Changes in lumbar lordosis modify the role of the extensor muscles
Introduction
A recent review paper in Clinical Biomechanics [1] evaluated the biomechanical evidence to support some advantage for either stoop or squat style lifting. No unifying support could be found to preferentially advocate either. Perhaps the issue is more subtle – specifically the curvature of the lumbar spine during lifting, independent of the style of the lift may be important. Changes in lumbar lordosis have been documented to influence several aspects of spine mechanics and the potential for tissue damage. Specifically, a fully flexed lumbar spine, in contrast to a neutral posture, results in a reduced moment arm for the extensor muscles [2], a decreased tolerance to compressive load [3], and a transfer of load from muscle to passive tissues increasing the risk of injury to ligaments and more specifically increases for the risk of posterior disc herniation [4]. However, there may be yet another consideration regarding the negative effects of performing tasks with a fully flexed lumbar spine. The major lumbar extensors, namely longissimus thoracis and iliocostalis lumborum, do not run parallel to the compressive axis of the spine, but rather have an oblique orientation such that they support the anterior shear forces that result during forward flexion of the torso [5] (Fig. 1). Lumbar shear forces have been shown to be linked with elevated injury rates in industry [6]. When bending forward, one has the option of obtaining rotation from the hips, from the lumbar spine, or from a combination of both. Many have suggested it is safer to minimize spine flexion (neutral spine), requiring more hip rotation, when performing bending tasks such as lifting. In this work the question is asked “Does a change in lumbar curvature also affect the orientation angle of these major lumbar extensors thus modulating the ability to support shear forces?”.
The purpose of this work, was to document the effect, if any, of changing lumbar curvature on the fiber directions of these extensor muscles thereby influencing the ability to support shear loads on the spine. Traditionally, the action of these muscles is interpreted from cadaveric specimens, however, in this study, muscle fibers were imaged in vivo using high resolution ultrasound.
Section snippets
Subjects
Nine men and five women participated in the study. The mean height of participants was 170.7 (SD, 9.2) cm, mean weight of 73.3 (SD, 12.6) kg; with a range in age from 18 to 31 years (mean 23 (SD, 3.4) years). All subjects were healthy and not experiencing any disabling low-back pain within the previous year.
Initially, subjects stood in an upright relaxed position, with feet shoulder-width apart, while anthropometric measures were taken. Body width (in mm) was measured at the level of T10, L3
Results
Mean longissimus/iliocostalis fiber angles for each experimental position: full flexion, neutral lumbar curvature and relaxed standing were 10.7 (SD, 4.6°), 28.3 (SD, 4.7°) and 25.7 (SD, 5.3°), respectively. Post hoc tests revealed that longissimus/iliocostalis fiber angles were not different between standing vs. the neutral lumbar position (P > 0.05). However, a difference was found between fiber angles in the relaxed standing and the flexed positions (P < 0.001) and between the neutral lumbar and
Discussion
Fully flexing the lumbar spine reduces the cosine of the orientation of the longissimus/iliocostalis complex thereby compromising the ability of the lumbar extensors to support shear forces that result from torso flexion. Given several other negative effects that result when the spine is fully flexed, listed in the introduction of this paper, it would appear that this spine posture should be avoided when the spine is subjected to load. Thus, the argument whether to stoop or squat lift should
Acknowledgements
The authors gratefully acknowledge the financial support of the Natural Sciences and Engineering Research Council (NSERC), Canada, and H. Naylor for assistance with ultrasound measurements.
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