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Biomedical subjects

S M McGill

Publications and source records attributed to S M McGill.

At least 19 recordsLinked to original sources
PubMed1995

Muscle activity and low back loads under external shear and compressive loading.

STUDY DESIGN: This study analyzed anatomic and neural control characteristics of the trunk musculature. Subjects were exposed to external shear and compressive loads with equivalent moments to evaluate activation patterns and loading on the low back. OBJECTIVES: The migration of activity between the thoracic and lumbar erector spinae muscle groups was examined to determine whether the motor control system chooses to minimize joint loading by recognizing differences in moment, compression, and shear support requirements and assigning muscle activation in the most appropriate way. SUMMARY OF BACKGROUND DATA: Loads were applied either parallel or perpendicular to the low back to create compressive or shear forces. No previous study has attempted to isolate the response of the trunk musculature with the type of external load. METHODS: Eleven male subjects isometrically held an external load that was altered to create either a compressive or an anterior shear load on the low back but with equal extensor (reaction) moments (experiment 1). In a second experiment four men repeated the task with an increased range of applied loads (5-25 kg) together with measurements of intra-abdominal pressure. RESULTS: The tasks with a compressive external load resulted in significantly higher levels of activation for all seven electromyographic channels recorded. Intraabdominal pressure, compressive and shear joint forces were all higher in the compression loading method when equal loads and low back moments were compared. CONCLUSIONS: It was concluded that the motor control system does not arrange muscle activation levels in a way to minimize lumbar spinal loading at least for the relatively low levels of this study. Biomechanical models that use the objective criterion of minimum joint load may not be representative of the motor control system, at least in the low back.

Abdominal Muscles↗
PubMed1995

Frozen storage increases the ultimate compressive load of porcine vertebrae.

The use of freezing as a method of storage is commonplace in mechanical testing of biological tissues. The effects of freezing on tissues that comprise spinal segments have been examined separately, but little work has been done on intact specimens. We examined the effect of freezing on the structural properties of porcine cervical spines. The intact cervical spines of seven pigs (a total of 14 specimens--seven of C2-C4 and seven of C5-C7) were stored frozen (-20 degrees C) for 1 month. The ultimate compressive load, displacement, stiffness, and energy absorbed were obtained using a monotonic compressive load applied at 3,000 N/sec. The structural properties were compared with those of another 14 porcine cervical specimens (control group, matched for age and weight) that were tested in a fresh state. The frozen storage of the vertebral specimens significantly increased the ultimate compressive load (24%) and energy absorbed to failure (33%). The stiffness and displacement at failure were not affected. We concluded that the use of freezing as a storage medium should be of concern when the resulting measures are used to quantify the ultimate compressive load of the spinal motion segments.

Animals↗
PubMed1995

Isovelocity investigation of the lengthening behaviour of the erector spinae muscles.

The purpose of this study was to investigate the force-velocity (F/v) relationship for the erector spinae muscles in submaximal activation movements, with particular attention to their response during lengthening movements and at lower shortening contraction velocities. Dynamic models that predict lower back muscle forces require reasonable representations of the modulating effect of instantaneous velocity. Ten males were observed performing trunk flexion and extension in the sagittal plane under constant load. Contraction velocities were measured as the first derivative from a devise sensitive to changes in spine curvature, and controlled by a visual feedback system while a constant load was applied through a chest harness. The erector spinae exhibited a yielding phenomenon which causes an abrupt drop in force during constant velocity stretching under constant, submaximal, stimulation. The findings were consistent with previous isovelocity muscle lengthening experiments. Yielding appeared dependent on the level of load/activation supporting the theory of a "state-variable" F/v relationship. The eccentric behaviour of the lower erectors (L3) seemed independent of velocity and length, while that of the upper erectors (T9) showed a dependence on length. At lower concentric velocities, concavity in torque-velocity curves was noted after a "threshold" velocity. The findings of this study strongly reinforce the notion that the F/v length relationship is not a continuous hyperbolic relationship during muscle shortening and that the commonly modelled force augmentation effect of lengthening is incorrect, at least for submaximal activation of the extensors of the lower back.

