The Involuntary Motion of the Sacrum Between the Ilia.

It is an anatomical fact that the sacrum (tail bone) moves between the ilia (bones of the pelvis). It simply would not be possible for humans to walk or run if the sacral-iliac joints did not allow for motion.

Yet for centuries it was believed by scientists that the pelvis was a solid array of bone and that there was no independent motion of the sacrum. This erroneous belief was still held by most of the medical community in the first part of the 20th century. History may in fact be repeating itself in the debate over cranial bone motion, just as the sacrum was once considered immobile.

  • In the mid-1950s Weisl1 delineated the biomechanical dimensions of the sacral-iliac joint.
  • Kissling2 measured sacral-iliac motion in subjects performing different types of postural motion.
  • Walker3 describes motion around a number of different axes of sacral motion (a concept well integrated into osteopathic training programs).

According to the PRM model, the sacrum also moves involuntarily between the ilia, around a respiratory axis. What remains controversial about this type of sacral motion, is the driving force or cause.

  • Mitchell and Pruzzo4-6 have demonstrated a horizontal axis for sacral motion located anterior to the second sacral segment. The impetus for this motion around this axis was simply breathing. This study associates sacral motion with the normal respiratory excursion, postulating possible physiologic functions other than ambulatory activity.
  • Frymann7 reminds us that the breathing motion of humans (the respiratory excursion) is yet a separate and distinct motion from that induced by the PRM.


It is now a well-established fact the sacrum moves between the ilia. Ambulatory and respiratory motions have each been demonstrated. Further research is required to understand the relationship between involuntary motion of the sacrum and motion of the cranial bones.


  1. Weisl H. The movement of the sacro-iliac joint. Acta Anat. 1955;23:80-91.
  2. Kissling RO, Jacob HA. The mobility of the sacroiliac joint in healthy subjects. Bull Hosp Joint Disease. 1996;54(3):158-164.
  3. Walker JM. The sacroiliac joint: a critical review. Physical Therapy. 1992;72:903-916.
  4. Mitchell FL Jr, Pruzzo NL. Investigation of voluntary and primary respiratory mechanisms. J Am Osteopath Assoc. 1971;70:149-153.
  5. Mitchell FL Jr. Voluntary and involuntary respiration and the craniosacral mechanism. Osteopath Ann. 1977;5:52-59.
  6. Pruzzo NA. Lateral lumbar spine double-exposure technique and associated principles. J Am Osteopath Assoc. 1970;69:84-86.
  7. Frymann VM. A study of the rhythmic motions of the living cranium. J Am Osteopath Assoc. 1971;70:1-18.