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Mammalian spinal biomechanics: postural support in seated macaques

Julianna Gal^*

School of Applied Sciences, University of Glamorgan, Trefforest, Pontypridd CF37 1DL, Wales, UK

View larger version (129K): [in a new window]   Fig. 1. A photograph showing a typical example of the external dorsal curvature of seated macaques (Macaca sylvana). The flexed dorsal curvature of the animal on the right (the `groomer') was used to estimate the approximate total flexion angle of the lumbar column from tangents drawn to the upper and lower lumbar regions (indicated schematically in Fig. 3). This total flexion angle was then partitioned equally amongst all the lumbar—lumbar intervertebral joints. This photograph was reproduced with the kind permission of the photographer (John Bracegirdle).

View larger version (16K): [in a new window]   Fig. 2. A tracing of a sagittal X-radiograph of the lower thoracic and lumbosacral spine (excised, unloaded, ligamentous) of a 2.34 kg male Macaca fascicularis. In contrast to the in vivo dorsal curve during sitting (see Figs 1 and 3), here the lumbosacral spine is hyper-extended. P, S, L and T indicate the pelvis and the sacral, lumbar and thoracic vertebrae, respectively. The length of the lumbar column (L1-L6) is approximately 10cm. The calculation in this study focuses on the lumbar column; the angles between these unloaded lumbar intervertebral joints were estimated by measuring the angles between lines drawn parallel to the long axes of the vertebral centra.

View larger version (19K): [in a new window]   Fig. 3. An outline of the `groomer' animal (from Fig. 1) with the lumbosacral X-radiograph (from Fig. 2) scaled down and superimposed to illustrate the contrast between the unloaded excised hyper-extended and the seated flexed, dorsal profiles. The length of the lumbar column (L1-L6) is approximately 10 cm. Tangents drawn to the approximate regions of L1 and L6 in the seated animal are shown, and the angle subtended, approximately -20°, was used to estimate the mean change in lumbar—lumbar intervertebral joint angle from the unloaded excised to the loaded in vivo state. The black circle indicates the approximate position of the centre of mass of the head, forelimbs and upper body in this particular seated posture. W is an estimate of the `weight' of the collective head, upper body and forelimb masses, which exerts a flexion moment about the lumbar spine. The moment arm of W is indicated by the dashed line and is approximately 10 cm (0.10 m) in this animal.