A method for measuring external loads during dynamic lifting exertions

Recovering from Laboratory-Induced slips and trips causes high levels of lumbar muscle activity and spine loading

Slips, trips, and falls are some of the most substantial and prevalent causes of occupational injuries and fatalities, and these events may contribute to low-back problems. We quantified lumbar kinematics (i.e., lumbar angles relative to pelvis) and kinetics during unexpected slip and trip perturbations, and during normal walking, among 12 participants (6F, 6 M). Individual anthropometry, lumbar muscle geometry, and lumbar angles, along with electromyography from 14 lumbar muscles were used as input to a 3D, dynamic, EMG-based model of the lumbar spine. Results indicated that, in comparison with values during normal walking, lumbar range of motion, lumbosacral (L5/S1) loads, and lumbar muscle activations were all significantly higher during the slip and trip events. Maximum L5/S1 compression forces exceeded 2700 N during slip and trip events, compared with ∼ 1100 N during normal walking. Mean values of L5/S1 anteroposterior (930 N), and lateral (800 N) shear forces were also substantially larger than the shear force during the normal walking (230 N). These observed levels of L5/S1 reaction forces, along with high levels of bilateral lumbar muscle activities, suggest the potential for overexertion injuries and tissue damage during unexpected slip and trip events, which could contribute to low back injuries. Outcomes of this study may facilitate the identification and control of specific mechanisms involved with low back disorders consequent to slips or trips.

Immune responses to low back pain risk factors

$\underline{Objective}$: Investigate effects of interactions between biomechanical, psychosocial and individual risk factors on the body's immune inflammatory responses. $\underline{Background}$: Current theories for low back pain causation do not fully account for the body's response to tissue loading and tissue trauma. $\underline{Methods}$: Two groups possessing a preference for the sensor or intuitor personality trait performed repetitive lifting combined with high or low mental workload on separate occasions. Spinal loading was assessed using an EMG-assisted subject-specific biomechanical model and immune markers were collected before and after exposure. $\underline{Results}$: Mental workload was associated with a small decrease in AP shear. Both conditions were characterized by a regulated time-dependent immune response making use of markers of inflammation, tissue trauma and muscle damage. Intuitors' creatine kinase levels were increased following low mental workload compared to that observed in Sensors with the opposite trend occurring for high mental workload. $\underline{Conclusions}$: A temporally regulated immune response to lifting combined with mental workload exists. This response is influenced by personality and mental workload.

Putting mind and body back together: a human-systems approach to the integration of the physical and cognitive dimensions of task design and operations

As human factors and ergonomics professionals we should be considering the total context within which the person must operate when performing a task, providing a service, or using a product. We have traditionally thought of the person as having a cognitive system and a physical system and much of our scientific literature has been myopically focused on one or the other of these systems while, in general, totally ignoring the other. However, contemporary efforts have begun to recognize the rich interactions occurring between these systems that can have a profound influence on performance and dictate overall system output. In addition, modern efforts are beginning to appreciate the many interactions between the various elements of the environment that can influence the components of the human systems. The next level of sophistication in the practice of human factors and ergonomics must begin to consider the totality of the human-system behavior and performance and must consider systems design interactions which result from these collective effects. Only then will we be able to truly optimize systems for human use.

Rescuer’s Position and Energy Consumption, Spinal Kinetics, and Effectiveness of Simulated Cardiac Compression

Background Cardiopulmonary resuscitation is often performed in compromised conditions and for long periods.

Objective To compare energy expenditure, compression effectiveness, and kinetics of the spine during simulated chest compression with the rescuer in different positions.

Methods A 3-group design with 36 nurses (26 females) and 20 male emergency medical technicians was used. Participants performed chest compressions on a mannequin while kneeling on the floor, standing, or kneeling on the bed at the edge of the mattress (bed mount). Oxygen consumption and effectiveness of chest compression were recorded. Muscle moment and power at the lumbosacral joint were determined by recording motions of the lower limbs and pelvis with an electromagnetic tracking device and measuring ground reaction forces with a force plate.

Results A total of 80% of chest compressions delivered by male rescuers (vs 40% delivered by females) were effective, irrespective of position. Male rescuers consumed less oxygen when delivering chest compressions while standing than while kneeling (P = .03), but effective compression ratio also was lower. In female rescuers, effective compressions correlated positively with oxygen consumption in the standing (r = 0.42, P = .04) and bed-mount (r = 0.53, P = .008) positions. Administering chest compressions while standing involved a larger moment magnitude and required more power than doing so while kneeling.

Conclusion Administering chest compressions while standing demands more power but consumes less oxygen than doing so while kneeling, perhaps because fewer cardiac compressions delivered while standing are effective.

