Trunk positioning accuracy in the frontal and sagittal planes

Abstract

The accuracy with which the head and spine could be positioned in the frontal and sagittal planes relative to the pelvis was measured and compared in ten healthy adult males. Subjects were tested with eyes closed, while standing with their pelvis externally restrained. The positions of markers, attached to the back of the head and over each of the T1, T6, T11, and L3 spinous processes, were measured to the nearest mm using strain-gaged flexible beam transducers. Subjects were tested for their accuracy in sensing return of the trunk to an initial neutral position under different test conditions. Results showed that positioning was 16-45% more accurate in the frontal than in the sagittal plane, although the difference did not reach statistical significance. T1 could be centered to within 7 and 10 mm in the frontal and sagittal planes, respectively. No significant differences were found between active and passive positioning accuracies. Presence of an external trunk moment did not significantly affect trunk positioning accuracy, although it systematically caused overshoot of the neutral position. Lastly, lateral trunk shifts exceeding 12 mm may be classified as abnormal in young adults.

Stability of the human spine in neutral postures

“…These were: a realistic distribution of the upper body weight [13,24,26,27], rotation at the pelvis [8,11,22], initial positioning of T1 [11,17], and the role of some muscles [4]. The TLS models with reasonable structural properties [2, 22-24, 26, 27] used in this study were both sufficiently practical to make possible a large number of nonlinear analyses coupled with constraint equations and sufficiently accurate to determine the stability response in upright postures under relatively small axial compressive forces.…”

“…With the T1 vertebra prevented from moving horizontally from its initial position [11,17], horizontal reaction at T1 was taken as the cost function dependent on the magnitude of the pelvic rotation. The required values of optimal pelvic rotation were then evaluated at different loads by minimizing the cost function.…”

“…Experimental studies on the positioning accuracy of the head in a free standing posture indicate that the T1 vertebra can be centered horizontally to within 10 mm [17]. Studies on asymptomatic subjects [11,25] and lowback patients [11] have suggested a possible correlation between sacral slope (angle between the plumb line and the line parallel to the back of proximal sacrum), lordosis, kyphosis, and the sagittal position of the T1 vertebra in both groups.…”

“…The likely role of muscles is then evaluated by two idealized muscle models. In the proposed approach, the neural controller is postulated to maintain T1 in a steady position in the horizontal plane [11,17]. Full exploitation of the passive loadbearing capacity of the TLS is postulated to minimize the muscular effort required in maintaining neutral postures.…”

Synergy of the human spine in neutral postures

“…First, the effect of T1 horizontal displacement in a sagittal direction 40 mm anterior and 20 mm posterior to its initial position [20,28] is investigated with S1 fixed. The T1 position is controlled by the action of the global muscles, with the local muscles inactive.…”