Synergy of the human spine in neutral postures

Abstract

IntroductionAn understanding of mechanisms for the load-resisting capacity of the human spine and load sharing between the passive ligamentous spine and the active muscle tissues in neutral posture is essential for investigation of spinal functioning in normal and pathologic conditions. Several approaches have been taken in previous numerical models for static analysis of the lumbar spine: maximum moment-generating capacity models [7,38], transverse section equilibrium models [36], and stability criterion models [5,10,16]. Experimental measurements of, for instance, electromyographic activities in spinal muscles and Abstract The neutral position of the spine is the posture most commonly sustained throughout daily activities. Previous investigations of the spine focused mainly on maximal exertions in various symmetric and asymmetric postures. This report proposes a new synergetic approach for analysis of the spine in neutral postures and evaluates its performance. The model consists of passive components, the osteoligamentous spine, and active components, the spinal muscles. The muscle architecture includes 60 muscles inserting onto both the rib cage and lumbar vertebral bodies. The passive spine is simulated by a finite element model, while kinematic constraints and optimization are used for resolution of a redundant muscle recruitment problem. Although the passive spine alone exhibits little resistance to a vertical load, its load-bearing capacity in neutral posture is significantly enhanced by the muscles, i.e., the passive spine and its muscles must be considered as a synergetic system. The proposed method is used to investigate the response of the spine when the T1 vertebra displaces 40 mm anteriorly and 20 mm posteriorly from its initial position. The sacrum is fixed at all times and the T1 displacements are achieved by the action of muscles. The results suggest that relatively small muscle activations are sufficient to stabilize the spine in neutral posture under the body weight. The results also indicate that muscles attaching onto the rib cage are important for control of the overall spinal posture and maintenance of equilibrium. The muscles inserting onto the lumbar vertebrae are found mainly to enhance the stability of the spine. The proposed method also predicts forces and moments carried by the passive system. Flexion moments ranging from 8000 Nmm to 15,000 Nmm, corresponding to decreases in lordosis of 6°and 7.5°respectively, are found to be carried by the passive spine at the thoracolumbar junction when the T1 vertebra is 40 mm anterior to its initial position.

How do anterior/posterior translations of the thoracic cage affect the sagittal lumbar spine, pelvic tilt, and thoracic kyphosis?

“…During thoracic Tz translations, subjects had a mean flexion/extension of less than 4.2°at T1-T12, indicating that these endpoints remained vertically aligned (Table 1). However, Kiefer et al [15] modeled T1-T12 as one rigid body and the large changes in thoracic kyphosis reported here, especially in the lower thoracic region (total change in Cobb angle of 26°at T1-T12 in Table 3) indicate that the rigidity of the thoracic cage needs to be re-evaluated. Perhaps only the thoracic kyphosis from T1 to T10 should be modeled as a rigid body.…”

“…Recently, Kiefer et al [15] reported the importance of keeping T1, T12 and S1 vertically aligned in the sagittal plane to minimize muscle forces and moments in upright neutral posture. For these reasons, we measured thoracic cage translations as horizontal displacements of T12 to S1, while keeping T1 vertically aligned with T12.…”

Spinal muscle forces, internal loads and stability in standing under various postures and loads—application of kinematics-based algorithm

“…A sagittally symmetric T1-S1 beam-rigid body model [21,22,32,43] is used. It is made of six deformable beams to represent T12-S1 discs and seven rigid elements to represent T1-T12 (as a single body) and lumbosacral vertebrae (L1 to S1).…”

“…To overcome this deficiency by satisfying the equilibrium in different directions at all lumbar levels, and accounting for the passive ligamentous resistance, a linear finite element model has been reported to evaluate muscle recruitment and internal lumbar loads during maximum and sub-maximum efforts [13,14,52,54]. Recently, we introduced and applied a novel iterative, kinematics-based approach, in which the a priori known kinematics of the spine at different levels under given external loads, along with passive properties, were exploited in a nonlinear finite element model to evaluate unknown muscle and internal loads, resulting in a synergistic solution of the entire active-passive system [21,22,49]. The relative validity of various models and the accuracy in their predictions under various loading and postural conditions, though naturally dependent on their assumptions (e.g., the choice of cost functions or strategies to distribute reactive moments amongst spinal muscles), need yet to be established.…”

“…The passive ligamentous thoracolumbar and lumbar spines are known to exhibit large displacements or hypermobility (i.e., instability for an imperfect system such as the spine) under compression loads <100 N [10,22,41,42,43]. Bearing in mind that these forces are only a small fraction of those carried by the spine in activities of daily living; the question thus arises as to how then the system is stabilized in vivo?…”