Spinal loading during manual materials handling in a kneeling posture

Stooped, restricted, kneeling, and other awkward postures adopted during manual materials handling have frequently been associated with LBP onset. However, lift assessment tools have focused on materials handling performed in an upright, or nearly upright standing posture. Unfortunately, many of the tools designed to analyze standing postures are not easily adapted to jobs requiring restricted postures. Therefore, the objective of this study was to evaluate spinal loading during manual materials handing in kneeling postures and determine if those loads can be predicted using simple regression. An EMG-driven biomechanical model, previously validated for upright lifting, was adapted for use in kneeling tasks. Subjects knelt under a 1.07m ceiling and lifted luggage of six weights (6.8, 10.9, 15.0, 19.1, 23.1 and, 27.2kgf) to one of four destination heights (0, 25.4, 53.3, 78.7cm). Spine loading was significantly affected by both destination height and load weight. Destination height increased compression, AP shear and lateral shear by an average of 14.5, 3.7 and 6.6N respectively per cm height increase. Load weight increased compression, AP shear and lateral shear by an average of 83.8, 27.0 and 13.1N respectively per kgf lifted. Regression equations were developed to predict peak spine loading using subject height, load weight and destination height with R(2) values of 0.62, 0.51 and 0.57 for compression, AP and lateral shear respectively.

Active trunk stiffness during voluntary isometric flexion and extension exertions

OBJECTIVE

Compare muscle activity and trunk stiffness during isometric trunk flexion and extension exertions.

BACKGROUND

Elastic stiffness of the torso musculature is considered the primary stabilizing mechanism of the spine. Therefore, stiffness of the trunk during voluntary exertions provides insight into the stabilizing control of pushing and pulling tasks.

METHODS

Twelve participants maintained an upright posture against external flexion and extension loads applied to the trunk. Trunk stiffness, damping, and mass were determined from the dynamic relation between pseudorandom force disturbances and subsequent small-amplitude trunk movements recorded during the voluntary exertions. Muscle activity was recorded from rectus abdominus, external oblique, lumbar paraspinal, and internal oblique muscle groups.

RESULTS

Normalized electromyographic activity indicated greater antagonistic muscle recruitment during flexion exertions than during extension. Trunk stiffness was significantly greater during flexion exertions than during extension exertions despite similar levels of applied force. Trunk stiffness increased with exertion effort.

CONCLUSION

Theoretical and empirical analyses reveal that greater antagonistic cocontraction is required to maintain spinal stability during trunk flexion exertions than during extension exertions. Measured differences in active trunk stiffness were attributed to antagonistic activity during flexion exertions with possible contributions from spinal kinematics and muscle lines of action.

APPLICATION

When compared with trunk extension exertions, trunk flexion exertions such as pushing tasks require unique neuromuscular control that is not simply explained by differences in exertion direction. Biomechanical analyses of flexion tasks must consider the stabilizing muscle recruitment patterns when evaluating spinal compression and shear loads.

The influence of age on isometric endurance and fatigue is muscle dependent: a study of shoulder abduction and torso extension

The present study examined differences in isometric muscle capacity between older (55-65 years) and younger (18 - 25 years) individuals. A total of 24 younger and 24 older participants (gender balanced within each group) performed sustained shoulder abductions and torso extensions to exhaustion at 30%, 50% and 70% of individual maximal voluntary contraction (MVC). Along with endurance time, manifestations of localized fatigue were determined based on changes in surface electromyographic signals obtained from the shoulder (middle deltoid) and the torso (multifidus and longissimus thoracis) muscles. Strength recovery was monitored using post-fatigue MVCs over a 15-min period. Compared to the younger group, older individuals exhibited lower muscular strength, longer endurance time and slower development of local fatigue. Age effects on fatigue were typically moderated by effort level, while effects of gender appeared to be marginal. Non-linear relationships between target joint torque and endurance time were observed, with effects of age differing between shoulder abduction and torso extension. Overall, the effects of age on endurance and fatigue were more substantial and more consistent for the shoulder muscle than for the torso muscles and were likely related to differences in muscle fibre type composition. For strength recovery rates, no significant age or gender effects were found in either experiment. In summary, this study suggests that differences in isometric work capacity do exist between older and younger individuals, but that this effect is influenced by effort level and the muscle tested.

