The transverse abdominis is active in both nutation and counternutation but primarily in nutation, where it increases the lumbar lordosis and pulls the ilia posteriorly.
Observation reveals that, by its attachments to the spinouses, through the thoracolumbar fascia, it rotates the lumbar spine contralaterally and, by its attachments to the anterior ilium, it rotates the ilium ipsilaterally; both actions are movements of nutation.
Origin: costal cartilages of the lower 6 ribs, diaphragm, and the thoracolumbar fascia between the 12th rib and the iliac crest, anterior 2/3 of the iliac crest, lateral 1/3 of the inguinal ligament, and fascia over Iliacus muscle.
Insertion: pubic crest and posterior layer of the aponeurosis of the internal oblique.
- May be the primary muscle responsible for spinal stabilization
- Controls the abdominal contents
- Increases Internal Abdominal Pressure (IAP) in coordination with the diaphragm and pelvic floor musclesP1064
- Maintains abdominal tone P1064
- Controls flexion (through the posterior layer of thoracolumbar fascia) Approximates the lumbar spinous processes, producing lumbar extension/increased lordosis p92
- Functions independently of other abdominal muscles through the central nervous system  
- Normally the first muscle activated In limb movement, but this activation is significantly delayed in peoplewith back pain [6, 7] p63-64, which may be due to reflex inhibition p66-68, which, in turn, may be due to anutation lesion
- Active during both inspiration and expiration but is active earlier during forced expiration than during forced inspiration 
- Approximates the spinous processes of the lumbar vertebrae by tensioning the thoracolumbar fascia during expiration  p56
- Draws the sacroiliac joint surfaces towards each other 
- Observation reveals that it pulls the innominate posteriorly, rotating the ilium ipsilaterally.
- Observation reveals that, by its attachments to the spinous processes, through the thoracolumbar fascia, it rotates the lumbar spine contralaterally.
The importance of the transversus abdominis is the primary muscle responsible for spinal stabilization can be seen through some of the following research studies.
Hodges [6, 7] and Richardson et al. p63-64 demonstrated that in people without back pain, the transverse abdominis was the first muscle active, by several hundred milliseconds, prior to arm or leg movement, regardless of speed, direction of movement, or anticipation. However, in people with back pain, the transverse abdominis usually contracted several hundred milliseconds after the prime mover, which may contribute to more instability, pain and dysfunction. These studies show the importance of the transverse abdominis in providing spinal stabilization in preparation for limb movement.
Richardson et al.p66-68discuss the above relationships and mention possible causes of the delayed response of the transversus abdominis and other abdominal muscles, including central nervous system influences, such as opposing muscle tightness, vestibular dysfunction, trunk muscle dysfunction, and reflex inhibition. Reflex inhibition has been linked to effusion, pain, ligament stretch, and capsular compression. Effusion, pain, and ligament stretch occur in the nutation lesion of the sacroiliac joint and capsular compression occurs when a sacroiliac belt is placed too tightly on an injured sacroiliac joint. Thus, it is reasonable that a sacroiliac nutation lesion may lead to a delayed response of the transversus abdominis and subsequent instability during arm or leg movement, including giving way.
Further, during relaxed breathing, the onset timing of the transverse abdominis was identical in inspiration and expiration. However, upon deeper breathing, the transverse abdominis was active earlier in expiration than inspiration p53. This indicates that the transverse abdominis is active in both nutation and counternutation but primarily in nutation. Also, because the transverse abdominis pulls the lumbar spine and innominates into extension, it provides a nutation effect.
In a normal, uninjured sacroiliac joint, Snijders , suggested that the transverse abdominis muscle can provide a force to enhance self-bracing of the sacroiliac joint. His statement refers to the fact that this muscle draws the joint surfaces towards each other, towards nutation, which is considered to be the position of stability. He does not take into account the effect of the muscle on a sacroiliac joint in which the ligaments that restrain nutation are sprained and the transverse abdominis pulls the joint towards the lesion; in this case, the transverse abdominis and other nutators will be inhibited, so the joint surfaces are not drawn together sufficiently to stabilize the SI joint; instead the counternutators surrounding muscles attempt to stabilize the SIJ by pulling it into counternutation.
