Comparative study of Mm. Multifidi in lumbar and thoracic spine.

Imbalance of Mm. Multifidi may play a role in spinal disorders such as scoliosis in the thoracic spine, and lumbar disc herniation and lower back pain in the lumbar spine. Even though changes in these muscles are related to the etiology of these disorders, their anatomy is still poorly understood, especially in the upper regions of the spine. With the aim of gaining a better understanding of the anatomy of Mm. Multifidi in the lumbar and thoracic spine, 12 fresh and two embalmed cadavers were dissected. Our results indicate that Mm. Multifidi present differences in lumbar and thoracic spines concerning their deepness, fibre trajectory, muscle length, muscle mass and tendinous tissue. In the lumbar spine Mm. Multifidi are a superficial, thick and fleshy mass, and their fibres are more vertical in relation to the spinous processes. In the thoracic spine Mm. Multifidi are deeper, thinner, and their fibres are more tendinous and oblique than in the lumbar spine. These differences have implications on Mm. Multifidi architecture and consequently for their function in these two regions of the spine.     

The functions of the lumbar spine during stepping in the cat

To examine the functional roles played by the lumbar spine during overground stepping, seven adult cats were run in electromyographic (EMG) experiments. Recordings were made bilaterally from mm. iliocostalis, longissimus dorsi and multifidus at a single vertebral level (L₃) and from m. rectus abdominis. Stepping movements were monitored synchronously either by videotape or by high speed cinematography. During alternate use of the hindlimbs (walking and trotting), both epaxial and abdominal muscles were active bilaterally and biphasically. During in-phase use of the hindlimbs (galloping and half-bounding), single bursts of activity were observed. Phasic bursts of activity in rectus abdominus were reciprocal to those of epaxial muscles. Second bursts of activity in either group were noted infrequently. Recordings from the same back muscle at several vertebral levels indicated little difference from these patterns. Movements of the lumbar spine during galloping and half-bounding steps, both angular and linear, are easily correlated with muscle activity patterns. Movements of the lumbar spine during walking and trotting show no particular pattern. Only small angular and linear movements are found. It is concluded that the lumbar spine contributes substantially to step length and limb speed during galloping and half-bounding steps and the epaxial and abdominal musculature may also act as elastic bodies. During walking and trotting steps, the epaxial muscles are proposed to act to stabilize the pelvic girdle to provide a firm base for limb muscles which arise on the pelvis and are synchronously active.     

sEMG activity of masticatory, neck, and trunk muscles during the treatment of scoliosis with functional braces. A longitudinal controlled study

Background

Studies on the relationship between occlusal problems and the spine are of increasing interest. In this study, we monitored the sEMG activity of masticatory, neck, and trunk muscles during the treatment of scoliosis in young patients, and compared the data with a control of untreated group.

Subjects and methods

Twelve white Caucasian patients (nine males and three females; mean age of 8.0 ± 1.5 years) with scoliosis and Class I occlusion (without crowding) were included in this study (study group). Fifteen healthy subjects (nine males and six females; mean age of 9.5 ± 0.8 years) were recruited as control group. The subjects were visited before they underwent the treatment of scoliosis, as well as after 3 (T1) and 6 months (T2) of their treatment for scoliosis. The patients were instructed to wear the device during sleep and during the day, according to the protocol given by their orthopedic.

Results

The treated group showed statistically significant changes in the sEMG activity of masticatory, neck, and trunk muscles, both at rest and during MVC of the mandible with respect to T0. The masseter and the anterior temporalis showed a significant improvement in the asymmetry index from T0 to T2. On the other hand, subjects in the control group did not register much change.

Conclusion

Our findings suggest that the use of a functional device for the treatment of scoliosis induces a significant reduction in the asymmetry index of the trunk muscles, as well as a significant increase in the contractility of masticatory muscles.     

Analysis of functional scoliosis by means of an anisotropic beam model of the human spine

An upright, muscle-relaxed human spine, suffering from a mild functional scoliosis, caused by a small difference in leg length, is modeled as an anisotropic, elastic beam. The lower end of the beam is built-in in a fixed body, i.e., the laterally tilted pelvis. The upper end is rigidly attached to a rigid body, i.e., the supported upper part of the trunk, which is supposed to move freely in the frontal plane. It is shown that the characteristic scoliotic curvature of the spine, observed on an X-ray picture, can be reproduced by means of buckling analysis of the beam model, using realistic values of geometric and loading parameters and a properly chosen bending stiffness, which is found to be in reasonable agreement with earlier experimental findings. The analysis also shows that the muscle-relaxed upright equilibrium position of the spine is mechanically unstable.     

