Anatomy and Biomechanics of the Back Muscles in the Lumbar Spine With Reference to Biomechanical Modeling

Lone Hansen, PhD; Mark de Zee, PhD; John Rasmussen, PhD; Thomas B. Andersen, PhD; Christian Wong, PhD; Erik B. Simonsen, PhD

Disclosures

Spine. 2006;31(17):1888-1899. 

In This Article

Spinal Kinematics

The principal motions of the spine are flexion, extension, lateral flexion, and rotation. The range of motion of a single segment is difficult to measure clinically. Variations among individuals are considerable and also age dependent. A reduced range of motion in full flexion and lateral bending, but not in axial twist, was found when elderly were compared to younger subjects.[19] In addition, males had more mobility in flexion-extension, while females were more mobile in lateral flexion.[20] When modeling the lumbar spine, knowledge of the axes of rotation is important, considering actions and forces of the muscles. During flexion and extension of the lumbar spine, each vertebra undergoes an arcuate motion in relation to the next lower vertebra, which is caused by a combination of rotation and translation in the sagittal plane.[21] For any arch of movement defined by a given starting position and a given end position of the moving vertebra, the center of movement is known as the instantaneous axis of rotation.[22]

In normal conditions, the instantaneous axis of rotation of flexion-extension and lateral flexion lies within the disc and generally in the posterior part of the disc.[23,24] Pearcy and Bogduk[25] determined a mean location of the instantaneous axis of rotations of flexion-extension in the lumbar spine, allowing calculation of the moments exerted by any muscle at any lumbar level. Ranges of segmental motion are given in Table 1 .

When considering the spine as a whole, it consist of multiple vertebrae with a wealth of possible movements. However, the tissues connecting the vertebrae constrain the movement considerably. Thus, the spine relies on its ability to constrain movement between the vertebrae to patterns, which are compatible with the limitations on tissue strain. In other words, the relative movement of the vertebrae follows a pattern, a spinal rhythm also called coupled motions, imposed by the elasticity of the passive tissues and the coordinated actions of the spinal muscles. Miyasaka et al[28] investigated the influence of ligament stiffness and transverse process thickness on the range of motion of the spine, and confirmed the influence of the passive tissues on the kinematics. Cholewicki et al[29] investigated the effects of posture and structure on coupled rotations in the lumbar spine using a model. They concluded that the lumbar lordosis and intrinsic mechanical properties of the spine were equally important in prediction of coupled rotations.

Harrison et al[30,31] found that thoracic translation was coupled with the lumbar curve and pelvic tilt. Furthermore, on both, Tully et al,[32] and Lee and Wong[33] discovered a clear lumbo-femoral rhythm during hip flexion, meaning that there is a concurrent motion of the lumbar spine-pelvis and hip during normal hip flexion. These findings support the idea that that spinal kinematics is related to the behavior of elastic beams, and Stokes et al[34] pursued this notion and proceeded to characterize spinal movement using a stiffness matrix.

The relevance of coupled motions for musculoskeletal modeling is that it might actually simplify the description of the motion, which is especially true in relation to musculoskeletal modeling using inverse dynamics in which the key is to describe the motion accurately. In theory, one could describe the angle change between each vertebra, but these kinds of motion data are difficult to obtain in vivo . Coupled motions give the possibility to use only a couple of degrees of freedom, while the remaining degrees of freedom are given by the coupled motions equations. Of course, a prerequisite is that one knows these coupled motions equations. Adopting the idea that the spine kinematically functions as an elastic beam embedded into the sacrum leads to a description of its movement as a second order polynomial.

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