Utility of traditional and alternative EMG-based measures of fatigue during low-moderate level isometric efforts

Traditional electromyographic (EMG) measures (e.g., amplitude, mean and median frequencies of the power spectra) have demonstrated inconsistent abilities in monitoring localized muscle fatigue at relatively low effort levels. In the present study, several alternative EMG-based fatigue indices were evaluated, derived using a logarithmic representation of the power spectrum, the fractal dimension of the raw signal, and a Poisson distribution fit to the power spectrum. These methods, along with traditional approaches, were applied to EMG data obtained from three separate experiments. In the first two experiments, 24 participants performed sustained isometric shoulder abductions and torso extensions at 30% of maximum voluntary strength (MVC). In the third experiment, another group of 12 participants performed similar shoulder exercises at 15% and 30% MVC, with repeatability assessed at 15% MVC. Both traditional and alternative EMG measures were analyzed for their 'utility', in terms of sensitivity to fatigue, variability, repeatability, and predictive ability. Results demonstrated that parameters derived from fractal analysis and the Poisson distribution demonstrated high utility. These alternative approaches appear promising as fatigue indices for low level isometric tasks.

Interface stability influences torso muscle recruitment and spinal load during pushing tasks

Handle or interface design can influence torso muscle recruitment and spinal load during pushing tasks. The objective of the study was to provide insight into the role of interface stability with regard to torso muscle recruitment and biomechanical loads on the spine. Fourteen subjects generated voluntary isometric trunk flexion force against a rigid interface and similar flexion exertions against an unstable interface, which simulated handle design in a cart pushing task. Normalized electromyographic (EMG) activity in the rectus abdominus, external oblique and internal oblique muscles increased with exertion effort. When using the unstable interface, EMG activity in the internal and external oblique muscle groups was greater than when using the rigid interface. Results agreed with trends from a biomechanical model implemented to predict the muscle activation necessary to generate isometric pushing forces and maintain spinal stability when using the two different interface designs. The co-contraction contributed to increased spinal load when using the unstable interface. It was concluded that handle or interface design and stability may influence spinal load and associated risk of musculoskeletal injury during manual materials tasks that involve pushing exertions.

Co-contraction recruitment and spinal load during isometric trunk flexion and extension

BACKGROUND

Pushing and pulling tasks account for 20% of occupational low-back injury claims. Primary torso muscle groups recruited during pushing tasks include rectus abdominis and the external obliques. However, analyses suggest that antagonistic co-contraction of the paraspinal muscles is necessary to stabilize the spine during flexion exertions. The study quantified co-contraction and spinal load differences during isometric flexion and extension exertions. The goal was to provide insight into the mechanisms requiring greater co-contraction during trunk flexion exertions compared to extension exertions.

METHODS

Electromyographic (EMG) signals were recorded from the trunk muscles of healthy volunteers during isometric trunk flexion and extension exertions. A biomechanical model was implemented to estimate total muscle force from the measured EMG and trunk moment data. A similar model estimated the muscle forces necessary to achieve equilibrium while minimizing the sum of squared muscle forces. The difference in these forces represented co-contraction. Spinal load attributed to co-contraction was computed.

RESULTS

Average co-contraction during flexion exertions was approximately twice the value of co-contraction during extension, i.e. 28% and 13% of total muscle forces respectively. Co-contraction accounted for up to 47% of the total spinal load during flexion exertions. Consequently, spinal compression during the flexion tasks was nearly 50% greater than during extension exertions despite similar levels of trunk moment.

INTERPRETATION

Co-contraction must be considered when evaluating spinal load during pushing exertions. Results underscore the need to consider neuromuscular control of spinal stability when evaluating the biomechanical risks.

The Effect of a Repetitive, Fatiguing Lifting Task on Horizontal Ground Reaction Forces

There are many outdoor work environments that involve the combination of repetitive, fatiguing lifting tasks and less-than-optimal footing (muddy/slippery ground surfaces). The focus of the current research was to evaluate the effects of lifting-induced fatigue of the low back extensors on lifting kinematics and ground reaction forces. Ten participants performed a repetitive lifting task over a period of 8 minutes. As they performed this task, the ground reaction forces and whole body kinematics were captured using a force platform and magnetic motion tracking system, respectively. Fatigue was verified in this experiment by documenting a decrease in the median frequency of the bilateral erector spinae muscles (pretest-posttest). Results indicate significant (p< 0.05) increases in the magnitude of the peak anterior/posterior (increased by an average of 18.3%) and peak lateral shear forces (increased by an average of 24.3%) with increasing time into the lifting bout. These results have implications for work environments such as agriculture and construction, where poor footing conditions and requirements for considerable manual materials handling may interact to create an occupational scenario with an exceptionally high risk of a slip and fall.

Functional impairment as a predictor of spine loading

STUDY DESIGN

Spine loadings during a variety of lifting exertions were compared with individual torso kinematic abilities. Relationships were evaluated between these measures.

