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Biomedical subjects

A B Schultz

Publications and source records attributed to A B Schultz.

At least 73 records · Page 4Linked to original sources
PubMed1988

The mechanical role of the trunk and lower extremities in a seated weight-moving task in the sagittal plane.

A two-dimensional, sagittally-symmetric biomechanical model was developed to analyze the joint moments required to stabilize the trunk in a seated, dynamic, weight-moving task. Kinematic and reaction force data were measured while subjects moved a hand-held weight (0-4 kgf) at shoulder level to and fro at 1 Hz. These data were then used for model input and validation purposes. A second, simpler model was used to simulate how joint loads varied with weight held, trunk inclination, and movement frequency. The results for this seated task demonstrate a) significant trunk, hip, knee, and ankle joint moments (37, 13, 4, 13 percent of maximum strength values, respectively) were required, b) considerable intersubject differences in mean joint moments (more than 66 percent) were found, which primarily were due to subtle differences in body segment kinematics and lower extremities use, and c) the important role of the lower extremities in stabilizing the trunk in the seated posture.

Adult↗
PubMed1988

Biomechanics of the human spine and trunk.

This chapter has reviewed the past 30 years of experimental biomechanical studies of the spine and trunk. In the last 10 years, computers have allowed the development of simulation techniques and models to predict spine and muscle loading in most static and quasi-static activities. Some problems remain, however, particularly with activities involving bending and twisting and those that entail maximal efforts. Current research is focused on trying to validate models for the analysis of dynamic activities involving simple planar motions. Although the body segment kinematics and external support forces in complex motions can be measured fairly easily with modern motion analysis equipment, models that correctly predict the internal trunk forces have yet to be fully developed and validated. These models will be useful in studying how, why, and where failure of the soft and bony tissues is most likely to occur in a given activity, and whether it is related to work or athletics. The challenge for the future is to develop models that adequately reflect the anatomical sophistication of the spine and trunk. Thus the stress and strain distributions in any trunk musculoskeletal component, whether the posterior wall of the annulus, a muscle slip of the semi-spinalis group, or the lumbosacral endplate, will be able to be found. These results can then be combined with models of cumulative trauma response to successfully identify potential failure sites.

Aging↗
PubMed1988

Lumbar disc degeneration: correlation with age, sex, and spine level in 600 autopsy specimens.

Using data from 16 published reports, the authors correlated macroscopic disc degeneration grades with age, sex, and spine level in 600 lumbar intervertebral discs from 273 cadavers (ages: 0-96 years). Male discs were more degenerated than female discs at most ages; significantly so in the second, fifth, sixth, and seventh decades. On average, L4-L5 and L3-L4 level discs showed more degeneration than discs at other lumbar levels. These macroscopic findings corroborate radiographic data from epidemiologic studies. The calculations suggest that higher mechanical stress, perhaps combined with longer nutritional pathways, may be responsible for the earlier degeneration of male discs.

Adolescent↗
PubMed1987

Load-displacement behavior of sacroiliac joints.

We measured the load-displacement behavior of both single and paired sacroiliac (SI) joints in fresh cadaver specimens obtained from eight adults between the ages of 59 and 74 years. With both ilia fixed, static test loads were applied to the center of the sacrum along and about axes parallel and normal to the superior SI endplate. Test forces up to 294 N were applied in the superior, inferior, anterior, posterior, and lateral directions. Moments up to 42 N-m were applied in flexion, extension, lateral bending, and axial torsion. Displacements of the center of the sacrum were measured 60 s after each load increment was applied, using dial gauges and an optical lever system. The tests were then repeated with only one ilium fixed. Finally, the three-dimensional location and overall geometry of each SI joint were measured. For an isolated left joint at the maximum test loads, the mean (SD) sacral displacements in the direction of the force ranged from 0.76 mm (1.41) in the medial to 2.74 mm (1.07) in the anterior direction. The mean rotations in the directions of the moments ranged from 1.40 degrees (0.71) in right lateral bending to 6.21 degrees (3.29) in clockwise axial torsion viewed from above. We also examined load-displacement behavior under larger loads. Single sacroiliac joints resisted loads from 500 to 1440 N, and from 42 to 160 N-m without overt failure.

Aged↗
PubMed1987

Load displacement behavior of the human lumbo-sacral joint.

The three-dimensional load displacement behavior of nine fresh adult L5-S1 spine motion segments was studied. Static test forces up to 160 N in anterior, posterior, and lateral shear, test forces up to 320 N in compression, and test moments up to 15.7 Nm in flexion, extension, lateral bending, and torsion were used. The six displacements of the center of the inferior L5 endplate were measured 15 and 60 s after the load was applied. Specimens were then retested after posterior element excision. The results show that at the maximum test force, intact specimen mean (SD) displacements ranged from 1.65 mm (0.63 mm) in lateral shear to 2.21 mm (0.87 mm) in posterior shear. Posterior element excision resulted in an average 1.66-fold increase in shear translations. At the maximum moment, rotations ranged from 3.38 degrees (1.03 degrees) in torsion to 7.19 degrees (1.77 degrees) in flexion. Posterior element excision resulted in an average 2.09-fold increase in bending rotations and a 2.74-fold increase in the average torsional rotation. In general, these L5-S1 joints were stiffer than more cranial lumbar segments in flexion, extension, and lateral bending and were less stiff in torsion tests.