Adult↗
PubMed1995

The psoas major muscle: a three-dimensional geometric study.

The purpose of this study was to use anatomical data obtained from cadavers, and geometrical scaling data obtained from MRI scans of living subjects, to assess the line of action and mechanical function of the psoas major muscle in three dimensions about each lumbar spine level. In addition, the line of action of the psoas major was documented as a function of lordosis. A total of seven cadavers were dissected from which fibre/tendon architecture was measured, while MRI scans were performed on 15 males to obtain centroid paths and area scales of the muscle over its length. In this way, the curving path of muscle line of action was accommodated together with force and moment predictions that recognized the presence of a tendon at lower lumbar levels (up to L3 in some subjects) significantly increasing the stress. Results confirm that the mechanics of the psoas cannot be adequately represented with a series of straight line vectors from vertebral origins to insertion. Moreover, the mechanical action of the psoas major does not change as a function of lumbar spine lordosis as the muscle path of action changes in accordance with changes in spine posture. Functionally, contrary to claims, the psoas cannot act as a 'derotator' of the spine, does not impose large shear forces on the spine in any posture except at L5-S1, and cannot have major affects to 'control lordosis'. It has the potential to stabilize the lumbar spine with compressive loading and with bilateral activation, to laterally flex it, and can create large anterior shear forces but only at L5-S1.

Adult↗
PubMed1995

Comparison of muscle forces and joint load from an optimization and EMG assisted lumbar spine model: towards development of a hybrid approach.

The purpose of this study was to determine whether the same estimates of individual muscle and L4/L5 lumbar joint compressive forces result from an optimization (OPT) compared to an electromyography (EMG) assisted approach for solving the inderminate moment equilibrium equations in the same anatomical model. Four male subjects performed near maximum, isometric, ramp efforts in trunk flexion, extension and lateral bending in a testing apparatus. The EMG approach was sensitive to subject and trial differences in the magnitudes of individual muscle forces needed to produce the same reaction moment. In contrast, the OPT method converged on a similar estimate of muscle forces for all subjects and trials producing the same moment. The OPT method predicted lower L4/L5 joint compression values, on average, by 32, 43 and 23% in trunk extension, flexion and lateral bending, respectively, because, unlike the EMG method, it could not predict co-contraction of anatomically antagonistic muscles. We incorporated the OPT method's advantage of forcing an equilibrium in the reaction moments into the EMG method in a new approach we have called 'EMG assisted optimization' (EMGAO). Muscle force estimates from the EMG and EMGAO methods differed from those from the OPT method, on average, by 123% (RMS) for flexion and extension and by 218% for lateral bends. Data from the two approaches result in different conclusions about spine mechanics. We have more confidence in the EMG assisted methods because they respond to variation in muscle synergy and co-contraction patterns commonly observed in different trials and subjects for the same reaction moments.

Adult↗
PubMed1995

Loads on spinal tissues during simultaneous lifting and ventilatory challenge.

Muscles of the torso have been implicated to play a role in stabilization of the low back, and to assist in ventilation. This motivated an investigation to combine a load challenge to the low back with a breathing challenge, similar to that which a worker might experience when shovelling snow. Perhaps modulation of muscle activity needed to facilitate breathing may compromise the margin of safety of tissues that depend on constant muscle activity for support. Eight young healthy males dynamically lifted, and isometrically held, large loads (73-95 kg) and breathed a 10% CO2 gas mixture to elevate breathing (both with and without hand-held loads). Individual tissue forces were calculated using an anatomically detailed, dynamic model of the torso that was sensitive to individual variation by utilizing myoelectric signals, intra-abdominal pressure, ventilation rate and spine kinematics, obtained from each subject, as input. For large loads in the hands, most subjects appeared to stabilize the trunk with large muscle forces relegating the responsibility of creating lung air flow to the diaphragm. When reasonably small low-back demands were coupled with a breathing challenge and higher ventilation rates two out of eight subjects demonstrated entrainment of abdominal activity to breathing that resulted in additional cyclic low-back compressive loading of the order of 1000 N. Ergonomists should consider the additional tissue loading from physiologically demanding tasks and the related ventilation challenge, together with the tissue loads required to support external objects and maintain body posture.

