Kinetic potential of the lumbar trunk musculature about three orthogonal orthopaedic axes in extreme postures.

Many studies have examined the mechanics of the lumbar spine in various planes, but only a limited number of three-dimensional investigations have been reported. Analysis of the low back during complex, dynamic postures demands rigorous representation of the trunk musculature. The data of this study demonstrated the force and torque contributions of approximately 50 laminas of various trunk muscles to flexion-extension, lateral bending, and axial twisting torque at the L4-L5 joint. This analysis was conducted with the spine in an upright standing posture and when fully flexed (60 degrees), laterally bent (25 degrees), and axially twisted (10 degrees) together with two examples of combined postures. Maximum moment potential, muscle length excursions, and the resultant compressive, anteroposterior shear, and lateral shear forces on the joint were also computed. The results indicate that the position of the vertebrae and their orthopaedic axes, which are a function of spinal posture, are an important factor in the reasonable determination of joint compressive, lateral shear, and anteroposterior shear loads. Muscle length changes that exceeded 20% of their respective length during upright standing were not observed during a full axial twist, but were observed in portions of the abdominal obliques during lateral bending, and in some extensors during full flexion. Extreme postures tended to change the torque potential of some muscles and influence joint load. Various portions of erector spinae were observed to have appreciable potential to generate torque about all three orthopaedic axes. This observation supports the notion held by some therapists that conditioning of the erector spinae is of utmost importance.