Influence of posture on the relation between surface electromyogram amplitude and back muscle moment: consequences for the use of surface electromyogram to measure back load

Sensitivity of kinematics-based model predictions to optimization criteria in static lifting tasks

The effect of eight different cost functions on trunk muscle forces, spinal loads and stability was investigated. Kinematics-based approach combined with nonlinear finite element modeling and optimization were used to model in vivo measurements on isometric forward flexions at approximately 40 degrees and approximately 65 degrees in sagittal plane with or without a load of 180N in hands. Four nonlinear (summation stress(3), summation stress(2), summation force(2) and muscle fatigue) and four linear (summation stress, summation force, axial compression and double-linear) criteria were considered. Predicted muscle activities were compared with measured EMG data. All predictions, irrespective of the cost function used, satisfied required kinetic, kinematics and stability conditions all along the spine. Four criteria (summation stress(3), summation stress(2), fatigue and double-linear) predicted muscle activities that qualitatively matched measured EMG data. The fatigue and double-linear criteria were inadequate in predicting greater forces in larger muscles with no consideration for their moment arms. Nearly the same stability margin was computed under these four cost functions. At the lower lumbar levels, the compression forces differed by <20% and the shear forces by <14% as various cost functions were considered. Smaller axial compression and anterior shear forces (by less than or approximately equal 6%) were computed when only the active components rather than the total muscle forces were taken as unknown in the summation stress(3) cost function. Overall, one single cost function of summation stress(2) or summation stress(3) rather than a multi-criteria one was found sufficient and adequate in yielding plausible results comparable with measured EMG activities and disc pressure.

Biomechanics of changes in lumbar posture in static lifting

STUDY DESIGN

In vivo measurements and model studies are combined to investigate the role of lumbar posture in static lifting tasks.

OBJECTIVES

Identification of the role of changes in the lumbar posture on muscle forces, internal loads, and system stability in static lifting tasks with and without load in hands.

SUMMARY OF BACKGROUND DATA

Despite the recognition of the causal role of lifting in spinal injuries, the advantages of preservation or flattening of the lumbar lordosis while performing lifting tasks is not yet clear.

METHODS

Kinematics of the spine and surface EMG activity of selected muscles were measured in 15 healthy subjects under different forward trunk flexion angles and load cases. Apart from the freestyle lumbar posture, subjects were instructed to take either lordotic or kyphotic posture as well. A kinematics-based method along with a nonlinear finite element model were interactively used to compute muscle forces, internal loads and system stability margin under postures, and loads considered in in vivo investigations. RESULTS.: In comparison with the kyphotic postures, the lordotic postures increased the pelvic rotation, active component of extensor muscle forces, segmental axial compression and shear forces at L5-S1, and spinal stability margin while decreasing the passive muscle forces and segmental flexion moments.

CONCLUSION

Alterations in the lumbar lordosis in lifting resulted in significant changes in the muscle forces and internal spinal loads. Spinal shear forces at different segmental levels were influenced by changes in both the disc inclinations and extensor muscle lines of action as the posture altered. Considering internal spinal loads and active-passive muscle forces, the current study supports the freestyle posture or a posture with moderate flexion as the posture of choice in static lifting tasks.

Relationships of EMG to effort in the trunk under isometric conditions: force-increasing and decreasing effects and temporal delays

BACKGROUND

Electromyograms are used in increasingly sophisticated biomechanical analyses to estimate forces within the trunk to prevent and evaluate painful spinal conditions. However, even under nominally isometric conditions the relationship between EMG and effort is complex. This study quantified influences of pulling direction, increasing versus decreasing effort and electromechanical delay on the EMG/effort relationships for principal lower trunk muscle groups in isometric pulling tasks, to determine whether the observed differences between increasing versus decreasing effort relationships were consistent with electromechanical delay or activation differences.

METHODS

Twenty-three healthy subjects (15 male, 8 female; mean age 32 years; mean bodymass 74.5 kg) each stood in an apparatus to stabilize the pelvis and performed ramped isometric efforts with a harness around the thorax connected to each of a series of five anchor points on the wall, for angles of pull at each 45 degrees increment from 0 degrees to 180 degrees to the anterior direction. A load cell recorded the generated force for a 5 s timed increase up to a voluntary maximum, a 1s 'dwell', and a 5 s relaxation back to zero effort. EMG signals were recorded via electrodes (surface, except indwelling for multifidus) from right and left rectus abdominis, internal and external obliques, longissimus, iliocostalis and L2 and L4 level multifidus. EMG signals were rectified with a 250 ms root-mean-square moving average filter. Effort-increasing and effort-decreasing sections of recordings were analyzed separately.