OBJECTIVE

To determine if trunk kinematic status (functional impairment) is indicative of spine loading increases in patients with low back pain (LBP) compared to asymptomatic individuals.

SUMMARY OF BACKGROUND DATA

Recurrent LBP is a common and costly problem that may be related to increased spine loads in those individuals with LBP. Previous studies suggest that patients with LBP had greater loading than their asymptomatic counterparts when performing work. However, we know little about how to identify when a patient with LBP can resume lifting tasks without having exaggerated spine loading.

METHODS

Sixty-two patients with LBP and 61 who were asymptomatic were evaluated for signs of kinematic compromise (i.e., inability to generate normal trunk kinematic patterns) during a prelift test. All subjects were then asked to perform a variety of lifting exertions that varied in lift origin (region), lift asymmetry position, and weight lifted. An electromyography-assisted model was used to evaluate spine loading in each subject during the lifting exertions. Statistical models were used to assess the relationship between kinematic compromise and spine loading.

RESULTS

Patients with LBP had greater spine loading as well as greater kinematic compromise. The degree of kinematic compromise was related to the degree of spine loading increases in those individuals with LBP. A statistical model was developed that was able to describe 87% of the variability in compression, 61% in anteroposterior shear, and 65% in lateral shear.

CONCLUSIONS

Those patients with greater kinematic compromise used higher levels of antagonistic muscle coactivation that not only reduced trunk motion but also resulted in increases in spine loading. Given the degree of kinematic compromise and the lifting task conditions, a method has been devised to predict the increase in spine loading above and beyond that of an asymptomatic individual when performing typical materials handling tasks.

Low-back biomechanics and static stability during isometric pushing

Pushing and pulling tasks are increasingly prevalent in industrial workplaces. Few studies have investigated low-back biomechanical risk factors associated with pushing, and we are aware of none that has quantified spinal stability during pushing exertions. Data recorded from 11 healthy participants performing isometric pushing exertions demonstrated that trunk posture, vector force direction of the applied load, and trunk moment were influenced (p < .01) by exertion level, elevation of the handle for the pushing task, and foot position. A biomechanical model was used to analyze the posture and hand force data gathered from the pushing exertions. Model results indicate that pushing exertions provide significantly (p < .01) less stability than lifting when antagonistic cocontraction is ignored. However, stability can be augmented by recruitment of muscle cocontraction. Results suggest that cocontraction may be recruited to compensate for the fact that equilibrium mechanics provide little intrinsic trunk stiffness and stability during pushing exertions. If one maintains stability by means of cocontraction, additional spinal load is thereby created, increasing the risk of overload injury. Thus it is important to consider muscle cocontraction when evaluating the biomechanics of pushing exertions. Potential applications of this research include improved assessment of biomechanical risk factors for the design of industrial pushing tasks.

Paraspinal muscle reflex dynamics

Neuromuscular control of spinal stability may be represented as a control system wherein the paraspinal muscle reflex acts as feedback response to kinetic and kinematic disturbances of the trunk. The influence of preparatory muscle recruitment for the control of spinal stability has been previously examined, but there are few reported studies that characterize paraspinal reflex gain as feedback response. In the current study, the input-output dynamics of paraspinal reflexes were quantified by means of the impulse response function (IRF), with trunk perturbation force representing the input signal and EMG the output signal. Surface EMGs were collected from the trunk muscles in response to a brief anteriorly directed impact force applied to the trunk of healthy participants. Reflex behavior was measured in response to three levels of force impulse, 6.1, 9.2 and 12.0 Ns, and two different levels of external trunk flexion preload, 0 and 110 N anterior force. Reflex EMG was quantifiable in response to 91% of the perturbations. Mean reflex onset latency was 30.7+/-21.3 ms and reflex amplitude increased with perturbation amplitude. Impulse response function gain, G(IRF), was defined as the peak amplitude of the measured IRF and provided a consistent measure of response behavior. EMG reflex amplitude and G(IRF) increased with force impulse. Mean G(IRF) was 2.27+/-1.31% MVC/Ns and demonstrated declining trend with flexion preload. Results agree with a simple systems model of the neuromechanical feedback behavior. The relative contribution of the reflex dynamics to spinal stability must be investigated in future research.

Spine loading in patients with low back pain during asymmetric lifting exertions

BACKGROUND CONTEXT

Recurrent low back pain (LBP) is a common and costly problem that might be related to increased spine loads in those with LBP. However, we know little about how the spine is loaded when those with LBP perform lifting exertions.

PURPOSE

Document spine loading patterns of patients with LBP performing symmetric and asymmetric lifting exertions compared with asymptomatic individuals performing the same tasks.

