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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
PubMed2001

Spinal shrinkage during repetitive controlled torsional, flexion and lateral bend motion exertions.

This experiment analysed the spinal shrinkage due to repetitive exertions confined to each of three separate axes (twist, lateral bend, flexion). While the experiment was performed twice with small technique modifications in the twisting task (and thus two data collections were performed), the essential components were as follows. A total of 20 subjects were loaded with an equal moment of 20 Nm in each of the three axes, on 3 separate days (one axis per day). Subjects performed each task for 20 min at 10 repetitions min(-1), where stadiometer measurements of standing height were taken prior to and immediately following the 20 min exertion. The twisting task demonstrated significant spinal shrinkage (1.81 and 3.2 mm in the two experiments) between the pre- and post-stature measurements while no clear effect emerged for the other two tasks. These data suggest that repetitive torsional motions impose a larger cumulative loading on the spine when compared with controlled lateral or flexion motion tasks of a similar moment.

Adult↗
PubMed2001

Low back joint loading and kinematics during standing and unsupported sitting.

The aim was to examine lumbar spine kinematics, spinal joint loads and trunk muscle activation patterns during a prolonged (2 h) period of sitting. This information is necessary to assist the ergonomist in designing work where posture variation is possible -- particularly between standing and various styles of sitting. Joint loads were predicted with a highly detailed anatomical biomechanical model (that incorporated 104 muscles, passive ligaments and intervertebral discs), which utilized biological signals of spine posture and muscle electromyograms (EMG) from each trial of each subject. Sitting resulted in significantly higher (p<0.001) low back compressive loads (mean +/- SD 1698 +/- 467 N) than those experienced by the lumbar spine during standing (1076 +/- 243 N). Subjects were equally divided into adopting one of two sitting strategies: a single 'static' or a 'dynamic' multiple posture approach. Within each individual, standing produced a distinctly different spine posture compared with sitting, and standing spine postures did not overlap with flexion postures adopted in sitting when spine postures were averaged across all eight subjects. A rest component (as noted in an amplitude probability distribution function from the EMG) was present for all muscles monitored in both sitting and standing tasks. The upper and lower erector spinae muscle groups exhibited a shifting to higher levels of activation during sitting. There were no clear muscle activation level differences in the individuals who adopted different sitting strategies. Standing appears to be a good rest from sitting given the reduction in passive tissue forces. However, the constant loading with little dynamic movement which characterizes both standing and sitting would provide little rest/change for muscular activation levels or low back loading.

Adult↗
PubMed2001

Intervertebral disc herniation: studies on a porcine model exposed to highly repetitive flexion/extension motion with compressive force.

OBJECTIVE: To determine whether repeated motion with low magnitude joint forces, and flexion/extension moments consistently produce herniation in a non-degenerated, controlled porcine spine motion segment. DESIGN: Combined loading (flexion/extension motions and compressive forces) was applied to in vitro porcine functional spinal units. Biomechanical and radiographic characteristics were documented. BACKGROUND: While most studies performed in vitro have examined uniaxial or fixed position loading to older specimens, there have been few studies that have examined whether 'healthy' intervertebral discs can be injured by low magnitude repeated combined loading. METHODS: Porcine cervical spine motion segments (C3-C4) were mounted in a custom jig which applied axial compressive loads with pure flexion/extension moments. Dynamic testing was conducted to a maximum of 86400 bending cycles at a rate of 1 Hz with simultaneous torques, angular rotations, axial deformations recorded for the duration of the test. RESULTS: Herniation (posterior and posterior-lateral regions of the annulus) occurred with relatively modest joint compression but with highly repetitive flexion/extension moments. Increased magnitudes of axial compressive force resulted in more frequent and more severe disc injuries. CONCLUSIONS: The results support the notion that intervertebral disc herniation may be more linked to repeated flexion extension motions than applied joint compression, at least with younger, non-degenerated specimens. Relevance. While intervertebral disc herniations are observed clinically, consistent reproduction of this injury in the laboratory has been elusive. This study was designed to examine the biomechanical response and failure mechanics of spine motion segments to highly repetitive low magnitude complex loading.