Biomechanical Phenomena↗
PubMed1987

The activity of individual trunk muscles during heavy physical loading.

The myoelectric activity of ten trunk muscles were recorded, using intramuscular electrodes, when ten subjects made maximal and 50% of maximal static exertions in standing postures. Exertions were made in flexion, extension, and left and right lateral bending. Three heavy-lifting tasks also were studied. A biomechanical model was used to predict the forces in the trunk muscles, and the predictions then were compared to the measurements. The abdominal muscles were all active in attempted flexion, while the erector spinae muscles were inactive. In attempted extension, the erectors were maximally active, but considerable activity was present in the abdominal muscles as well. The highest activity levels recorded in the oblique abdominal muscles were in lateral bending. There were high degrees of correlation between the measured muscle activities and predicted muscle tensions for the erector spinae and rectus abdominus muscles, while the correlation coefficients for the oblique abdominal muscles were lower (0.4-0.7). The study indicates that inclusion of antagonistic activity is an important consideration to improve model predictions. The oblique abdominal muscles appear to be more active, in general, than predicted. For the longitudinal trunk muscles, the predictions are excellent throughout.

Abdominal Muscles↗
PubMed1986

Mechanical properties of lumbar spine motion segments under large loads.

The mechanical behavior of fourteen fresh human lumbar motion segments taken at autopsy from males with an average age of 29 yr was studied. Forces up to 1029 N were applied in anterior, posterior and lateral shear; and moments up to 95 Nm were applied in flexion, extension, lateral bending and torsion. In response to these loads endplate displacements up to 9 mm and rotations up to 18 degrees were measured. Stiffness values ranged from 53 to 140 N mm-1 in response to the shear forces and 6-11 Nm degree-1 in response to the moments. Lumbar motion segments can develop significant passive resistances to loads in situations where they are allowed to undergo substantial deformations.

Adolescent↗
PubMed1986

Lumbar spine orthosis wearing. I. Restriction of gross body motions.

The effects of wearing commonly prescribed low-back braces and corsets on restriction of gross body motions were investigated. A lumbosacral corset, a chairback brace, and a molded plastic thoracolumbosacral orthosis (TLSO) were studied. Four trunk movements (flexion, extension, lateral bending, and twisting) were examined in five healthy adult men when standing and sitting. All three orthoses restricted at least some gross body motion to approximately two thirds to one half of no-orthosis values. All three orthoses failed to provide restrictions of at least 10% in at least one motion. Mean motion restriction across all eight movements studied in all five subjects were largest when wearing the TLSO and least when wearing the corset. Gross body motion restrictions relieve lumbar trunk muscle and spine loads.

Adult↗
PubMed1986

Material constants for a finite element model of the intervertebral disk with a fiber composite annulus.

A simple axisymmetric finite element model of a human spine segment containing two adjacent vertebrae and the intervening intervertebral disk was constructed. The model incorporated four substructures: one to represent each of the vertebral bodies, the annulus fibrosus, and the nucleus pulposus. A semi-analytic technique was used to maintain the computational economies of a two-dimensional analysis when nonaxisymmetric loads were imposed on the model. The annulus material was represented as a layered fiber-reinforced composite. This paper describes the selection of material constants to represent the anisotropic layers of the annulus. It shows that a single set of material constants can be chosen so that model predictions of gross disk behavior under compression, torsion, shear, and moment loading are in reasonable agreement with the mean and range of experimentally measured disk behaviors. It also examines the effects of varying annular material properties.

Collagen↗
PubMed1986

Valsalva maneuver biomechanics. Effects on lumbar trunk loads of elevated intraabdominal pressures.

The ability of a partial or full Valsalva maneuver (voluntary pressurization of the intraabdominal cavity) to unload the spine was investigated in four subjects. During the performance of five isometric tasks, intraabdominal and intradiscal pressures and surface myoelectric activities in three lumbar trunk muscle groups were measured. The tasks were carried out without voluntary pressurization of the intraabdominal cavity and then when the subjects performed partial and full Valsalva maneuvers. A biomechanical model analysis of each task was made to help interpret the experimental measurements. Intraabdominal pressure was found not to be an indicator of spine load in these experiments. The Valsalva maneuvers did raise intraabdominal pressure, but in four of the five tasks increased rather than decreased lumbar spine compressions occurred.

Abdomen↗
PubMed1986

Lumbar spine orthosis wearing. II. Effect on trunk muscle myoelectric activity.