STUDY DESIGN

Spine loadings during lifting exertions that varied in asymmetric origin as well as horizontal and vertical distance from the spine were compared between asymptomatic subjects and patients with LBP.

METHODS

Sixty-two patients with LBP and 61 asymptomatic individuals performed a variety of lifting exertions that varied in lift origin horizontal and vertical position (region), lift asymmetry position and weight lifted. An electromyography-assisted model was used to evaluate spine loading in each subject during the lifting exertions. Differences in spine loading between the LBP and asymptomatic subjects were noted as a function of the experimental variables.

RESULTS

Patients with LBP experienced greater spine compression and shear forces when performing lifting tasks compared with asymptomatic individuals. The least taxing conditions resulted in some of the greatest differences between LBP and asymptomatic individuals.

CONCLUSIONS

Greater levels of antagonistic muscle coactivation resulted in increases in spine loading for patients with LBP. Specific lifting conditions that tend to exacerbate loading can be identified by means of physical workplace requirements. These findings may impact acceptable return-to-work conditions for those with LBP.

Spine loading as a function of gender

STUDY DESIGN

In vivo laboratory studies were conducted to investigate the spine loads imposed on men and women during a series of lifting tasks that varied in the degree of lifting control required by the subject.

OBJECTIVE

To identify and understand differences in spine loading and musculoskeletal control strategies between men and women performing lifts of varying task complexity.

SUMMARY OF BACKGROUND DATA

Few studies have examined differences in spine loading as a function of individual factors such as subject gender. Furthermore, no biomechanical studies have attempted to quantify and understand how differences in anthropometry between genders might influence muscle recruitment and subsequent spine loads. Because the modern workplace seldom discriminates between genders in job assignments, it is important to understand how differences in spine loading and potential low back disorder risk might be associated with gender differences.

METHODS

For this study, 140 subjects participated in two separate experiments requiring different degrees of musculoskeletal motion control during sagittal plane lifting. The two experiments consisted of 35 men and 35 women performing lifts in which motion was isolated to the torso and 35 men and 35 women completing whole-body free-dynamic whole body lifts. An electromyography-assisted model was used to evaluate spine loading under these conditions.

RESULTS

Absolute spine compression generally was greater for the men. Under the highly controlled (isolated torso) conditions, most differences were attributed solely to differences in body mass. Under a whole-body free-dynamic condition, significant differences in muscle coactivations resulted in greater relative compression and anterior-posterior shear spine loading for the women.

CONCLUSIONS

Differences in spine loadings as a function of gender under the more controlled lifting conditions were primarily a function of different body masses. However, loading pattern differences existed between the genders under whole-body free-dynamic conditions as a result of kinematic compensations and increases in muscle cocontraction, with women generally experiencing greater relative loads. When spine tolerance differences are considered, one would expect that females would be at greater risk of musculoskeletal overload during lifting tasks.

Lower torso muscle activation patterns for high-magnitude static exertions: gender differences and the effects of twisting

STUDY DESIGN

Surface electromyographic signals were collected from 14 lower torso muscles while participants resisted high-magnitude static trunk moments applied in a variety of directions.

OBJECTIVES

To obtain a description of muscle activations in response to large moment magnitudes and axial twisting, including levels of agonistic and antagonistic muscle cocontraction. To assess differences in lower torso muscle activation patterns associated with gender and trial repetition.

SUMMARY OF BACKGROUND DATA

Back pain is associated with mechanical loads in the back. Biomechanical modeling of these loads is facilitated by knowledge of typical muscle activation patterns. Previous efforts in obtaining such data have often limited their scope to low-magnitude exertions or relatively simple scenarios.

METHODS

Eight male and eight female participants, matched by height and mass, performed static exertions in an apparatus that immobilized their lower body while the activation levels of seven bilateral torso muscles were measured using surface electromyography. Activation patterns were analyzed to assess differences resulting from a variety of factors.

RESULTS

No significant differences in activation patterns were found between genders or repetitions, but moment magnitude and direction elicited substantial differential responses. Good repeatability was found between trial repetitions, as indicated by intraclass correlation coefficients (>0.65). Significant synergistic muscle coactivation, large intersubject variability (mean coefficient of variation 82.2%), and consistent levels of antagonism ranging from 10% to 30% maximum voluntary exertions were observed.

CONCLUSIONS

Individuals of different genders, but similar anthropometry, have comparable muscular reactions to complex torso loads, suggesting similar motor control strategies. Future spine models should consider that the variability in muscle recruitment patterns is larger between subjects than within subjects. High-magnitude exertions, especially those with moment loads in more than one plane, require most muscles to be active (>5%) and moderate levels of antagonism.