Animals↗
PubMed2001

Spinal manipulation causes variable spine kinematic and trunk muscle electromyographic responses.

STUDY DESIGN: Analytic cohort with a convenience sample in a research clinic. OBJECTIVES: To determine the influence of a spinal manipulation on trunk kinematics and associated trunk myoelectric activity. SUMMARY OF BACKGROUND: While the mechanism of spinal manipulation is unknown, it has been theorized to influence spinal range of motion and trunk muscle activity. METHODS: Trunk kinematics were measured in low back pain patients (n = 14) during simple range of motion tasks in three planes, while trunk muscle electromyogram signals were recorded bilaterally from paraspinal and abdominal musculature. Kinematics and electromyogram signals were assessed pre-post manipulation. Electromyogram activity was also assessed pre-post manipulation during quiet stance. RESULTS: While no consistent kinematic or electromyographic changes occurred following manipulation across the population, individual changes were observed. The largest changes (> 6 degrees ) in range of motion occurred in the sagittal plane of three patients experiencing the greatest amount of pain. During quiet stance 17 muscles across all subjects exhibited changes in muscle activity following manipulation. Sixteen of those changes were decreases in muscle amplitude. CONCLUSIONS: This study offers some preliminary data on the short-term effects of manipulation on lumbar range of motion and dynamic electromyogram. The findings suggest that the response to manipulation is variable and dependent on the individual, with no change in some to the largest changes seen in the more pained patients. Relevance. Basic science investigations into the mechanisms and biomechanical influences of spinal manipulation are few. This study attempts to address issues of measureable functional change with manipulative therapy.

Adult↗
PubMed2001

Spinal posture and prior loading history modulate compressive strength and type of failure in the spine: a biomechanical study using a porcine cervical spine model.

OBJECTIVE: The purpose of this study was to investigate the effect of posture and loading history on the compressive strength and site of failure in the spine. DESIGN: An in vitro experiment was performed using a porcine cervical model that provided a homogeneous population of young healthy spines. BACKGROUND: The distribution of stresses amongst the many load bearing tissues of the spine is altered throughout the day by posture and the history of loading, but it is not clear how this modulates tissue damage or the risk of injury. METHODS: 48 porcine cervical spines were harvested and dissected into motion segments containing two vertebrae and the intervening disc (C3/4 and C5/6). Compressive loads and rotational torques (flexion/extension) were applied so that the effects of four loading histories (hydrated, neutral dehydration, flexed dehydration, superhydrated) and two failure postures (neutral, flexed) could be examined. Levels of dehydration were based on those reported over the course of a day. Dissection techniques and X-rays were used to document tissue damage. RESULTS. Specimens had a lower yield point (43--63%) and ultimate compressive strength (23--47%) when in a flexed posture than when in a neutral posture. When injured in a neutral posture, superhydrated specimens had a lower strength (22--29%) than dehydrated specimens. Loading history also modulated the site of failure. CONCLUSIONS: The spine may be more prone to injury early in the morning when the discs are at their greatest level of hydration and/or when they are in a fully flexed posture.

Animals↗
PubMed2001

Low back stability: from formal description to issues for performance and rehabilitation.

Low back stability: from formal description to issues for performance and rehabilitation. Exerc. Sport Sci. Rev. Vol. 29, No. 1, pp. 26-31, 2001. The concept of stability, together with notions of design and the application of stabilization exercise, is briefly synthesized. The objective is to challenge muscle systems to achieve sufficient functional stability but in a way that spares the spine of excessive exacerbating load.

Biomechanical Phenomena↗
PubMed2001

Quantification of the differences in electromyographic activity magnitude between the upper and lower portions of the rectus abdominis muscle during selected trunk exercises.