Adult↗
PubMed1994

Transfer of loads between lumbar tissues during the flexion-relaxation phenomenon.

STUDY DESIGN AND METHODS: This study used an anatomically detailed model of the lumbar tissues, driven from biologic signals of vertebral displacement and myoelectric signals, to estimate individual muscle and passive tissue force-time histories during the performance of the "flexion-relaxation" maneuver. Eight male university students performed three trials each of the "flexion-relaxation" maneuver with six pairs of surface myoelectric electrodes monitoring the right side of the trunk musculature, an electromagnetic device to record lumbar flexion, and videotape to record body segment displacement. OBJECTIVES: To examine the loads on individual tissues during the transfer of moment support responsibility from predominantly active muscle to predominantly passive tissue. SUMMARY OF BACKGROUND DATA: No previous studies, to the authors' knowledge, have examined individual tissue loading during the flexion-relaxation maneuver. RESULTS: Although most subjects were able to "relax" their lumbar extensors in full flexion, activity remained in the thoracic extensors and abdominals. Tissue load predictions suggested that while the lumbar extensor muscles were neurally "relaxed" (i.e., myoelectric silence), substantial elastic forces would assist the passive tissues in extensor moment support. On average, subjects sustained almost 3 kN in compressive load on the lumbar spine and about 755 N of anterior shear during full flexion with only 8 kg held in the hands. CONCLUSIONS: The "relaxation" of lumbar extensor muscles appeared to occur only in an electrical sense because they generated substantial force elastically through stretching. Loading of the interspinous and supraspinous ligaments, in particular, was high relative to their failure tolerance.

Adult↗
PubMed1994

EMG assisted optimization: a hybrid approach for estimating muscle forces in an indeterminate biomechanical model.

There are two basic approaches to estimate individual muscle forces acting on a joint, given the indeterminacy of moment balance equations: optimization and electromyography (EMG) assisted. Each approach is characterized by unique advantages and liabilities. With this in mind, a new hybrid method which combines the advantages of both of these traditional approaches, termed 'EMG assisted optimization' (EMGAO), was described. In this method, minimal adjustments are applied to the individual muscle forces estimated from EMG, so that all moment equilibrium equations are satisfied in three dimensions. The result is the best possible match between physiologically observed muscle activation patterns and the predicted forces, while satisfying the moment constraints about all three joint axes. Several forms of the objective function are discussed and their effect on individual muscle adjustments is illustrated in a simple two-dimensional example.

Algorithms↗
PubMed1992

Lumbar posterior ligament involvement during extremely heavy lifts estimated from fluoroscopic measurements.

The mechanical role of the lumbar posterior ligaments during lifting tasks remains controversial. This study was designed to assess the ligament and disc contribution in resisting trunk flexion moment during extremely heavy lifts performed by national class powerlifters. Direct measurements of lumbar vertebrae kinematics in sagittal plane were obtained from videofluoroscopy utilizing multiple digitizing, correction for optical distortions and digital filtering. Four experienced powerlifters executed three trials, resulting in about 72 mA s of total radiation exposure. In the first trial, joint angles were measured when subjects fully flexed their spines to a point where the passive tissues resisted the flexor moment creating myoelectric silence in the extensor musculature. Next, two conventional deadlift style lifts were executed with the barbell load ranging from 183.7 to 210.9 kg. Four vertebral corners were digitized at a sampling rate of 30 Hz. The relative intervertebral joint angles, distance between the ligament attachment points, shearing and compressive displacements were calculated from a rigid body motion approach. Analysis revealed that except for one trial of one subject, they accomplished their lifts with an amount of lumbar flexion between 1.5 and 13 degrees less than they demonstrated during full flexion. Resultant ligament lengths at the beginning of the lifts ranged from 56.1 to 99.8% of their lengths when the trunk was fully flexed. It was concluded that ligaments did not strain sufficiently to contribute substantial resistance to the trunk flexion moment, relegating this responsibility to the musculature.