Functional morphology of the pharyngeal jaw apparatus in perciform fishes: An experimental analysis of the haemulidae

A new mechanical model for function of the pharyngeal jaw apparatus in generalized perciform fishes is developed from work with the family Haemulidae. The model is based on anatomical observations, patterns of muscle activity during feeding (electromyography), and the actions of directly stimulated muscles. The primary working stroke of the pharyngeal apparatus involves simultaneous upper jaw depression and retraction against a stabilized and elevating lower jaw. The working stroke is characterized by overlapping activity in most branchial muscles and is resolved into three phases. Four muscles (obliquus dorsalis 3, levator posterior, levator externus 3/4, and obliquus posterior) that act to depress the upper jaws become active in the first phase. Next, the retractor dorsalis, the only upper jaw retracting muscle, becomes active. Finally, there is activity in several muscles (transversus ventrales, pharyngocleithralis externus, pharyngohyoideus, and protractor pectoralis) that attach to the lower jaws. The combined effect of these muscles is to elevate and stabilize the lower jaws against the depressing and retracting upper jaws. The model identifies a novel mechanism of upper jaw depression, here proposed to be the primary component of the perciform pharyngeal jaw bite. The key to this mechanism is the joint between the epibranchial and toothed pharyngobranchial of arches 3 and 4. Dorsal rotation of epibranchials 3 and 4 about the insertion of the obliquus posterior depresses the lateral border of pharyngobranchials 3 and 4 (upper jaw). The obliquus dorsalis 3 muscle crosses the epibranchial-pharyngo-branchial joint in arches 3 and 4, and several additional muscles effect epibranchial rotation. Five upper jaw muscles cause upper jaw depression upon electrical stimulation: the obliquus dorsalis 3, levator posterior, levator externus 3/4, obliquus posterior, and transversus dorsalis. This result directly contradicts previous interpretations of function for the first three muscles. The presence of strong depression of the upper pharyngeal jaws explains the ability of many generalized perciform fishes to crush hard prey in their pharyngeal apparatus.     

A biomechanical analog of curve progression and orthotic stabilization in idiopathic scoliosis

A biomechanical analog of curve progression and orthotic stabilization in idiopathic scoliosis has been developed using the classical theory of curved beam-columns. The interaction of the spinal musculature and other supporting structures is incorporated in the model using an equivalent flexural rigidity. The stability of a given scoliotic curve relative to a normal spine is described in terms of the so-called critical load ratio (Pc/Pe). This dimensionless quantity appears in the exact solution of the governing differential equation and boundary conditions. It is defined as the ratio of the load bearing capacity of a scoliotic spine (Pc) to that of a normal spine where the load bearing capacity of a normal spine is defined as Euler's buckling load (Pe). The computation of Pc/Pe is based upon a maximum allowable moment criterion. This model is used to study the effect of the degree of initial curvature and curve pattern in the frontal plane on the stability of untreated idiopathic scoliosis. Although restricted to two-dimensions, the model appears to demonstrate the synergistic effects of end support, transverse loading, and curve correction on improvement in relative stability of an orthotically supported scoliotic curve. The results of this study are in qualitative agreement with clinical findings that are based on long-term studies of natural history of idiopathic scoliosis and of patients undergoing orthotic management for scoliosis.     

Effects of tracheal airway occlusion on hyoid muscle length and upper airway volume

Complex relationships exist among electromyograms (EMGs) of the upper airway muscles, respective changes in muscle length, and upper airway volume. To test the effects of preventing lung inflation on these relationships, recordings were made of EMGs and length changes of the geniohyoid (GH) and sternohyoid (SH) muscles as well as of tidal changes in upper airway volume in eight anesthetized cats. During resting breathing, tracheal airway occlusion tended to increase the inspiratory lengthening of GH and SH. In response to progressive hypercapnia, the GH eventually shortened during inspiration in all animals; the extent of muscle shortening was minimally augmented by airway occlusion despite substantial increases in EMGs. SH lengthened during inspiration in six of eight animals under hypercapnic conditions, and in these cats lengthening was greater during airway occlusion even though EMGs increased. Despite the above effects on SH and GH length, upper airway tidal volume was increased significantly by tracheal occlusion under hypercapnic conditions. These data suggest that the thoracic and upper airway muscle reflex effects of preventing lung inflation during inspiration act antagonistically on hyoid muscle length, but, because of the mechanical arrangement of the hyoid muscles relative to the airway and thorax, they act agonistically to augment tidal changes in upper airway volume. The augmentation of upper airway tidal volume may occur in part as a result of the effects of thoracic movements being passively transmitted through the hyoid muscles.     

