title: Psoas muscle architectural design, in vivo sarcomere length range, and passive tensile properties support its role as a lumbar spine stabilizer.
abstract: METHODS
  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.
  
  
  BACKGROUND
  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 13 cadaver specimens, permitting architectural measurements of physiological cross-sectional area (PCSA), normalized fiber length (Lf), and Lf:muscle length (Lm) ratio. 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 ± 1.32 cm2, average Lf was 12.70 ± 2 cm, and average Lf: Lm was 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 of fiber bundles was 55.3 ± 11.8 kPa.
  
  
  CONCLUSIONS
  Analysis of PM architecture demonstrates that its average Lf and passive biomechanical properties resemble those of the lumbar erector spinae muscles. In addition, 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 a dynamic stabilizer of the lumbar spine.
authors.affiliation: Department of Orthopaedic Surgery, University of California Medical Center, San Diego, CA, USA.
authors.forename: Gilad J
authors.initials: GJ
authors.lastname: Regev
authors.forename: Choll W
authors.initials: CW
authors.lastname: Kim
authors.forename: Akihito
authors.initials: A
authors.lastname: Tomiya
authors.forename: Yu Po
authors.initials: YP
authors.lastname: Lee
authors.forename: Hossein
authors.initials: H
authors.lastname: Ghofrani
authors.forename: Steven R
authors.initials: SR
authors.lastname: Garfin
authors.forename: Richard L
authors.initials: RL
authors.lastname: Lieber
authors.forename: Samuel R
authors.initials: SR
authors.lastname: Ward
mesh_terms: D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D001696:Biomechanical Phenomena; D002102:Cadaver; D064211:Connectin; D005260:Female; D006801:Humans; D008159:Lumbar Vertebrae / Q000502:physiology*; D008297:Male; D008875:Middle Aged; D009119:Muscle Contraction / Q000502:physiology; D018485:Muscle Fibers, Skeletal / Q000378:metabolism / Q000502:physiology; D009124:Muscle Proteins / Q000378:metabolism; D020033:Protein Isoforms / Q000378:metabolism; D011494:Protein Kinases / Q000378:metabolism; D016658:Psoas Muscles / Q000033:anatomy & histology* / Q000378:metabolism / Q000502:physiology*; D012518:Sarcomeres / Q000502:physiology*
chemical_list: D064211:Connectin; D009124:Muscle Proteins; D020033:Protein Isoforms; C577285:TTN protein, human; D011494:Protein Kinases