The effects of wearing commonly prescribed low-back braces and corsets on myoelectric signal levels in the erector spinae and oblique abdominal muscles were investigated. A lumbosacral corset, a chairback brace, and a molded thoracolumbosacral orthosis (TLSO) were studied. Nineteen tasks involving sitting and standing were performed by five healthy adult men. Myoelectric signal levels measured when wearing each orthosis were compared with those measured when performing the same task while wearing no orthosis. The changes in mean myoelectric signal levels ranged from a 9% reduction to a 44% increase when the lumbosacral corset was worn, from a 27% reduction to a 25% increase when the chairback brace was worn, and from a 38% reduction to a 19% increase when the TLSO was worn.

Adult↗
PubMed1985

Quantitative studies of the flexion-relaxation phenomenon in the back muscles.

In quiet standing positions involving substantial trunk flexion, myoelectric activity in the back muscles diminishes to low levels. Aspects of that "flexion-relaxation" phenomenon were explored through measurements of myoelectric activities in 11 young men during performance of 19 isometric tasks in flexed positions. Biomechanical model analyses were used to predict the internal loads imposed on the lumbar trunk structures during those performances. Flexion-relaxation consistently occurred in quiet flexed standing, but marked increases in myoelectric activity were found on imposition of external loads in flexed positions. Increases in myoelectric activity per unit increase in back muscle contraction force increase were nearly the same as those found in upright postures. Whether or not flexion-relaxation occurs, large trunk flexions load the spine heavily.

Adult↗
PubMed1985

Trunk position sense in the frontal plane.

Twenty healthy volunteers (ages 18 to 25 years) were tested for their ability to sense the lateral position of the top of their thoracic spine. When moved slowly from side-to-side in the frontal plane with vision occluded and pelvis immobilized, they could sense the position of a midline point on the skin at the T1 level to within 3 mm of a mean center position in relaxed standing tests, and to within 9 mm in supine tests. When subjects centered themselves actively, or additionally, contracted trunk flexor or extensor muscles to predetermined levels of activity, no increase in trunk positioning accuracy was found. The effect of a lateral pelvic tilt or lateral trunk moment had little effect on trunk positioning accuracy, but always induced a characteristic trunk offset. No differences were found in any of these results between males or females, or gymnasts or nongymnasts.

Adolescent↗
PubMed1985

Stiffness properties and geometry of lumbar spine posterior elements.

This paper reports measurements made in five fresh cadaver lumbar spine motion segments of the load-deformation properties of the posterior element soft tissues. These properties were measured and the stiffness corresponding to them were calculated for loading in anterior, posterior and lateral shear; in longitudinal tension and compression; and in flexion, extension, lateral bending and axial torsion. In addition, measurements were made in six motion segments of the positions of the inferior facet joint centers relative to the vertebral body centers, and of the orientations of the facet joint surfaces.

Adolescent↗
PubMed1984

Biomechanical factors in the progression of idiopathic scoliosis.

Idiopathic scoliosis is present when, in upright positions of the trunk, the spine curves to the side for unknown reasons. This paper reviews evidence concerning some biomechanical factors that might underlie the progression of such curves. The review concentrates on studies conducted in our laboratories. Arguments are made, based on biomechanical analyses and experiments, that progression occurs because of defects in the postural control system of the spine.

Adolescent↗
PubMed1984

Mechanical response of a simple finite element model of the intervertebral disc under complex loading.

A simple axisymmetric finite element model of a human spine segment containing two adjacent vertebrae and the intervening intervertebral disc was constructed. The bodies and disc were modeled by three substructures; one to represent each of the vertebral bodies, the annulus fibrosus, and the nucleus pulposus. A semi-analytic technique was used to maintain the computational economies of a two-dimensional analysis when non- axisymmetric loads were imposed on the model. The response of the model to compression, shear, torsion and bending loads applied to the superior vertebral body was examined to determine the effects of disc geometry and material properties on response. Comparisons of model responses with experimentally measured responses were made to estimate material property values for which model behaviors are in agreement with measured behaviors.

Biomechanical Phenomena↗
PubMed1984

Effectiveness of braces in mild idiopathic scoliosis.

Despite the wide use of bracing for the treatment of idiopathic scoliosis, controlled studies apparently have not been performed to examine whether bracing, in fact, alters the natural history of spine lateral curves. We studied 255 female patients, ages 8-17 years, with idiopathic scoliosis who had curves with initial Cobb measures from 15-30 degrees. They were divided into two groups: one group consisted of 144 patients who had received a Milwaukee or Boston brace; and the other, a control group, consisted of 111 patients who remained untreated through a mean period of 1.9 years. The groups had similar mean ages, ages of menarche and curve severities. The results showed a slight but nonsignificant trend, suggesting that bracing reduced the overall probability of progression in the braced curves. However, noting that nearly 75% of the control group curves were nonprogressive, it is possible that a similar proportion of the braced curves need not have been braced. Moreover, bracing failed to prevent eight curves in seven patients (5%) from progressing. These curves progressed at a mean rate of 8 degrees per year. Our retrospective results suggest that bracing probably is not necessary in a large proportion of patients who meet current, clinical criteria for bracing. Given the limitations of retrospective studies like this one, a controlled prospective trial of bracing effectiveness in idiopathic scoliosis seems warranted.

Adolescent↗