FINDINGS

The EMG/effort relationship had a statistically significantly greater gradient as the effort was increasing than when decreasing for 28 of 70 muscle-angle permutations. This difference in gradient was found to explain a significant part of the apparent lag between effort generated and EMG signal that averaged between 261 and 658 ms before and between 31 and 196 ms for different muscles after the slope difference was taken into account.

INTERPRETATION

The findings were consistent with the notion that the motor unit recruitment differs in increasing versus decreasing isometric efforts, probably because of a small stretching of the muscle as its tension increases. The residual temporal delay was thought to represent electromechanical delay.

Between-days reliability of electromyographic measures of paraspinal muscle fatigue at 40, 50 and 60% levels of maximal voluntary contractile force

OBJECTIVE

To ascertain which percentage of maximal voluntary contractile force of the paraspinal muscles, when tested in a functional position, is most reliable for assessing electromyographic (EMG) fatigue changes.

SUBJECTS

Ten healthy volunteers with no history of low back pain (six males).

MAIN OUTCOME MEASURES

The surface EMG signal during 60-second isometric contractions of the paraspinal muscles at 40, 50 and 60% levels of maximal voluntary contractile force was captured and analysed. Each contraction level was assessed on two occasions, at least three days apart. The initial median frequency, the decline in median frequency slope and the increase in root mean square values were assessed for between-days reliability, using intraclass correlation coefficients (ICCs) and standard errors of measurements (SEM). Normalized median frequency and root mean square values were also assessed.

RESULTS

At 40% of maximal voluntary contraction, little or no EMG fatigue changes occurred in any of the observed parameters. At 50% maximal voluntary contraction the initial mean frequency and root mean square changes proved highly reliable, with ICCs ranging from 0.74 to 0.86 and 0.75 to 1.00 respectively. Normalizing the root mean square data reduced the reliability, but this was still acceptable with ICCs 0.70-0.83. The median frequency decline slope proved less reliable with ICCs 0.24-0.74 for raw and 0.26-0.77 for normalized data. At 60% maximal voluntary contraction the initial mean frequency proved as reliable as initial median frequency at 50% with ICCs 0.70-0.89. The raw and normalized root mean squares (ICCs 0.43-0.89 and 0.30-0.87 respectively) and raw and normalized median frequency (ICCs 0.27-0.51 and 0.24-0.53 respectively) changes were less reliable than at 50% MVC. Overall, the reliability is better at the L4/5 than at the L2/3 level.

CONCLUSION

Outcome measures taken at 50% maximal voluntary contraction are the most reliable in functional testing the paraspinal muscles of healthy volunteers. With initial median frequency and root mean square values being more reliable parameters than median frequency decline. At the L4/5 level, however, all parameters were acceptably reliable at 50% of maximum effort. However the between-subject variability of the median frequency decline and root mean square incline slopes suggest that these parameters are not yet fully suitable for monitoring fatigue changes during prolonged isometric contraction.

Factors in breathing maneuvers that affect trunk electromyogram during manual lifting

STUDY DESIGN

A fully randomized experiment was conducted in a laboratory with a breath-by-breath monitor to control accurately the two factors of breathing maneuvers: breathholding duration and air volume within the thoracic cavity.

OBJECTIVES

To resolve the controversy in previous reports about the effect of breathholding on the trunk electromyogram, and to verify the hypothesis that not only the factor of glottis closure, but also that of the air volume inside the thoracic cavity affects the trunk muscular activities during lifting.

SUMMARY OF BACKGROUND DATA

Breathing was shown to affect spinal loading. However, there still is a debate about the effect of breathholding on trunk muscular activation during activities. It is possible that variations in air volume influence this effect.

METHODS

Seven healthy, volunteer men participated in lifting tasks, in which lifting moment was standardized. Three breathing maneuvers were used: sustained breathholding with tidal volume of air, sustained breathholding with functional residual volume, and intended breath-nonholding involving inspiration within tidal capacity. Data on the surface electromyographic activation of the external oblique muscle, rectus abdominis, erector spinae, latissimus dorsi, air volume inside the cavity, and the duration of the one breath held in the last lift were collected and analyzed.

RESULTS

Of the four muscles investigated, the breathing maneuvers affected only the external oblique muscle. The effect of sustained breathhold during lifting was the significantly increased activation of this muscle (P < 0.05). The effect of decreased air volume held was further increased activation. Intention to inspire normally during lifting decreased external oblique activation, but increased compensatory diaphragmatic effort, measured as inspiratory flow acceleration.

CONCLUSIONS

Both the factors of the breathheld state and sustained air volume were verified to affect the external oblique activation during lifting. The current study emphasizes that both factors should be controlled in studies analyzing trunk electromyogram during activities. Otherwise, these breathing variations will be a confounding factor on electromyogram results.