The Vestibular-Evoked Postural Response of Adolescents with Idiopathic Scoliosis Is Altered

Adolescent idiopathic scoliosis is a multifactorial disorder including neurological factors. A dysfunction of the sensorimotor networks processing vestibular information could be related to spine deformation. This study investigates whether feed-forward vestibulomotor control or sensory reweighting mechanisms are impaired in adolescent scoliosis patients. Vestibular evoked postural responses were obtained using galvanic vestibular stimulation while participants stood with their eyes closed and head facing forward. Lateral forces under each foot and lateral displacement of the upper body of adolescents with mild (n = 20) or severe (n = 16) spine deformation were compared to those of healthy control adolescents (n = 16). Adolescent idiopathic scoliosis patients demonstrated greater lateral displacement and net lateral forces than controls both during and immediately after vestibular stimulation. Altered sensory reweighting of vestibular and proprioceptive information changed balance control of AIS patients during and after vestibular stimulation. Therefore, scoliosis onset could be related to abnormal sensory reweighting, leading to altered sensorimotor processes.     

Analysis of the interaction between vertebral lateral deviation and axial rotation in scoliosis

There is a lack of clear biomechanical analyses to explain the interaction of the lateral and axial deformity of the spine in idiopathic scoliosis. A finite element model which represented an isolated ligamentous spine with realistic elastic properties and idealized geometry was used to analyse this interaction. Three variations of this model were used to investigate two different hypotheses about the etiology of scoliosis and to define the forces required to produce a scoliosis deformity. The first hypothesis is that coupling within a motion segment produces the interaction between lateral deviation and axial rotation. The second hypothesis is that posterior tethering by soft tissues in the growing spine produces the observed interaction. Modeling of both hypotheses failed to produce the clinically observed pattern of interaction. Therefore, to find which biomechanical forces were required to produce an idealized scoliosis, prescribed displacements were applied to the model. Production of a double curve scoliosis of 10 degrees Cobb angles required lateral forces on the order of 20 N acting 40 mm anterior to the vertebral body centers. There do not appear to be any anatomic structures capable of producing such forces. Therefore, it seems unlikely that scoliosis deformity can be explained in terms of forces acting on the spine, and understanding of its origins may come from examination of other mechanisms such as asymmetric thoracic growth, or asymmetric vertebral development.     

Instrumented forceps for measuring tensile forces in the rod of the VDS implant during correction of scoliosis.

Ventral derotation spondylodesis (VDS) is the standard in ventral scoliosis surgery. Especially in the thoracic spine, there are no alternatives to VDS with compression and derotation as its correction forces. However, pull-out of the end-vertebra screw during correction of scoliosis with the VDS implant is a common complication involving particularly the cranial end-vertebra screw in the thoracic region. This complication requires an extension of the fusion length or reduces at least the outcome of the correction. There are no in vivo data on correction forces in ventral scoliosis surgery. Thus the correction depends on the skill and experience of the surgeon. An instrumented forceps developed and built to measure forces in the longitudinal rod allows axial tensile forces to be determined in the longitudinal rod during surgery. The instrumented forceps has the advantage of reducing the risk of screw pull-out. Furthermore, viscoelastic behavior of the spine can be measured during ventral correction. In addition, knowledge of the correction forces improves our biomechanical understanding of the spine, especially during correction of scoliosis. Intraoperative force measurement is in no way detrimental to the patient.     

