The science concerned with the action of forces, internal or external, on the living body (1)

The human vertebral column consists of 24 separate (presacral) vertebrae and two composite vertebrae, the sacrum (5 pieces) and coccyx (4 pieces) for a total of 33 segments.

The spine serves many functions: stable base for head and internal organs; attachments for ligaments, tendons, rib cage and pelvis; links upper and lower extremities; provides trunk mobility; and protects the spinal cord. (2)

Thoracic and sacral kyphosis develops in utero and are present in the newborn.

The secondary curves of the spine develop with cervical lordosis, generally at 6-8 months, by means of prone head righting and turning. Lumbar lordosis develops next, at 11-14 months, as a result of weight bearing from standing and walking.

The normal contours of the standing adult are 7 cervical vertebrae in lordosis, 12 thoracic vertebrae in kyphosis, 5 lumbar vertebrae in lordosis and 9 fused sacral-coccygeal segments in kyphosis.

Generally there are six articulations between each adjacent vertebra: the superior body with the disc above; the inferior body with the disc below; two superior facets articulating with the inferior facets of the rostral vertebra; and two inferior facets articulating with superior facets of the caudal vertebra.

The cervical region differs primarily in the first two segments. The occipital-atlanto complex has no disc, among other differences, and the atlanto-axial (C1-C2) complex also lacks a disc, but has a superior protrusion called the dens, or odontoid process. The thoracic region has rib and sternum articulations from T1-T9 and rib only articulations at T10-T12. The sacrum and coccyx are fused, and the sacrum articulates with the ilium at S1-S3.

The cervical region has a large vertebral foramen, foramen transversarium for vertebral arteries, and a small, narrow body that ends inferiorly in a concave shape and short, slender spinous processes. The thoracic region has long, overlapping spinous processes, and articulations for ribs. The lumbar region features large diameter bodies with increased height, broad, thick, hatchet-shaped spinous processes, and horizontal transverse processes.

Orientation of the facet joints, structure of the vertebra, disc size and fluid content, ligamentous and bony articulations and muscular attachments.

The cervical spine features two distinct movements: OA provides 10-30° flexion-extension (head nodding) and AA (C1-2) provides about 90° rotation. The thoracic spine is primarily rotation and is limited by the rib cage. The lumbar spine offers flexion and extension, but is the most stable are due to weight bearing. (2)

The basic motions of the spine are flexion, extension, lateral flexion and rotation. These motions rarely occur in isolation. Most often they occur as "coupled motions." The amount of motion that occurs at most joints, however, is quite small.

Increasing mobility generally leads to less stability, and vice-versa, increasing stability generally leads to decreased mobility.

How does this information apply biomechanically to the human body?

Through these mechanisms, the seated individual’s trunk control is modulated by muscle recruitment that is direction-specific (to the perturbation) and demonstrates a caudal-to-cranial order. (3)

In a study of 10 children with spastic diplegia, dysfunction presented during modulation of forward perturbations resulted in activation of all ventral muscles, cranial-caudal recruitment and excessive antagonistic co-activation. (5)

The seated position is one of the primary risk factors for low back pain. Studies indicate the position of the seating surface can prevent or reduce the onset of LBP. (6-13)

In vitro studies indicate the critical load (Pcr= inherent passive stability of the osteoligamentous spine) of the thoracolumbar spine is 20N* and the cervical spine 10.5N. This is represented as the stiffness of the column and is represented mathematically by a bending moment. (14) The average Pcr of the lumbar spine is 8.6 times that of the average cervical spine (14,18)

Dorsal muscle activity (esp. longissimus and iliocostalis of the lumbar extensors) is important for stability in a seated position, but is compromised with full forward flexion of the lumbar spine. Many other muscles (paraspinals including multifidi, quadratus lumborum, psoas and abdominal wall muscles) have force vectors that are less affected by lumbar flexion. (19)

Trunk muscle EMG studies show decreased activation with the use of lumbar supports and seat back inclination (13), while studies of office chairs showed that sagittal curvature of the lumbar spine was clearly affected by the seat tilt, backrest recline, backrest profile and seat-back angle. (7)

Numerous studies have indicated the decrease in lumbar lordosis from standing to sitting. (4,7-9,12,20) Without appropriate placement and size, a lumbar support may inadvertently displace the pelvis and create a posteriorly rotated pelvis. Lumbar support for comfort, control of pelvis rotation at PSIS is in order.

Working in a seated position for 6.5h, shrinkage occurred in the thoracic spine, with virtually no change in lumbar pre-test height. When compared to standing work, the shrinkage is greater in the lumbar spine and equal in the thoracic spine. (9)

Comfort is usually defined in seating research as discomfort, the presence of a distracting bodily sensation, (20) or biomechanical stress acting on the body. (12)

Comfort (or more often, discomfort) is most often measured subjectively by the user and correlated to an objective measurement device. This is taken as a static measurement, or more precisely, a non-continuous measurement. Sitting, however, is a dynamic activity. (20)

Because sitting is a dynamic activity, comfort or discomfort may be more accurately measured by continuous monitoring of in-chair movement. This has been shown to correlate linearly with discomfort as seated time progresses. Changes in the center of pressure can be measured over time, and independent of task, can be a reliable indicator of sitting discomfort. (20)

Pressure mapping as a clinical tool can provide: client and caregiver education; selection and comparison of support surfaces; assessment of changes in position or equipment (21); objective, quantitative outcome measurement; and research data collection.

A preponderance of the published research on seating comfort has been performed in the occupational and transportation settings. Findings can be useful when applied to 80% of wheelchair users, but may be more difficult with the complex seated user, as pure comfort may be combined with functional needs.

REFERENCES

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