Changes in lumbar lordosis modify the role of the extensor muscles.
METHODS
Fiber angles of longissimus thoracis and iliocostalis lumborum at L3 were documented in vivo, using high resolution ultrasound, with the lumbar spine in neutral curve and when fully flexed.


OBJECTIVE
To evaluate the effect of changes in lumbar curvature on the mechanics of these muscles.


BACKGROUND
Full flexion modifies the failure tolerance of the lumbar spine, determines the load distribution among muscle and passive tissues, and modulates the types of tissue damage that occur. Related to this issue are the possible changes in muscle line of action with full flexion which changes the ability of the spine to support shear loads.


METHODS
Nine normal men and 5 normal women were scanned in three positions: (1) an upright standing posture; (2) with the hips flexed to approximately 30 degrees and the spine fully flexed; (3) hips flexed but the spine returned to a neutral curvature.


RESULTS
Mean longissimus/iliocostalis fiber angles for upright standing, hips flexed-spine flexed, and hips flexed-spine neutral lordosis were 25. 7 degrees, 10.7 degrees and 28.3 degrees, respectively.


CONCLUSIONS
Anterior shear load on the lumbar spine has been recently shown to be highly related to the risk of reporting a back injury. Bending forward allowing the spine to fully flex changes the line of action of the largest lumbar extensor muscles compromising their role to support anterior shear forces. Relevance Fiber angles of longissimus thoracis and iliocostalis lumborum were documented with high resolution ultrasound at L3, with the spine in neutral curvature and fully flexed. Full lumbar flexion changes the line of action of these muscle compromising their role to support anterior shear forces on the spine - anterior shear forces have been recently documented to be highly related to the risk of reporting a back injury.
McGill
S M
SM
Faculty of Applied Health Sciences, Department of Kinesiology, Occupational Biomechanics Laboratory, University of Waterloo, Waterloo, N2L 3G1, Ontario, Canada. mcgill@healthy.uwaterloo.ca
Hughson
R L
RL
Parks
K
K
D000293:Adolescent; D000328:Adult; D019567:Back Injuries / Q000209:etiology; D001696:Biomechanical Phenomena; D005260:Female; D006621:Hip Joint / Q000502:physiology; D006801:Humans; D008141:Lordosis / Q000000981:diagnostic imaging / Q000503:physiopathology*; D008159:Lumbar Vertebrae / Q000000981:diagnostic imaging / Q000503:physiopathology*; D008297:Male; D009119:Muscle Contraction / Q000502:physiology; D018485:Muscle Fibers, Skeletal / Q000000981:diagnostic imaging / Q000648:ultrastructure; D018482:Muscle, Skeletal / Q000000981:diagnostic imaging / Q000503:physiopathology*; D011187:Posture / Q000502:physiology; D016059:Range of Motion, Articular / Q000502:physiology; D012307:Risk Factors; D013223:Statistics as Topic; D014463:Ultrasonography; D014743:Videotape Recording; D016474:Weight-Bearing / Q000502:physiology