BACKGROUND AND PURPOSE: Controversy exists around exercises and clinical tests that attempt to differentially activate the upper or lower portions of the rectus abdominis muscle. The purpose of this study was to assess the activation of the upper and lower portions of the rectus abdominis muscle during a variety of abdominal muscle contractions. SUBJECTS: Subjects (N = 11) were selected from a university population for athletic ability and low subcutaneous fat to optimize electromyographic (EMG) signal collection. METHODS: Controlling for spine curvature, range of motion, and posture (and, therefore, muscle length), EMG activity of the external oblique muscle and upper and lower portions of rectus abdominis muscle was measured during the isometric portion of curl-ups, abdominal muscle lifts, leg raises, and restricted or attempted leg raises and curl-ups. A one-way repeated-measures analysis of variance was used to test for differences in activity between exercises in the external oblique and rectus abdominis muscles as well as between the portions of the rectus abdominis muscle. RESULTS: No differences in muscle activity were found between the upper and lower portions of the rectus abdominis muscle within and between exercises. External oblique muscle activity, however, showed differences between exercises. DISCUSSION AND CONCLUSION: Normalizing the EMG signal led the authors to believe that the differences between the portions of the rectus abdominis muscle are small and may lack clinical or therapeutic relevance.

Analysis of Variance↗
PubMed2000

Stadiometry: on measurement technique to reduce variability in spine shrinkage measurement.

OBJECTIVE: To test the effect of two measurement techniques for repeated measures of spine height using stadiometry following five experimental activity conditions.DESIGN. Six subjects were repeatedly measured while they stepped in and out of the stadiometer for each pair of measures and again on another day when they remained in place in the stadiometer for all 10 measures.RESULTS. There was much greater variability in height measures with the "in-out" method while the "in place" method demonstrated a steady shrinkage over the 3-3.5 min required to obtain the repeated measures. RelevanceContrary to popular practice, leaving a subject in the stadiometer during repeated measures includes the shrinkage that occurs over the 3-3.5 min of measurement when standing and reduces random variation due to posture change.

Anthropometry↗
PubMed2000

Changes in lumbar lordosis modify the role of the extensor muscles.

STUDY DESIGN: Fiber angles of longissimus thoracis and iliocostalis lumborum at L3 were documented in vivo, using high resolution ultrasound, with the lumbar spine in neutral curve and when fully flexed. OBJECTIVES: To evaluate the effect of changes in lumbar curvature on the mechanics of these muscles. BACKGROUND: Full flexion modifies the failure tolerance of the lumbar spine, determines the load distribution among muscle and passive tissues, and modulates the types of tissue damage that occur. Related to this issue are the possible changes in muscle line of action with full flexion which changes the ability of the spine to support shear loads. METHODS: Nine normal men and 5 normal women were scanned in three positions: (1) an upright standing posture; (2) with the hips flexed to approximately 30 degrees and the spine fully flexed; (3) hips flexed but the spine returned to a neutral curvature. RESULTS: Mean longissimus/iliocostalis fiber angles for upright standing, hips flexed-spine flexed, and hips flexed-spine neutral lordosis were 25. 7 degrees, 10.7 degrees and 28.3 degrees, respectively. CONCLUSIONS: Anterior shear load on the lumbar spine has been recently shown to be highly related to the risk of reporting a back injury. Bending forward allowing the spine to fully flex changes the line of action of the largest lumbar extensor muscles compromising their role to support anterior shear forces. Relevance Fiber angles of longissimus thoracis and iliocostalis lumborum were documented with high resolution ultrasound at L3, with the spine in neutral curvature and fully flexed. Full lumbar flexion changes the line of action of these muscle compromising their role to support anterior shear forces on the spine - anterior shear forces have been recently documented to be highly related to the risk of reporting a back injury.

Adolescent↗
PubMed2000

Lumbar erector spinae oxygenation during prolonged contractions: implications for prolonged work.

