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.