Adult↗
PubMed1992

A myoelectrically based dynamic three-dimensional model to predict loads on lumbar spine tissues during lateral bending.

This work describes a dynamic model of the low back that incorporates extensive anatomical detail of the musculo-ligamentous-skeletal system to predict the load time histories of individual tissues. The dynamic reaction moment about L4/L5 was determined during lateral bending from a linked-segment model. This reaction moment was partitioned into restorative components provided by the disc, ligament strain, and active-muscle contraction using a second model of the spine that incorporated a detailed representation of the anatomy. Muscle contraction forces were estimated using both information from surface electromyograms, collected from 12 sites, and consideration of the modulating effects of muscle length, cross-sectional area and passive elasticity. This modelling technique is sensitive to the different ways in which individuals recruit their musculature to satisfy moment constraints. Time histories of muscle forces are provided. High muscle loads are consistent with the common clinical observation of muscle strain often produced by load handling. Furthermore, the coactivation measured in muscles on both sides of the trunk suggests that muscles are recruited to satisfy the lateral bending reaction torque in addition to performing other mechanical roles such as spine stabilization. If an estimate of the intervertebral joint compression is desired for assessment of lateral bends in industry, then a single equivalent lateral muscle with a moment arm of approximately 3.0-4.0 cm would conservatively capture the effects of muscle co-contraction quantified in this study.

Abdominal Muscles↗
PubMed1992

The influence of lordosis on axial trunk torque and trunk muscle myoelectric activity.

Force contributions from the facet complex and posterior ligaments during the generation of axial torque are a function of lordosis, and it has been speculated that these forces together with muscular contributions play a role in axial trunk twisting. This study investigated the electromyographic activity of the trunk musculature and torque-generating capacity of the lumbar spine under the conditions of normal lordosis, hyperlordosis, and hypolordosis. Eleven male subjects volunteered for this study. The subjects performed isometric twisting efforts and maximum dynamic twisting efforts at 30 degrees/sec. The myoelectric activity levels (normalized to maximal amplitude obtained from nontwist activities) were quite low despite maximal efforts to generate axial torque (for example: approximately 60% maximum voluntary contraction for latissimus dorsi and even lower for the abdominals). Furthermore, changes in lordosis did not produce any consistent changes in muscle activity, although a hyperlordotic spine produced significantly smaller axial torques, and a hypolordotic spine smaller still. Larger torques were measured during all three conditions of lordosis, as the subjects rotated toward an untwisted position, and lower torques as the subjects rotated away. The opposite trend was observed, however, in myoelectric activity of the agonistic side of latissimus dorsi, the thoracic level of erector spine, and the lumbar level of erector spinae, i.e., larger amplitudes were observed as the trunk was twisted away from the untwisted position. These data suggest that tissues other than muscle (i.e., passive tissue) contribute significantly to axial torque production and that the flexed and twisted spine is less able to resist applied axial torques, possibly increasing the risk of torsional injury.

Abdominal Muscles↗
PubMed1991

Electromyographic activity of the abdominal and low back musculature during the generation of isometric and dynamic axial trunk torque: implications for lumbar mechanics.