Spine loading characteristics of patients with low back pain compared with asymptomatic individuals

STUDY DESIGN

Patients with low back pain and asymptomatic individuals were evaluated while performing controlled and free-dynamic lifting tasks in a laboratory setting.

OBJECTIVE

To evaluate how low back pain influences spine loading during lifting tasks.

SUMMARY OF BACKGROUND DATA

An important, yet unresolved, issue associated with low back pain is whether patients with low back pain experience spine loading that differs from that of individuals who are asymptomatic for low back pain. This is important to understand because excessive spine loading is suspected of accelerating disc degeneration in those whose spines are damaged already.

METHODS

In this study, 22 patients with low back pain and 22 asymptomatic individuals performed controlled and free-dynamic exertions. Trunk muscle activity, trunk kinematics, and trunk kinetics were used to evaluate three- dimensional spine loading using an electromyography- assisted model in conjunction with a new electromyographic calibration procedure.

RESULTS

Patients with low back pain experienced 26% greater spine compression and 75% greater lateral shear (normalized to moment) than the asymptomatic group during the controlled exertions. The increased spine loading resulted from muscle coactivation. When permitted to move freely, the patients with low back pain compensated kinematically in an attempt to minimize external moment exposure. Increased muscle coactivation and greater body mass resulted in significantly increased absolute spine loading for the patients with low back pain, especially when lifting from low vertical heights.

CONCLUSIONS

The findings suggest a significant mechanical spine loading cost is associated with low back pain resulting from trunk muscle coactivation. This loading is further exacerbated by the increases in body weight that often accompany low back pain. Patient weight control and proper workplace design can minimize the additional spine loading associated with low back pain.

A non-MVC EMG normalization technique for the trunk musculature: Part 1. Method development

Normalization of muscle activity has been commonly used to determine the amount of force exerted by a muscle. The most widely used reference point for normalization is the maximum voluntary contraction (MVC). However, MVCs are often subjective, and potentially limited by sensation of pain in injured individuals. The objective of the current study was to develop a normalization technique that predicts an electromyographic (EMG) reference point from sub-maximal exertions. Regression equations predicting maximum exerted trunk moments were developed from anthropometric measurements of 120 subjects. In addition, 20 subjects performed sub-maximal and maximal exertions to determine the necessary characteristic exertions needed for normalization purposes. For most of the trunk muscles, a highly linear relationship was found between EMG muscle activity and trunk moment exerted. This analysis determined that an EMG-moment reference point can be obtained via a set of sub-maximal exertions in combination with a predicted maximal exertion (expected maximum contraction or EMC) based upon anthropometric measurements. This normalization technique overcomes the limitations of the subjective nature for the MVC method providing a viable assessment method of individuals with a low back injury or those unwilling to exert an MVC as well as could be extended to other joints/muscles.

A non-MVC EMG normalization technique for the trunk musculature: Part 2. Validation and use to predict spinal loads

Estimates of the amount of force exerted by a muscle using electromyography (EMG) rely partially upon the accuracy of the reference point used in the normalization technique. Accurate representations of muscle activities are essential for use in EMG-driven spinal loading models. The expected maximum contraction (EMC) normalization method was evaluated to explore whether it could be used to assess individuals who are not capable of performing a maximum exertion such as a person with a low back injury. Hence, this study evaluated the utility of an EMG normalization method (Marras and Davis, A non-MVC EMG normalization technique, Part 1, method development. Journal of Electromyography and Kinesiology 2000) that draws upon sub-maximal exertions to determine the reference points needed for normalization of the muscle activities. The EMC normalization technique was compared to traditional MVC-based EMG normalization by evaluating the spinal loads for 20 subjects (10 males and 10 females) performing dynamic lifts. The spinal loads (estimated via an EMG-assisted model) for the two normalization techniques were very similar with differences being <8%. The model performance variables indicated that both normalization techniques performed well (r(2)>0.9 and average error below 6%) with only the muscle gain being affected by normalization method as a result in different reference points. Based on these results, the proposed normalization technique was considered to be a viable method for EMG normalization and for use in EMG-assisted models. This technique should permit the quantitative evaluation of muscle activity for subjects unable to produce maximum exertions.

Lumbar-pelvic coordination is influenced by lifting task parameters

STUDY DESIGN

Low back kinematics, including relative lumbar and pelvic motions, were quantified during controlled lifting tasks.

OBJECTIVES

To evaluate the influence of load and lifting velocity on lumbar-pelvic (LP) coordination.

SUMMARY OF BACKGROUND DATA

Sagittal trunk extension is achieved through the coordinated motion of the pelvis and lumbar spine. There are no data to indicate whether lifting task design influences lumbar-pelvic coordination.