Morphing the feature-based multi-blocks of normative/healthy vertebral geometries to scoliosis vertebral geometries: development of personalized finite element models

Personalized Finite Element (FE) models and hexahedral elements are preferred for biomechanical investigations. Feature-based multi-block methods are used to develop anatomically accurate personalized FE models with hexahedral mesh. It is tedious to manually construct multi-blocks for large number of geometries on an individual basis to develop personalized FE models. Mesh-morphing method mitigates the aforementioned tediousness in meshing personalized geometries every time, but leads to element warping and loss of geometrical data. Such issues increase in magnitude when normative spine FE model is morphed to scoliosis-affected spinal geometry. The only way to bypass the issue of hex-mesh distortion or loss of geometry as a result of morphing is to rely on manually constructing the multi-blocks for scoliosis-affected spine geometry of each individual, which is time intensive. A method to semi-automate the construction of multi-blocks on the geometry of scoliosis vertebrae from the existing multi-blocks of normative vertebrae is demonstrated in this paper. High-quality hexahedral elements were generated on the scoliosis vertebrae from the morphed multi-blocks of normative vertebrae. Time taken was 3 months to construct the multi-blocks for normative spine and less than a day for scoliosis. Efforts taken to construct multi-blocks on personalized scoliosis spinal geometries are significantly reduced by morphing existing multi-blocks.     

Scoliosis and dental occlusion: a review of the literature

Background

Idiopathic scoliosis is a deformity without clear etiology. It is unclear wether there is an association between malocclusion and scoliosis. Several types of occlusion were described in subjects with scoliosis, mostly case-reports.

Objectives

The aim of this review was to evaluate the type of occluslins more prevalent in subjects with scoliosis

Search strategy

All randomised and controlled clinical trials identified from the Cochrane Oral Health Group Trials Register, a MEDLINE search using the Mesh term scoliosis, malocclusion, and relevant free text words, and the bibliographies of papers and review articles which reported the outcome of orthodontic treatment in subjects with scoliosis that were published as abstracts or papers between 1970 and 2010.

Selection criteria

All randomised and controlled clinical trials published as full papers or abstracts which reported quantitative data on the outcomes malocclusion in subjects with scoliosis.

Data collection and analysis

Data were extracted without blinding to the authors, age of patients or type of occlusion.

Main results

Using the search strategy eleven observational longitudinal studies were identified. No randomized clinical trials were recorded. Twenty-three cross-sectional studies were recorderd, and the others studies were reviews, editorials, case-reports, or opinions. The clinical trials were often not controlled and were about the cephalometric evaluation after treatment with the modified Milwuakee brace, followed by the orthodontic treatment of the class II relationship with a functional appliance. Clinical trials also included the study of the associations between scoliosis and unilateral crossbite, in children with asymmetry of the upper cervical spine. This association was also investigated in rats, pigs and rabbits in clinical trials. The other associations between scoliosis and occlusion seems to be based only on cross-sectional studies, case-reports, opinions.

Authors' conclusions

Based on selected studies, this review concludes that there is plausible evidence for an increased prevalence of unilateral Angle Class II malocclusions associated with scoliosis, and an increased risk of lateral crossbite, midline deviation in children affected by scoliosis. Also, documentation of associations between reduced range of lateral movements and scoliosis seem convincing. Data are also mentioned about the association between plagiocephaly and scoliosis.     

Biomechanical spinal growth modulation and progressive adolescent scoliosis – a test of the 'vicious cycle' pathogenetic hypothesis: Summary of an electronic focus group debate of the IBSE

There is no generally accepted scientific theory for the causes of adolescent idiopathic scoliosis (AIS). As part of its mission to widen understanding of scoliosis etiology, the International Federated Body on Scoliosis Etiology (IBSE) introduced the electronic focus group (EFG) as a means of increasing debate on knowledge of important topics. This has been designated as an on-line Delphi discussion. The text for this debate was written by Dr Ian A Stokes. It evaluates the hypothesis that in progressive scoliosis vertebral body wedging during adolescent growth results from asymmetric muscular loading in a "vicious cycle" (vicious cycle hypothesis of pathogenesis) by affecting vertebral body growth plates (endplate physes). A frontal plane mathematical simulation tested whether the calculated loading asymmetry created by muscles in a scoliotic spine could explain the observed rate of scoliosis increase by measuring the vertebral growth modulation by altered compression. The model deals only with vertebral (not disc) wedging. It assumes that a pre-existing scoliosis curve initiates the mechanically-modulated alteration of vertebral body growth that in turn causes worsening of the scoliosis, while everything else is anatomically and physiologically 'normal' The results provide quantitative data consistent with the vicious cycle hypothesis. Dr Stokes' biomechanical research engenders controversy. A new speculative concept is proposed of vertebral symphyseal dysplasia with implications for Dr Stokes' research and the etiology of AIS. What is not controversial is the need to test this hypothesis using additional factors in his current model and in three-dimensional quantitative models that incorporate intervertebral discs and simulate thoracic as well as lumbar scoliosis. The growth modulation process in the vertebral body can be viewed as one type of the biologic phenomenon of mechanotransduction. In certain connective tissues this involves the effects of mechanical strain on chondrocytic metabolism a possible target for novel therapeutic intervention.     

