The importance of pelvic nutation

The relatively small and poorly defined rotational and transnational movements that occur at the sacroiliac joint in the sagittal plane are called nutation and counter-nutation.

These movements perform two major functions, based on Donald A. Neumann, Kinesiology of The Musculoskeletal System, Foundations for Rehabilitation.

1. Stress relief within the pelvic ring

2. Stability during load transfer

Regarding the first function, I have created this flowchart in order to make it understandable:

walking --> reciprocal flexion-extension pattern --> out of phase pelvic rotation --> tension in muscles and ligaments --> oppositely directed torsion --> nutation/counternutation --> stress dissipation (stress that would otherwise occur in the pelvic ring it it were a solid structure).

Regarding the second function, it is important for us to realize that nutation is the closed pack position for the SI joint. This means that the majority of joint structures are under maximal tension in this position, thus the joint is more stable. Consequently, torques that favor nutation help stabilize the SI joint. These are: gravity, stretched ligaments and muscle activation.

Gravity: the body weight line passes vetrally to the sacrum causing a forward tilt & the femural head reaction force on the acetabulutm causes an iliac bacward tilt.

Stretched ligaments: interosseous and sacrotuberous ligaments stretch the joint when movement occurs.

Muscle activation: the following muscle make the sacrum to titl forward (S) or the iliac crest to move backward (I) or just stabilize the SI joint:

erector spinae (S)
multifidus (S)
rectus abd. (I)
obliq. abd. int. & ext.
transv. abd.
hamstrings (I)
gluteus max. (I)
lat. dorsi
iliacus (S)
piriformis