This study focused on the electromyographic activity of the trunk musculature, given the well-documented link between occupational twisting and the increased incidence of low back pain. Ten men and 15 women volunteered for this study, in which several aspects of muscle activity were examined. The first aspect assessed the myoelectric relationships during isometric exertions. There was great variability in this relationship between muscles and between subjects. Further, the myoelectric activity levels (normalized to maximal electrical activity) obtained from nontwist activities were not maximal despite maximal efforts to generate axial torque (e.g., rectus abdominis, maximum voluntary contraction; 22% external oblique, 52%; internal oblique, 55%; latissimus dorsi, 74%; upper erector spinae [T9], 61%; lower erector spinae [L3], 33%). In the second aspect of the study, muscle activity was examined during dynamic axial twist trials conducted at a velocity of 30 and 60 degrees/s. The latissimus dorsi and external oblique appeared to be strongly involved in the generation of axial torque throughout the twist range and activity in the upper erector spinae displayed a strong link with axial torque and direction of twist, even though they have no mechanical potential to contribute axial torque, suggesting a stabilization role. The third aspect of the study was comprised of the formulation of a model consisting of a three-dimensional pelvis, rib cage, and lumbar vertebrae and driven from kinematic measures of axial twist and muscle electromyograms. The relatively low levels of normalized myoelectric activity during maximal twisting efforts coupled with large levels of agonist-antagonist cocontraction caused the model to severely underpredict measured torques (e.g., 14 Nm predicted for 91 Nm measured). Such dominant coactivity suggests that stabilization of the joints during twisting is far more important to the lumbar spine than production of large levels of axial torque.

Abdominal Muscles↗
PubMed1991

Kinetic potential of the lumbar trunk musculature about three orthogonal orthopaedic axes in extreme postures.

Many studies have examined the mechanics of the lumbar spine in various planes, but only a limited number of three-dimensional investigations have been reported. Analysis of the low back during complex, dynamic postures demands rigorous representation of the trunk musculature. The data of this study demonstrated the force and torque contributions of approximately 50 laminas of various trunk muscles to flexion-extension, lateral bending, and axial twisting torque at the L4-L5 joint. This analysis was conducted with the spine in an upright standing posture and when fully flexed (60 degrees), laterally bent (25 degrees), and axially twisted (10 degrees) together with two examples of combined postures. Maximum moment potential, muscle length excursions, and the resultant compressive, anteroposterior shear, and lateral shear forces on the joint were also computed. The results indicate that the position of the vertebrae and their orthopaedic axes, which are a function of spinal posture, are an important factor in the reasonable determination of joint compressive, lateral shear, and anteroposterior shear loads. Muscle length changes that exceeded 20% of their respective length during upright standing were not observed during a full axial twist, but were observed in portions of the abdominal obliques during lateral bending, and in some extensors during full flexion. Extreme postures tended to change the torque potential of some muscles and influence joint load. Various portions of erector spinae were observed to have appreciable potential to generate torque about all three orthopaedic axes. This observation supports the notion held by some therapists that conditioning of the erector spinae is of utmost importance.

Biomechanical Phenomena↗
PubMed1991

Trunk muscle and lumbar ligament contributions to dynamic lifts with varying degrees of trunk flexion.

This study was done to assess the interplay between muscular and ligamentous sources of extensor moment during dynamic lifting with various loads and flexion angles of the trunk segment for 15 subjects lifting a total of 150 loads. Ligament forces predicted from an anatomically detailed biomechanical model did not generally contribute more than 60 Nm for most of the lifts because the lumbar spine was only flexed to a moderate and constant degree for each load condition. In contrast, additional moment demands associated with increases in hand load were supported by muscle. Although the compression forces on the L4-5 intervertebral disc were fairly insensitive to the interplay between the recruitment of muscle and ligament, the shear force was significantly higher with a greater degree of lumbar flexion. The risk of injury may be influenced more by the degree of lumbar flexion than the choice of stoop or squat technique.

Adult↗
PubMed1991

Lumbar spine loads during the lifting of extremely heavy weights.