Treatment of children scoliosis by corrective brace with regulated force effect

Rigid structural spine scoliosis of a child and even non progressive congenital scoliosis (e.g. isolated hemivertebra) can be treated by hypercorrective brace in full day regime. The article shows the new type of corrective brace with adjustable force effect. The brace consists of 3 stiff parts connected by joints and telescopes. The parts of brace are made from plastic according to plaster form of child trunk. The joints allow only mutual turning brace parts at frontal plane. The special telescopes were developed which operated with prescribed forces, it means the brace and trunk parts are mutually turned at prescribed moments. The article shows the algorithm for calculation of spine stress state, and spine curve correction for given brace with adjusted telescope forces. The second algorithm calculates the telescope forces for demanded spine curve correction. The computer program can be used for computer aid design of brace forces. The force effect of the new type of brace is demonstrated on a 14 months old boy with congenital scoliosis of lumbar spine (hemivertebra L1 and L3 on the right side). Curvature measured according to Coob was changed after application of this brace from 47.5° to 32.0°.     

Sarcomere length organization as a design for cooperative function amongst all lumbar spine muscles

The functional design of spine muscles in part dictates their role in moving, loading, and stabilizing the lumbar spine. There have been numerous studies that have examined the isolated properties of these individual muscles. Understanding how these muscles interact and work together, necessary for the prediction of muscle function, spine loading, and stability, is lacking. The objective of this study was to measure sarcomere lengths of lumbar muscles in a neutral cadaveric position and predict the sarcomere operating ranges of these muscles throughout full ranges of spine movements. Sarcomere lengths of seven lumbar muscles in each of seven cadaveric donors were measured using laser diffraction. Using published anatomical coordinate data, superior muscle attachment sites were rotated about each intervertebral joint and the total change in muscle length was used to predict sarcomere length operating ranges. The extensor muscles had short sarcomere lengths in a neutral spine posture and there were no statistically significant differences between extensor muscles. The quadratus lumborum was the only muscle with sarcomere lengths that were optimal for force production in a neutral spine position, and the psoas muscles had the longest lengths in this position. During modeled flexion the extensor, quadratus lumborum, and intertransversarii muscles lengthened so that all muscles operated in the approximate same location on the descending limb of the force-length relationship. The intrinsic properties of lumbar muscles are designed to complement each other. The extensor muscles are all designed to produce maximum force in a mid-flexed posture, and all muscles are designed to operate at similar locations of the force-length relationship at full spine flexion.     

Estimation of trunk muscle forces using the finite element method and in vivo loads measured by telemeterized internal spinal fixation devices.

Direct quantitative measurement of muscle forces is not possible. Forces in the trunk muscles were estimated for standing and flexion of the upper body using three-dimensional, nonlinear finite element models of the lumbar spine with and without an internal spinal fixation device. Muscle forces assumed were two pairs dorsally and one ventrally, each representing several muscles. Muscle forces in the model with internal fixators were varied in discrete steps until the implant loads calculated closely corresponded to those measured in a patient with an instrumented implant. The calculated angles between adjacent lumbar vertebrae were compared with corresponding values measured on X-ray films of a patient as well as with literature values and served as a second criterion for predicting muscle forces. For the model without an implant, the muscle forces of the first model were slightly varied until the lumbar spine shape and the intradiscal pressure were physiological. The abdomen was shown to have a considerable supporting function for flexion.     

The three dimensional analysis of the Sforzesco brace correction

Background

The historical view of scoliosis as a primary rotation deformity led to debate about the pathomechanic role of paravertebral muscles; particularly multifidus, thought by some to be scoliogenic, counteracting, uncertain, or unimportant. Here, we address lateral lumbar curves (LLC) and suggest a pathomechanic role for quadrates lumborum, (QL) in the light of a new finding, namely of 12th rib bilateral length asymmetry associated with idiopathic and small non-scoliosis LLC.