METHODS

Lumbar and pelvic motions were recorded from 18 healthy subjects while performing isokinetic lifting tasks of 0.1 kg and 10 kg. Coordinated motions of the pelvis (sacral spine) and low-thoracic spine were evaluated using eigenvector analyses and a ratio of lumbar and pelvic angles (L/P).

RESULTS

Eigenvector models of the lumbar-pelvic coordination accurately represented empirical coordination profiles. Weight significantly influenced lumbar-pelvic coordination. Trunk extension velocity demonstrated a small but statistically significant influence on lumbar-pelvic coordination. Weight and trunk flexion angle significantly influenced lumbar/pelvic angle ratios.

CONCLUSIONS

Trunk extension was achieved through simultaneous but nonlinear contributions from both the pelvis and lumbar spine throughout the range of motion. The lumbar spine accounted for 70% of the total, with increased pelvic contributions in flexed postures. Task weight increased the lumbar contribution to total trunk motion. When performing clinical evaluations of spinal kinematics, it is necessary to recognize that unloaded motions may not fully represent loaded behavior of spinal coordination.

Cost-benefit of muscle cocontraction in protecting against spinal instability

STUDY DESIGN

Lifting dynamics and electromyographic activity were evaluated using a biomechanical model of spinal equilibrium and stability to assess cost-benefit effects of antagonistic muscle cocontraction on the risk of stability failure.

OBJECTIVES

To evaluate whether increased biomechanical stability associated with antagonistic cocontraction was capable of stabilizing the related increase in spinal load.

SUMMARY OF BACKGROUND DATA

Antagonistic cocontraction contributes to improved spinal stability and increased spinal compression. For cocontraction to be considered beneficial, stability must increase more than spinal load. Otherwise, it may be possible for cocontraction to generate spinal loads that cannot be stabilized.

METHODS

A biomechanical model was developed to compute spinal load and stability from measured electromyography and motion dynamics. As 10 healthy men performed sagittal lifting tasks, trunk motion, reaction loads, and electromyographic activities of eight trunk muscles were recorded. Spinal load and stability were evaluated as a function of cocontraction and trunk flexion angle. Stability was quantified in terms of the maximum spinal load the system could stabilize.

RESULTS

Cocontraction was associated with a 12% to 18% increase in spinal compression and a 34% to 64% increase in stability. Spinal load and stability increased with trunk flexion.

CONCLUSIONS

Despite increases in spinal load that had to be stabilized, the margin between stability and spinal compression increased significantly with cocontraction. Antagonistic cocontraction was found to be most beneficial at low trunk moments typically observed in upright postures. Similarly, empirically measured antagonistic cocontraction was recruited less in high-moment conditions and more in low-moment conditions.

Variation in spinal load and trunk dynamics during repeated lifting exertions

OBJECTIVES

To quantify the variability in lifting motions, trunk moments, and spinal loads associated with repeated lifting exertions and to identify workplace factors that influence the biomechanical variability.

DESIGN

Measurement of trunk dynamics, moments and muscle activities were used as inputs into EMG assisted model of spinal loading.

BACKGROUND

Traditional biomechanical models assume repeated performance of a lifting task produces little variability in spinal load because the assessments overlook variability in lifting dynamics and muscle coactivity.

METHODS

Five experienced and seven inexperienced manual materials handlers performed 10 repeated lifts at each combination of load weight, task asymmetry and lifting velocity.

RESULTS

Box weight, task asymmetry and job experience influenced the magnitude and variability of spinal load during repeated lifting exertions. Surprisingly, experienced subjects demonstrated significantly greater spinal loads and within-subject variability in spinal load than inexperienced subjects. Trial-to-trial variability accounted for 14% of the total variation in compression overall and 32% in lateral shear load. Although the mean spinal load was safely below the NIOSH recommended limit; due to variability about the mean, more than 20% of the lifts exceeded the recommended limit.

CONCLUSION

Spinal load changed markedly from one exertion to the next despite identical task requirements. Trial-to-trial variability in kinematics, kinetics, and spinal load were influenced by workplace factors, and may play a role in the risk of low-back pain.

RELEVANCE

Ergonomic assessments considering only the mean value of spinal load overlook the fact that a large fraction of the lifts may exceed recommended levels.