An EMG technique for measuring spinal loading during asymmetric lifting

OBJECTIVES

To compare two methods of calibrating the erector spinae electromyographic signal against moment generation in order to predict extensor moments during asymmetric lifting tasks, and to compare the predicted moments with those obtained using a linked-segment model.

METHODS

Eight men lifted loads of 6.7 and 15.7 kg at two speeds, in varying amounts of trunk rotation. For each lift, the following were recorded at 60 Hz; the rectified and averaged surface electromyographic signal, bilaterally at T10 and L3, lumbar curvature using the 3-Space Isotrak, movement of body segments using a 4-camera Vicon system, and ground reaction forces using a Kistler force-plate. Electromyographic (EMG) and Isotrak data were used to calculate lumbosacral extensor moments using the electromyographic model, whereas movement analysis data and ground reaction forces were used to estimate net moments using the linked-segment model. For the electromyographic technique, predictions of extensor moment were based on two different sets of EMG-extensor moment calibrations: one performed in pure sagittal flexion and the other in flexion combined with 45 degrees of trunk rotation.

RESULTS

Extensor moments predicted by the electromyographic technique increased significantly with load and speed of lifting but were not influenced by the method of calibration. These moments were 7-40%greater than the net moments obtained with the linked-segment model, the difference increasing with load and speed.

CONCLUSIONS

The calibration method does not influence extensor moments predicted by the electromyographic technique in asymmetric lifting, suggesting that simple, sagittal-plane calibrations are adequate for this purpose. Differences in predicted moments between the electromyographic technique and linked-segment model may be partly due to different anthropometric assumptions and different amounts of smoothing and filtering in the two models, and partly due to antagonistic muscle forces, the effects of which cannot be measured by linked-segment models. RelevanceAsymmetric lifting is a significant risk factor for occupationally-related low back pain. Improved techniques for measuring spinal loading during such complex lifting tasks may help to identify work practices which place the spine at risk of injury.

Stoop or squat: a review of biomechanical studies on lifting technique

OBJECTIVE

To assess the biomechanical evidence in support of advocating the squat lifting technique as an administrative control to prevent low back pain.

BACKGROUND

Instruction with respect to lifting technique is commonly employed to prevent low back pain. The squat technique is the most widely advised lifting technique. Intervention studies failed to show health effects of this approach and consequently the rationale behind the advised lifting techniques has been questioned.

METHODS

Biomechanical studies comparing the stoop and squat technique were systematically reviewed. The dependent variables used in these studies and the methods by which these were measured or estimated were ranked for validity as indicators of low back load.

RESULTS

Spinal compression as indicated by intra-discal pressure and spinal shrinkage appeared not significantly different between both lifting techniques. Net moments and compression forces based on model estimates were found to be equal or somewhat higher in squat than in stoop lifting. Only when the load could be lifted from a position in between the feet did squat lifting cause lower net moments, although the studies reporting this finding had a marginal validity. Shear force and bending moments acting on the spine appeared lower in squat lifting. Net moments and compression forces during lifting reach magnitudes, that can probably cause injury, whereas shear forces and bending moments remained below injury threshold in both techniques.

CONCLUSION

The biomechanical literature does not provide support for advocating the squat technique as a means of preventing low back pain.

RELEVANCE

Training in lifting technique is widely used in primary and secondary prevention of low back pain, though health effects have not been proven. The present review assesses the biomechanical evidence supporting the most widely advocated lifting technique.

Pattern classification reveals intersubject group differences in lumbar muscle recruitment during static loading

OBJECTIVE: This paper examines interindividual differences in the patterns of torso muscle recruitment during 3-dimensional static moment loading of the lumbar spine. DESIGN: A mathematical model (artificial neural network) was used to differentiate individual patterns of muscle response. BACKGROUND: Traditionally, experimental myoelectric data is averaged over subjects, assuming an ideal mean response to a given loading. However, averaging may overlook important information and implications associated with interindividual variability. METHODS: In this study a simple classification tool in the form of a competitive neural network model is developed and used to evaluate lumbar muscle recruitment patterns. RESULTS: Subjects formed consistent and denumerable clusters, and could be categorized as either 'majority' or 'minority' type responders, based on their individual muscle response patterns as discerned from the output of the competitive network model. The practical significance of these differences is shown by comparison of muscle activity with more established optimization-based force predictions. Those subjects categorized as majority-type responders had muscle activity in better correspondence with optimization-based predicted forces. Subjects in minority categories displayed more variance in their response patterns and larger degrees of antagonistic cocontraction. CONCLUSIONS: The implications for deterministic (e.g. optimization-based) biomechanical modelling are discussed. It is speculated that interindividual muscle recruitment differences may be important for assessing individual musculoskeletal risk.

