidence to support mid-level or somewhat higher placement, as a rule-of-thumb, considering preferred gaze angle and musculoskeletal concerns. However, optimal placement may be lower for some individuals or tasks.

Trunk strength during asymmetric trunk motion

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.

Selective activation of the external oblique musculature during axial torque production

OBJECTIVE: To investigate whether different geographical regions of the external oblique musculature can activate at different levels and, if they do, to quantify the magnitude of these differences as a function of postural parameters during twisting exertions. DESIGN: Repeated measures design using electromyography on healthy subjects. BACKGROUND: The majority of the models currently used to assess spinal loading have represented the trunk musculature using single force vectors connecting a muscles point of origin to its point of insertion. However, for muscles with large areas of origin and/or insertion (such as the external obliques), this single vector modelling approach misrepresents the multiple vector reality which, in turn, underestimates the complex loads these muscles can develop. METHODS: Nine subjects performed sub-maximal isometric axial twisting exertions (20, 40 and 60% of maximum voluntary contraction) while assuming six different postures defined by three levels of axial rotation (-20 degrees, 0 degrees and 20 degrees ) and two levels of sagittal flexion (0 degrees and 20 degrees ). As the subjects performed these isometric exertions, the integrated electromyographic activity was sampled using surface electrodes at five different locations over the right and the left external oblique muscles. RESULTS: The results showed significant (p<0.05) regional differences in the activation profiles and these activation profiles changed as a function of trunk posture. CONCLUSIONS: The external oblique musculature is capable of differential activation and the activation profiles of the different regions are affected by the posture of the torso. RELEVANCE: These findings suggest that the external oblique muscle is capable of selective activation of different regions along its cross-section and should, therefore, be modelled using multiple vectors. The result can have a direct bearing on the calculated spine loading, especially lateral and anterior/posterior shear forces.

Continuous Assessment of Back Stress (CABS): A New Method to Quantify Low-Back Stress in Jobs with Variable Biomechanical Demands:

Jobs with a high degree of variability in manual materials handling requirements expose limitations in current low-back injury risk assessment tools and emphasize the need for a probabilistic representation of the biomechanical stress in order to quantify both acute and cumulative trauma risk. We developed a hybrid assessment methodology that employs established assessment tools and then represents their evaluations in a way that emphasizes the distributions of biomechanical stress. Construction work activities in the home building industry were evaluated because of the high degree of variability in the manual material handling requirements. Each task was evaluated using the Revised NIOSH Lifting Equation, The University of Michigan Three- Dimensional Static Strength Prediction Program™, and the Ohio State University Lumbar Motion Monitor Model. The output from each model was presented as time-weighted histograms of low-back stress, and the assessments were compared. The results showed considerable differences in what were considered high-risk activities, indicating that these 3 assessment tools consider the risk of low-back injury from different perspectives. The time-weighted distribution aspect of this methodology also contributed vital information toward the identification of high-risk activities. These results illustrate the necessity for more advanced low-back injury risk assessment techniques for jobs with highly variable manual materials handling requirements.

Intra-abdominal pressure during trunk extension motions

OBJECTIVE: This study was designed to help interpret the biomechanical role of intra-abdominal pressure during lifting type motions of the trunk. DESIGN: An in vivo study was performed in which intra-abdominal pressure was observed as subject trunks were subjected to different dynamic trunk loading conditions common during industrial lifting. BACKGROUND: There is a little consensus as to the biomechanical role of intra-abdominal pressure during lifting. Previous studies have suggested that: it may assist in load relief when lifting, may be involved in trunk stability, and/or may be used as a measure fo spine loading. Thus, in general, our understanding of intra-abdominal pressure is rather poor. METHODS: In this study intra-abdominal pressure was monitored using a radio pill in 114 subjects over a series of four experiments. Subjects trunks were subjected to different dynamic trunk symmetric and asymmetric trunk loading conditions that are common during industrial lifting tasks. RESULTS: The results indicated that (1) intra-abdominal pressure increased to significant levels (above 10 mmHg) only when more than 54 Nm of trunk torque were supported; (2) intra-abdominal pressure increases monotonically (up to 150 mmHg) as a function of trunk velocity; and (3) under concentric conditions intra-abdominal pressure increases as a function of greater asymmetry, whereas, under eccentric conditions the response changes to a much lesser extent as asymmetry changes. CONCLUSIONS: These findings suggest that intra-abdominal pressure appears to be more a by-product of trunk muscle coactivation. Any mechanical advantage gained from intra-abdominal pressure might be in the form of a preparatory action resulting from muscle coactivation that stiffens the trunk just prior to a rapid trunk extension exertion. This function may reinforce previous hypotheses regarding the stability role of intra-abdominal pressure. RELEVANCE: Intra-abdominal pressure has been observed during lifting for several decades, yet the biomechanical role of intra-abdominal pressure is poorly understood. This study has attempted to describe how intra-abdominal pressure behaves during lifting motions as the components of lifting are changed. The findings place in doubt biomechanical significance of intra-abdominal pressure. Thus, based upon this study, clinicians need not worry about interpreting intra-abdominal pressure, since it appears to be a by-product of muscle contraction and cocontraction.