Trunk muscle activities during asymmetric twisting motions

Axial twisting of the torso has been identified via epidemiologic studies as a significant risk factor for occupationally-related low back disorders. However, only recently have biomechanical studies been able to describe how twisting is accomplished through the use of the trunk musculature. These studies have been performed on subjects whose torso twists were performed in an upright posture. In this study, the electromyographic activity of ten trunk muscles was observed while 12 subjects performed twisting exertions in three different trunk postures. These postures included upright twisting, twisting while the trunk was flexed in the sagittal plane, and twisting while the trunk was flexed and rotated asymmetrically. In addition, twisting velocity and direction of motion were changed under the experimental conditions. Under upright twisting conditions, the twisting torque was generated easily and relatively efficiently through the employment of the oblique (internal and external) and latissimus dorsi muscles. When the trunk was flexed the activity of erector spinae muscles increased (about 10-15%) while the external oblique activity decreased (about 3-5%). Twisting while in asymmetric bent postures was accomplished with a reduction in oblique and latissimus dorsi muscle activities (approximately 5%) while the erector spinae muscle activity remained elevated. The change in muscle activity needed to balance the torso during twisting while bending also increased the amount of lateral torque that was produced by the trunk. These findings suggest that studies observing trunk muscle activities and trunk loading while subjects were in upright postures should be interpreted with caution when evaluating the activity of the trunk during occupational activities. Since many occupational twisting tasks are performed in awkward, asymmetric postures, application of results from upright twisting studies might underestimate the risk of these activities.

Evaluation of spinal loading during lowering and lifting

OBJECTIVE: To estimate the three-dimensional spinal loads during various lifting and lowering tasks. DESIGN: The in vivo measurements of the trunk dynamics, moments, and myoelectric activity were used as inputs into an electromyographic-assisted model used to predict the three-dimensional spinal loads. BACKGROUND: Previous studies of eccentric motions have investigated muscle activity, trunk strength, and trunk moments. A void in the body of knowledge exists in that none of these studies investigated spinal loading. METHODS: Ten subjects lifted (40 degrees of flexion to 0 degrees ) and lowered (0 degrees of flexion to 40 degrees ) boxes while positioned in a structure that restrained the pelvis and hips. The tasks were performed under isokinetic trunk velocities of 5, 10, 20, 40, and 80 deg s(-1) while holding a box with weights of 9.1, 18.2, and 27.3 kg. RESULTS: Lowering strength was found to be 56% greater than lifting strength. The lowering tasks produced significantly higher compression forces but lower anterior-posterior shear forces than the lifting tasks. The differences in the spinal loads produced by the two lifting tasks were attributed to differences in coactivity and unequal lifting moments (i.e. holding the box farther away from the body). CONCLUSIONS: The nature of the spinal loads that occur during lowering and lifting were significantly different. The difference in spinal loads may be explained by different lifting styles.

A method for measuring external spinal loads during unconstrained free-dynamic lifting

Biomechanical lifting models often require the knowledge of the applied trunk moments and forces for model validation purposes and/or to determine loading levels experienced at various joints of the body. Trunk kinetic data under dynamic exertions are commonly difficult to attain without restrictive anatomic/anthropometric assumptions and cost or constraining body motion. The main objectives of the study were to present a new technique for determining continuous three-dimensional forces and moments about the L5/S1 spinal joint, and to validate the technique and assess its applicability under lifting situations. A combination of a force plate and two electrogoniometers facilitated the determination of trunk kinetics about L5/S1. An apparatus was devised to allow the application of various actual moments that were compared to their corresponding predicted moments. The results showed that, over all the conditions considered, the average percent error in estimating the actual applied moment(s) was about 4% (2.3 S.D.), with a test-retest reliability approaching unity. Given such agreement, along with the relative ease and directness of the method, it is believed that this approach should be applicable under most lifting conditions. The technique offers a fairly accurate measure of trunk moments without the need for constraining the motion of any body joint.

Spine loading during trunk lateral bending motions

Increases in lateral trunk velocities have been identified as a mechanism for increasing the risk of low-back disorder. Previous studies have identified an increase in coactivation of the trunk musculature during lateral bends, but no studies have evaluated how spine loading changes as lateral trunk velocity increases. Twelve subjects were asked to lift loads laterally at one static and three dynamic velocities. Ten trunk muscle activities and trunk kinematics were documented and used as input parameters to an EMG-assisted model to evaluate spine loading. Muscle coactivation was observed in all lateral bends. Coactivation significantly increased during dynamic trials compared to the static trials. Coactivity increased spinal loads by as much as 25% compared to values predicted by models that did not consider coactivity. Movements to the right significantly increased spine loadings (252 N increase in compression) compared to movements to the left. Spine compression, A-P shear, and lateral shear all increased in the dynamic trials compared to the static conditions. Peak compression increased by an average of 525 N at 45 degrees s-1 compared to static loading. Compression and lateral shear increased monotonically as trunk velocity increased. It is expected that this combined (compression and lateral shear) loading is the mechanism for increased risk observed in industry.