Functional aspects of cross-legged sitting with special attention to piriformis muscles and sacroiliac joints.
BACKGROUND
Transversely oriented pelvic muscles such as the internal abdominal oblique, transversus abdominis, piriformis and pelvic floor muscles may contribute to sacroiliac joint stability by pressing the sacrum between the hipbones. Surface electromyographic measurements showed that leg crossing lowers the activity of the internal oblique abdominal muscle significantly. This suggests that leg crossing is a substitute for abdominal muscle activity. No previous studies addressed piriformis muscle and related pelvic structures in cross-legged sitting.


METHODS
Angles of pelvis and femur were measured in healthy subjects in standing, normal sitting and cross-legged sitting, and were used to simulate these postures on embalmed pelvises and measure piriformis muscle elongation. Deformations of pelvic ring and iliolumbar ligament caused by piriformis muscle force were measured on embalmed pelvises.


RESULTS
Cross-legged sitting resulted in a relative elongation of the piriformis muscle of 11.7% compared to normal sitting and even 21.4% compared to standing. Application of piriformis muscle force resulted in inward deformation of the pelvic ring and compression of the sacroiliac joints and the dorsal side of the pubic symphysis.


CONCLUSIONS
Cross-legged sitting is common. We believe that it contributes to sacroiliac joint stability. This study demonstrates the influence of the piriformis muscle on sacroiliac joint compression. The elongation of the piriformis muscle bilaterally by crossing the legs may be functional in the build-up of active or passive tension between sacrum and femur.
Snijders
Chris J
CJ
Department of Biomedical Physics and Technology, Erasmus MC, University Medical Center Rotterdam, P.O. Box 1738, 3000 DR Rotterdam, The Netherlands. c.snijders@erasmusmc.nl
Hermans
Paul F G
PF
Kleinrensink
Gerrit Jan
GJ
D000009:Abdominal Muscles / Q000502:physiology*; D000328:Adult; D001696:Biomechanical Phenomena; D005260:Female; D006615:Hip; D006801:Humans; D066298:In Vitro Techniques; D017116:Low Back Pain / Q000503:physiopathology; D008297:Male; D018482:Muscle, Skeletal / Q000502:physiology*; D010388:Pelvis; D011187:Posture / Q000502:physiology*; D012446:Sacroiliac Joint / Q000502:physiology*