View Full Version : Piriformis muscle action

Linda Mclean
11-08-2007, 02:48 AM
Hi everyone,

I have noticed that several textbooks suggest that the piriformis, normally
considered a lateral rotator (and abductor) of the hip, becomes a medial
rotator once the hip is flexed roughly to 90 degrees. I have found two
references that provide some evidence for this.

Snijders et al. (Clinical Biomechanics 21:116-121, 2006) showed on embalmed
cadavers that a cross-legged sitting position results in elongation of the
piriformis muscle, and Delp et al (Journal of Biomechanics 32: 493-501,
1999) showed that on four dissected skeletons, the moment arm length for
piriformis was 29mm and biased to lateral rotation with the hip in extension
but that the moment arm length was 14mm and biased to medial rotation with
the hip in 90 degrees of flexion.

Does anyone know of any other corroborating evidence for this idea?

Linda McLean, PhD

Associate Professor

Queen's University

School of Rehabilitation Therapy