Flexion relaxation during lifting: implications for torque production by muscle activity and tissue strain at the lumbo-sacral joint

During the full flexion phase of the back lift movement the lumbar part of the erector spinae muscle exhibits a reduced activity level (flexion relaxation). This study addresses the question how the required extension torque in the lumbo-sacral joint (L5/S1 joint) is balanced during the period in which apparently the lumbar erector spinae ceases to take its share. Six subjects participated in the experiment in which they performed seven lifting tasks. The load, the range of movement, and the phase in which the load was handled (lifting or lowering) were varied. A dynamic linked segment model was applied to determine the momentary torques acting at the L5/S1 joint, while the EMGs of the lumbar and thoracic part of the erector spinae muscle were measured. Furthermore, the lengths between markers on the lumbar and thoracic part of the trunk were determined to reveal changes in length during the movement. The dynamic EMGs were normalized to trunk angle-dependent maximal levels. The L5/S1 joint torques were analysed and combined with the normalized EMG data and the kinematics of the trunk, which are assumed to indicate the elongation of passive tissues. Although in the normalization procedure the change of the length-force relationship of the erector spinae was taken into account, the dynamic lumbar EMG activity decreased to a low-activity level (the phenomenon of flexion relaxation). This coincided with a 25% increase in lumbar length suggesting that passive tissue strain provided part of the required extension torque. In the tasks where a barbell was handled a significant increase in EMG level of the thoracic part of the erector spinae occurred just before the flexion relaxation at the lumbar level. Apparently, the extensor function of the lumbar part is then taken over by the thoracic part of the erector spinae muscle. This suggests that an intricate coordinating mechanism is operative that apportions the load to be balanced over active--(lumbar and thoracic part of the erector spinae) and passive structures (post vertebral ligaments).

A finite element musculoskeletal model of the shoulder mechanism

The finite element method described in this study provides an easy method to simulate the kinetics of multibody mechanisms. It is used in order to develop a musculoskeletal model of the shoulder mechanism. Each relevant morphological structure has been represented by an appropriate element. For the shoulder mechanism two special-purpose elements have been developed: a SURFACE element representing the scapulothoracic gliding plane and a CURVED-TRUSS element to represent muscles which are wrapped around bony contours. The model contains four bones, three joints, three extracapsular ligaments, the scapulothoracic gliding plane and 20 muscles and muscle parts. In the model, input variables are the positions of the shoulder girdle and humerus and the external load on the humerus. Output variables are muscles forces subject to an optimization procedure in which the mechanical stability of the glenohumeral joint is one of the constraints. Four different optimization criteria are compared. For 12 muscles, surface EMG is used to verify the model. Since the optimum muscle length and force-length relationship are unknown, and since maximal EMG amplitude is length dependent, verification is only possible in a qualitative sense. Nevertheless, it is concluded that a detailed model of the shoulder mechanism has been developed which provides good insight into the function of morphological structures.

The relationship between EMG activity and extensor moment generation in the erector spinae muscles during bending and lifting activities

The relationship between EMG activity and extensor moment generation in the erector spinae muscles was investigated under isometric and concentric conditions. The full-wave rectified and averaged EMG signal was recorded from skin-surface electrodes located over the belly of the erector spinae at the levels of T10 and L3, and compared with measurements of extensor moment. The effects of muscle length and contraction velocity were studied by measuring the overall curvature (theta) and rate of change of curvature (d theta/dt) of the lumbar spine in the sagittal plane, using the '3-Space Isotrak' system. Isometric contractions were investigated with the subjects pulling up on a load cell attached to the floor. Hand height was varied to produce different amounts of lumbar flexion, as indicated by changes in lumbar curvature. The extensor moment was found to be linearly related to EMG activity, and the 'gradient' and 'intercept' of the relationship were themselves dependent upon the lumbar curvature at the time of testing. Concentric contractions were investigated with the subjects extending from a seated toe-touching position, at various speeds, while the torque exerted on the arm of a Cybex dynamometer was continuously measured. Under these conditions the EMG signal (E) was higher than the isometric signal (E0) associated with the same torque. E and E0 were related as follows: E0 = E/(1 + A d theta/dt), where A = 0.0014 exp (0.045P) and P = percentage lumbar flexion. This equation was used to correct the EMG data for the effect of contraction velocity. The corrected data were then used, in conjunction with the results of the isometric calibrations, to calculate the extensor moment generated by the erector spinae muscles during bending and lifting activities. The extensor moment can itself be used to calculate the compressive force acting on the lumbar spine.