Owing to the recent interest in torso stabilization exercises together with many questions regarding the duration of prolonged isometric holds in occupational settings, the authors attempted to assess the level of back muscle oxygenation during prolonged isometric contractions. Specifically, this study recorded relative oxygen saturation of haemoglobin/myoglobin using Near Infrared Spectroscopy (NIRS) in the L3 erector mass during prolonged isometric contractions at intensities from 2 to 30% of maximum voluntary contraction (MVC). It was hypothesized that available oxygen to these muscles is severely compromised even at moderate levels of activation observed in occupational work. Eight volunteers without a history of lower back pain or injury participated in this study. The exercise task involved isometric contraction of the lower erector spinae at five different levels of each subject's maximal voluntary contraction: 2, 5, 10, 20 and 30% MVC, presented in random order. Subjects were placed in a sitting position, with a curved plastic plate secured horizontally to the pelvis to minimize movement at the hip joint. During extensor exertions, they were restrained with a harness that was attached at chest level to a load cell. Each isometric contraction was performed for 30 s followed by 1 min of rest. All levels of contraction demonstrated reduction in oxygen. Given the concern for motion artefact on the NIRS signal, sham trials were conducted where the subjects went through the procedure of attaching the pulling cable but no active pull was performed. These trials showed no change in the NIRS signal. At this time NIRS appears to be the only non-invasive instrumentation available to indicate total available muscle oxygen during low level, prolonged work. Although the specific tissue volume sampled by NIRS cannot be positively identified, it appears that tissue oxygenation in the lumbar extensor musculature is reduced as a function of contraction intensity, even at levels as low as 2% of MVC. These data have implications for prolonged work where postures requiring isometric contractions are sometimes held for hours, and where musculoskeletal illness has been linked to prolonged contraction levels above 2%MVC--these data suggest a possible biological pathway.

Adult↗
PubMed2000

Abdominal muscle response during curl-ups on both stable and labile surfaces.

BACKGROUND AND PURPOSE: With the current interest in stability training for the injured low back, the use of labile (movable) surfaces, underneath the subject, to challenge the motor control system is becoming more popular. Little is known about the modulating effects of these surfaces on muscle activity. The purpose of this study was to establish the degree of modulating influence of the type of surface (whether stable or labile) on the mechanics of the abdominal wall. In this study, the amplitude of muscle activity together with the way that the muscles coactivated due to the type of surface under the subject were of interest. SUBJECTS: Eight men (mean age=23.3 years [SD=4.3], mean height=177.6 cm [SD=3.4], mean weight=72.6 kg [SD=8.7]) volunteered to participate in the study. All subjects were in good health and reported no incidence of acute or chronic low back injury or prolonged back pain prior to this experiment. METHODS: All subjects were requested to perform 4 different curl-up exercises-1 on a stable surface and the other 3 on varying labile surfaces. Electromyographic signals were recorded from 4 different abdominal sites on the right and left sides of the body and normalized to maximal voluntary contraction (MVC) amplitudes. RESULTS: Performing curl-up exercises on labile surfaces increased abdominal muscle activity (eg, for curl-up on a stable surface, rectus abdominis muscle activity was 21% of MVC and external oblique muscle activity was 5% of MVC; for curl-up with the upper torso on a labile ball, rectus abdominis muscle activity was 35% of MVC and external oblique muscle activity was 10% of MVC). Furthermore, it appears that increases in external oblique muscle activity were larger than those of other abdominal muscles. CONCLUSION AND DISCUSSION: Performing curl-ups on labile surfaces changes both the level of muscle activity and the way that the muscles coactivate to stabilize the spine and the whole body. This finding suggests a much higher demand on the motor control system, which may be desirable for specific stages in a rehabilitation program.

Abdominal Muscles↗
PubMed1999

Anterior shear of spinal motion segments. Kinematics, kinetics, and resultant injuries observed in a porcine model.

STUDY DESIGN: A basic study of 56 porcine specimens in anterior shear loading. OBJECTIVES: To determine some modulators of the biomechanics of spinal motion segments exposed to acute shear loading and to identify the resultant injuries. SUMMARY OF BACKGROUND DATA: Most research on spinal injury mechanisms has focused on compressive loading, leaving a void in understanding of the effect of shear loading on origin of injury. METHODS: Cervical spines (n = 56) of domestic pigs (6 months old) were loaded to failure in a specially designed jig that restricted their motion to primarily the shear plane. The specimens were tested at load rates of 100 N/sec or 10,810 N/sec and either in a flexed or neutral posture. In addition, the function of the individual structures of the motion segment were determined by serial dissection forming three groups: whole specimens, specimens with no posterior ligaments, and specimens with no posterior ligaments or facet joints. Load-deformation curves were collected using analog-to-digital sampling rates of 50 and 100 Hz. The mode of failure was then documented through systematic dissection of the specimen and/or radiology techniques. Modeling approaches were then used to gain insight into the failure mechanisms. RESULTS: Dynamic loading (10,810 N/sec) and flexion of the specimens were found to increase the ultimate load at failure when compared with quasistatic loading (100 N/sec) and neutral postures. The disc resisted up to 70% of an applied load, with the pars interarticularis responsible for only 30% of the load. Nonetheless, the pars was the primary site of failure. Furthermore, higher load rates also caused endplate avulsion, specifically in the lateral borders of the anulus. CONCLUSIONS: The porcine model appears to reproduce injuries found in the data available on human lumbar material. Fractures in the pars interarticularis may not greatly weaken the joint, given the dominant role of the disc, but compromise its normal kinematics. Clinically, this may explain the occurrence of pars fractures, without total disability.