The reaction moments at the knee, hip, and L4/L5 joints, and the compressive and shearing forces on L4/L5 are documented in powerlifters competing in a national powerlifting championship. Analyses were made of 13 female and 44 male competitors. The joint moments and forces were estimated from a linked segment model (WATBAK) that incorporated functional low back extensor musculature with a moment arm of 6 cm and a line action that was oriented 5 degrees posteriorly to the L4/L5 compression axis. This oblique orientation of the extensor muscles reduced the anterior shearing load on the vertebral motion unit. Average compressive loads on L4/L5 were estimated up to 17,192 N while the highest average L4/L5 and hip moments were 988 and 1047 N.m, respectively. The sumo deadlift style resulted in a 10% reduction in the joint moment and 8% reduction in the load shear force at the L4/L5 level when compared with the conventional lifting style. Formulation of linear regression equations to predict the load lifted using reaction joint moments yielded substantial unexplained variability, though significant relationships were found. This analysis suggested that there is large variability in the pattern of loading joints among national class powerlifters.

Biomechanical Phenomena↗
PubMed1990

Measured and modelled static and dynamic axial trunk torsion during twisting in males and females.

Study of the mechanics of trunk twisting is of special interest given epidemiological evidence linking occupational twisting to increased incidence of low back pain. An anatomically detailed, three-dimensional model of the trunk (rib cage, pelvis, five lumbar vertebrae and 50 muscles), was used to predict maximum axial trunk torque. Predicted axial torques were compared with measured torques. Thirty-one (10 male and 21 female) subjects performed maximum effort isometric twisting exertions, at 0 degrees of twist and +/- 30 degrees of twist together with dynamic exertions, at 30 degrees s-1 and 60 degrees s-1. Females were able to generate approximately two-thirds of the torque of males (males, 97Nm; females 60Nm, isometric at 0 degrees). When the trunk was prerotated to 30 degrees, subjects were able to generate greater torque when the effort was toward the 0 degree position (approximately 105Nm by males and 68Nm by females). Experimental data indicated that velocity of rotation and amount of twist are important modulators of axial torque. Changes in muscle length were demonstrated to be minimal from model output as most muscle length changes during a twist from 0 degrees to 30 degrees, measured between the pelvis and the shoulder harness, were less than 1%, although some portions of the abdominal obliques underwent a length excursion of 5%. The small changes in the individual muscle force components that contribute to twist, i.e. the muscle unit vector about the axial twist axis and its moment arm that change as a function of twisted position, do not entirely account for the measured differences in torque, suggesting that additional mechanisms influence axial torque generation.

Adult↗
PubMed1990

The effect of an abdominal belt on trunk muscle activity and intra-abdominal pressure during squat lifts.

The purpose of this study was to determine whether abdominal belts such as those prescribed to industrial workers reduced trunk muscle activity and/or increased intra-abdominal pressure (IAP). In this study, six subjects lifted loads (72.7 to 90.9 kg) both with and without wearing a weightlifter belt. In addition, further trial conditions required that subjects lifted both with the breath held or continuously expiring on lifting effort. Dynamic hand loads were recorded together with intra-abdominal pressure (IAP) and abdominal, intercostal and low back EMG. Every subject demonstrated an increase in IAP when wearing the belt during both breathing conditions: 99 mmHg with no belt; 120 mmHg wearing belt (p less than 0.0001). However, it was also found that significant increases in IAP occurred (p less than 0.017) when the breath was held versus exhaling with or without the belt. One would expect that if the belt relieved either the direct compressive load on the spine or assisted IAP to produce an extensor moment then this would be reflected in diminished extensor muscle activity. Erector spinae activity tended to be lower with the breath held suggesting a reduced load on the lumbar spine although wearing a belt did not augment this reduction. In the case studies with subjects wearing an ergogenic corset designed for use by industrial manual materials handlers, perceptions of improved trunk stability were reported. However, the muscle activity and IAP results of this study during short duration lifting tasks make it difficult to justify the prescription of abdominal belts to workers.

Abdominal Muscles↗