Cresswell  found that the transverse abdominis was active in both phases of flexion and extension trunk movements, while the other abdominal muscles showed significantly more activity in flexion. Adding Valsalva’s Maneuver to flexion or extension showed that the transverse abdominis was more related to IAP than the other abdominal muscles. This study indicates that the transverse abdominis functions as a stabilizing muscle while the other abdominal muscles function more in joint movement. In an uninjured sacroiliac joint, nutation is considered the position of greatest stability. However, when the SIJ is injured and hypermobile (nutation lesion), nutation is the position of greatest instability.
Using Doppler imaging and vibration analysis, Richardson et al.  demonstrated that the transverse abdominis can act independently, and more effectively, from the rest of the trunk muscles to reduce laxity in the sacroiliac joint. As a nutation muscle, the transverse abdominis brings the sacrum and ilium closer together, enhancing stability, as long as the sacroiliac joint does not go past the normal range of motion. The motion was prevented by means of isometric exercises monitored by an inflatable pressure valve  p146-155.
Tesh et al. p56  demonstrated that the thoracolumbar fascia, by approximating the spinous processes of the lumbar spine, produced an extension moment (nutation). Since both the transversus abdominis and multifidus tense the thoracolumbar fascia during expiration, their combined action can be considered to assist spinal stability in a normal joint. Conversely, when malfunctioning, as in the nutation lesion, the resulting inhibition of this muscle can hinder spinal stability.
Damen  noted that “Due to the anterior attachment of the transversus abdominis muscle to the iliac crest, this muscle is ideally placed to act on the ilia to produce, in combination with stiff dorsal sacroiliac ligaments, compression of the SIJs.” It is suggested here that, since compression of the sacroiliac joints occurs in nutation, it can be assumed that this muscle promotes nutation. Because the transverse abdominis uses the dorsal ligaments as levers and because the dorsal ligaments would already be sprained in a nutation lesion, the transversus abdominis would be inhibited to keep stress off of these ligaments.
- Richardson, C., et al., Therapeutic Exercise for Spinal Segmental Stabilization in Low Back Pain. 1999: Churchill Livingstone.
- Standring, S., et al., eds. Gray’s Anatomy. 40th ed. 2008, Churchill Livingstone: London.
- Calais-Germain, B., Anatomy of Movement, ed. S. Anderson. 1993, Seattle, WA: Eastland Press.
- Cresswell, A.G., H. Grundstrom, and A. Thorstensson, Observations on intra-abdominal pressure and patterns of abdominal intra-muscular activity in man. Acta Physiol Scand, 1992. 144(4): p. 409-18.
- Richardson, C.A., et al., The relation between the transversus abdominis muscles, sacroiliac joint mechanics, and low back pain. Spine, 2002. 27(4): p. 399-405.
- Hodges, P.W. and C.A. Richardson, Contraction of the abdominal muscles associated with movement of the lower limb. Phys Ther, 1997. 77(2): p. 132-42; discussion 142-4.
- Hodges, P.W. and C.A. Richardson, Delayed postural contraction of transversus abdominis in low back pain associated with movement of the lower limb. J Spinal Disord, 1998. 11(1): p. 46-56.
- Tesh, K.M., J.S. Dunn, and J.H. Evans, The abdominal muscles and vertebral stability. Spine, 1987. 12(5): p. 501-8.
- Snijders, C.J., Transfer of Lumbosacral Load to Iliac Bones and Legs: Part 2 – Loading of the Sacroiliac Joints when Lifting in a Stooped Position. Clinical Biomechanics, 1993b. 8: p. 295-301.
- Damen, L., et al., Does a pelvic belt influence sacroiliac joint laxity? Clinical Biomechanics (Bristol, Avon), 2002. 17(7): p. 495-8.