Animals↗
PubMed1999

Atlas-axis facet asymmetry. Implications in manual palpation.

STUDY DESIGN: A basic study of six human cervical spines, documenting displacement with applied forces mimicking palpation. OBJECTIVES: To assess the issues of motion palpation of joint restrictions and the inferred link to disease. SUMMARY OF BACKGROUND DATA: Although several investigators have suggested that the issue of asymmetry and normal-abnormal function should be assessed, data are unavailable. METHODS: Atlas-axis specimens were harvested from six cadavers, cleaned of ligamentous and muscle tissue, and potted and secured with dental plaster. Forces (5-25 N) were applied along the mediolateral axis, and the corresponding displacement along three orthogonal axes were documented with infrared diodes and the Optotrak camera system (Northern Digital, Waterloo, Ontario, Canada). Specimen geometry and asymmetry were documented with plain radiographic film and a gimbal apparatus. RESULTS: Each of the six specimens displayed different behavior and differing degrees of asymmetry (e.g., facet inclination 17-35 degrees) so that each was analyzed as a case study. Asymmetrical and discontinuous force-displacement correlations were linked to anatomic asymmetry that appeared to be of natural occurrence. CONCLUSIONS: Asymmetrical joint geometry is common and causes asymmetrical joint dynamics. Thus, a clinician attempting to palpate vertebral motion would be misled by assuming that perceived restricted joint motion universally represented a finding potentially amenable to manipulation. For spine palpation to be a valid indicator for manipulation, the clinician applying it must first be able to differentiate between asymmetrical motion caused by vertebral fixation and that caused by asymmetrical joint anatomy.

Aged↗
PubMed1999

Lumbar spine stability can be augmented with an abdominal belt and/or increased intra-abdominal pressure.

The increased intra-abdominal pressure (IAP) commonly observed when the spine is loaded during physical activities is hypothesized to increase lumbar spine stability. The mechanical stability of the lumbar spine is an important consideration in low back injury prevention and rehabilitation strategies. This study examined the effects of raised IAP and an abdominal belt on lumbar spine stability. Two hypotheses were tested: (1) An increase in IAP leads to increased lumbar spine stability, (2) Wearing an abdominal belt increases spine stability. Ten volunteers were placed in a semi-seated position in a jig that restricted hip motion leaving the upper torso free to move in any direction. The determination of lumbar spine stability was accomplished by measuring the instantaneous trunk stiffness in response to a sudden load release. The quick release method was applied in isometric trunk flexion, extension, and lateral bending. Activity of 12 major trunk muscles was monitored with electromyography and the IAP was measured with an intra-gastric pressure transducer. A two-factor repeated measures design was used (P < 0.05), in which the spine stability was evaluated under combinations of the following two factors: belt or no belt and three levels of IAP (0, 40, and 80% of maximum). The belt and raised IAP increased trunk stiffness in all directions, but the results in extension lacked statistical significance. In flexion, trunk stiffness increased by 21% and 42% due to 40% and 80% IAP levels respectively; in lateral bending, trunk stiffness increased by 16% and 30%. The belt added between 9% and 57% to the trunk stiffness depending on the IAP level and the direction of exertion. In all three directions, the EMG activity of all 12 trunk muscles increased significantly due to the elevated IAP. The belt had no effect on the activity of any of the muscles with the exception of the thoracic erector spinae in extension and the lumbar erector spinae in flexion, whose activities decreased. The results indicate that both wearing an abdominal belt and raised IAP can each independently, or in combination, increase lumbar spine stability. However, the benefits of the belt must be interpreted with caution in the context of the decreased activation of a few trunk extensor muscles.

