INTRODUCTION: The multifidus muscle is a key stabilizer of the spine. Multifidus atrophy occurs in patients with low back pain and those with symptomatic herniated discs. Study of muscle behavior has generally been limited to EMG studies. Recently, validated musculoskeletal simulation software has been developed that permits analysis of muscle forces under a variety of activities. The AnyBody Modeling System™ contains more than 1000 individual muscle branches and is shown in Figure 1. This study examines the force required by the different branches of the multifidus for each of the principal motions.
METHODS: Lumbar flexion‐extension (80° flexion to 10° extension), lateral bending (10° in both directions) and axial rotation (12° in both directions) was analyzed using a model of a standing human (1.75 m height, 75 kg) under influence of gravity. The stiffness of each lumbar spinal segment in each direction was based on published cadaver stiffness measurements for normal segments. The analyzed parameters were the forces in all branches of the multifidus muscles of the model on the right side of the spine.
RESULTS: The peak force in a multifidus branch occurred during axial rotation (53N), with lesser forces in flexion/extension (20N) and lateral bending (17N). The multifidus force varied during the range of motion, and was maximal at the limit of axial rotation (12° of lumbar spine rotation) only in one direction (Fig 2), while a maximum force was seen at full flexion with a lesser peak at extension and reduced forces at neutral position (not shown).
DISCUSSION: Axial rotation motion induced the largest force in the multifidus. This result is consistent with previous work showing that axial rotation motion induces the greatest load on the facets and excessive axial rotation motion in patients with both low back pain and early disc degeneration.