Diab, M. 1999. Lexicon of Orthopaedic Etymology. London: Informa House. Hu, H., et al. 2011. Is the psoas a hip flexor in the active straight leg raise? European Spine Journal, 20,759–65. Kirchmair, L., et al. 2008. Lumbar plexus and psoas major muscle: Not always as expected. Regional Anesthesia and Pain Medicine, 33 (2), 109–14.