Methods

Group 1: The postero-anterior spinal radiographs of 14 children (girls 9, boys 5) aged 9–18, median age 13 years, with right lumbar idiopathic scoliosis (IS) and right LLC less that 10°, were studied. The mean Cobb angle was 12° (range 5–22°). Group 2: In 28 children (girls 17, boys 11) with straight spines, postero-anterior spinal radiographs were evaluated similarly to the children with the LLC, aged 8–17, median age 13 years. The ratio of the right/left 12th rib lengths and it’s reliability was calculated. The difference of the ratio between the two groups was tested; and the correlation between the ratio and the Cobb angle estimated. Statistical analysis was done using the SPSS package.

Results

The ratio’s reliability study showed intra-observer +/−0,036 and the inter-observer error +/−0,042 respectively in terms of 95 % confidence limit of the error of measurements. The 12th rib was longer on the side of the curve convexity in 12 children with LLC and equal in two patients with lumbar scoliosis. The 12th rib ratios of the children with lumbar curve were statistically significantly greater than in those with straight spines. The correlation of the 12th rib ratio with Cobb angle was statistically significant. The 12th thoracic vertebrae show no axial rotation (or minimal) in the LLC and no rotation in the straight spine group.

Conclusions

It is not possible, at present, to determine whether the 12th convex rib lengthening is congenitally lengthened, induced mechanically, or both. Several small muscles are attached to the 12th ribs. We focus attention here on the largest of these muscles namely, QL. It has attachments to the pelvis, 12th ribs and transverse processes of lumbar vertebrae as origins and as insertions. Given increased muscle activity on the lumbar curve convexity and similar to the interpretations of earlier workers outlined above, we suggest two hypotheses, relatively increased activity of the right QL muscle causes the LLCs (first hypothesis); or counteracts the lumbar curvature as part of the body’s attempt to compensate for the curvature (second hypothesis). These hypotheses may be tested by electrical stimulation studies of QL muscles in subjects with lumbar IS by revealing respectively curve worsening or correction. We suggest that one mechanism leading to relatively increased length of the right 12 ribs is mechanotransduction in accordance with Wolff’s and Pauwels Laws.

     

A reduced-order model of the spine to study pediatric scoliosis

Biomechanics and Modeling in Mechanobiology - The S-shaped curvature of the spine has been hypothesized as the underlying mechanical cause of adolescent idiopathic scoliosis. In earlier work, we...     

Altered spinal kinematics and muscle recruitment pattern of the cervical and thoracic spine in people with chronic neck pain during functional task

Knowledge on the spinal kinematics and muscle activation of the cervical and thoracic spine during functional task would add to our understanding of the performance and interplay of these spinal regions during dynamic condition. The purpose of this study was to examine the influence of chronic neck pain on the three-dimensional kinematics and muscle recruitment pattern of the cervical and thoracic spine during an overhead reaching task involving a light weight transfer by the upper limb. Synchronized measurements of the three-dimensional spinal kinematics and electromyographic activities of cervical and thoracic spine were acquired in thirty individuals with chronic neck pain and thirty age- and gender-matched asymptomatic controls. Neck pain group showed a significantly decreased cervical velocity and acceleration while performing the task. They also displayed with a predominantly prolonged coactivation of cervical and thoracic muscles throughout the task cycle. The current findings highlighted the importance to examine differential kinematic variables of the spine which are associated with changes in the muscle recruitment in people with chronic neck pain. The results also provide an insight to the appropriate clinical intervention to promote the recovery of the functional disability commonly reported in patients with neck pain disorders.     

Quantitative anatomy of the lumbar musculature

This paper describes the anatomy of the musculature crossing the lumbar spine in a standardized form to provide data generally suitable for static biomechanical analyses of muscle and spinal forces. The muscular anatomy from several sources was quantified and transformed to the mean bony anatomy of four young healthy adults measured from standing stereo-radiographs. The origins, insertions and physiological cross-sectional area (PCSA) of 180 muscle slips which act on the lumbar spine are given relative to the bony anatomy defined by the locations of 12 thoracic and five lumbar vertebrae, and the sacrum, and the shape and positions of the 24 ribs. The broad oblique abdominal muscles are each represented by six vectors and an appropriate proportion of the total PCSA was assigned to each to represent the muscle biomechanics.