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Psoas muscle architectural design, in vivo sarcomere length range, and passive tensile properties support its role as a lumbar spine stabilizer

Regev GJ, Kim CW, Tomiya A, Lee YP, Ghofrani H, Garfin SR, Lieber RL, Ward SR.
Spine, 2011 36(26):E1666-74. Epub 2011 Mar 14.


Study Design: Controlled laboratory and cross-sectional study designs.
Objective: To determine psoas major (PM) muscle architectural properties, in vivo sarcomere length operating range, and passive mechanical properties.
Summary of Background Data: PM is an important hip flexor but its role in lumbar spine function is not fully understood. Several investigators have detailed the gross anatomy of PM, but comprehensive architectural data and in vivo length-tension and passive mechanical behaviors have not been documented.
Methods: PM was isolated in thirteen cadaver specimens, permitting architectural measurements of physiological cross-sectional area (PCSA), normalized fiber length (Lf), fiber length-to-muscle length ratio (Lf/Lm). Sarcomere lengths were measured in vivo from intraoperative biopsies taken with the hip joint in flexed and extended positions. Single fiber and fiber bundle tensile properties and titin molecular weight were then measured from separate biopsies.
Results: Architecturally, average PCSA was 18.45 ± 1.32cm, average Lf was 12.70 ± 2cm, and average Lf/Lm was of 0.48 ± 0.06. Intraoperative sarcomere length measurements revealed that the muscle operates from 3.18 ± 0.20 µm with hip flexed at 10.7 ± 13.9° to 3.03 ± 0.22 µm with hip flexed at 55.9 ± 21.4°. Passive mechanical data demonstrated that the elastic modulus of the PM muscle fibers was 37.44 ± 9.11 kPa and bundles was 55.3 ± 11.8 kPa.
Conclusions: Analysis of PM architecture demonstrates that its average fiber length and passive biomechanical properties resemble those of the lumbar erector spinae muscles. Additionally, PM sarcomere lengths were confined to the descending portion of the length-tension curve allowing the muscle to become stronger as the hip is flexed and the spine assumes a forward leaning posture. These findings suggest that the human PM has architectural and physiologic features that support its role as both a flexor of the hip and as a dynamic stabilizer of the lumbar spine.

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