Abdomen↗
PubMed1999

Endurance times for low back stabilization exercises: clinical targets for testing and training from a normal database.

OBJECTIVE: To establish isometric endurance holding times, as well as ratios between torso extensors, flexors, and lateral flexors (stabilizers), for clinical assessment and rehabilitation targets. DESIGN: Simple measurement of endurance times in four tests performed in random order by a healthy cohort. To measure reliability, a subsample also performed the tests again 8 weeks later. SETTING: University laboratory. PARTICIPANTS: Seventy-five young healthy subjects (31 men, 44 women). RESULTS: Women had longer endurance times than men for torso extension, but not for torso flexion or for the "side bridge" exercise, which challenges the lateral flexors (stabilizers). Men could sustain the "side bridge" for 65% of their extensor time and 99% of their flexion time, whereas women could sustain the "side bridge" for only 39% of their extensor time and 79% of their flexion time. The tests proved reliable, with reliability coefficients of >.97 for the repeated tests on 5 consecutive days and again 8 weeks later. CONCLUSION: Healthy young men and women possess different endurance profiles for the spine stabilizing musculature. Given the growing support for quantification of endurance, these data of endurance times and their ratios between extensor, flexor, and lateral flexor groups in healthy normal subjects are useful for patient evaluation and for providing clinical training targets.

Adult↗
PubMed1999

Intra-abdominal pressure mechanism for stabilizing the lumbar spine.

Currently, intra-abdominal pressure (IAP) is thought to provide stability to the lumbar spine but the exact principles have yet to be specified. A simplified physical model was constructed and theoretical calculations performed to illustrate a possible intra-abdominal pressure mechanism for stabilizing the spine. The model consisted of an inverted pendulum with linear springs representing abdominal and erector spinae muscle groups. The IAP force was simulated with a pneumatic piston activated with compressed air. The critical load of the model was calculated theoretically based on the minimum potential energy principle and obtained experimentally by increasing weight on the model until the point of buckling. Two distinct mechanisms were simulated separately and in combination. One was antagonistic flexor extensor muscle coactivation and the second was abdominal muscle activation along with generation of IAP. Both mechanisms were effective in stabilizing the model of a lumbar spine. The critical load and therefore the stability of the spine model increased with either increased antagonistic muscle coactivation forces or increased IAP along with increased abdominal spring force. Both mechanisms were also effective in providing mechanical stability to the spine model when activated simultaneously. Theoretical calculation of the critical load agreed very well with experimental results (95.5% average error). The IAP mechanism for stabilizing the lumbar spine appears preferable in tasks that demand trunk extensor moment such as lifting or jumping. This mechanism can increase spine stability without the additional coactivation of erector spinae muscles.

Abdomen↗
PubMed1999

The importance of normalization in the interpretation of surface electromyography: a proof of principle.

OBJECTIVE: To demonstrate the errors in surface electromyogram (EMG) interpretation that can be made when the EMG signal is not normalized. DESIGN: A case study as a proof of principle. MAIN OUTCOME MEASURES: The EMG amplitude between the upper and lower portions of the rectus abdominis in one subject during a trunk curl when the EMG signal was normalized (expressed as a percentage of a maximum voluntary contraction) and the amplitude when the signal was expressed in raw, unnormalized arbitrary units or raw millivolts directly read from the instrumentation. RESULTS: Interpretation of the unnormalized EMG signal suggests that a large difference in neural activation of the upper and lower sections of the rectus abdominis is occurring. In this condition the average activity in the lower rectus is 60.9% of that in the upper portion. This interpretation is incorrect. When the EMG signal is normalized, the differences between the upper and lower segments are reduced. When normalized, the lower segment activity is equal to that of the upper segment. CONCLUSIONS: Because of the inherent EMG signal variability, clinical interpretation of surface EMG signals requires normalization of the signal for physiologic interpretation and for comparison between bilateral muscles and between the same muscle on different days and between different subjects.

Action Potentials↗