Transversus abdominis

Key points

The transversus abdominis is the deepest of the three flat abdominal muscles, running around the belly like the front of a belt¹.
It squeezes the abdomen, for example when breathing out hard, passing urine or stool, and giving birth¹.
A review of 27 studies found delayed muscle onsets in chronic low back pain². In one study of 15 people with pain and 15 without, this muscle switched on later with pain, whichever way the arm moved³.
Other laboratory studies found its early activity was uneven between sides and changed with movement direction, challenging the idea of a two-sided stiffening role^4,5.
A review of 15 studies found strong evidence that changes in this muscle’s contraction thickness or feedforward activity, over non-surgical programmes, were unrelated to changes in back pain or disability⁶.

Function

A belt around the belly

The side wall of the abdomen has three layers of muscle¹. The external oblique is the outer layer, the internal oblique the middle one, and the transversus abdominis the deepest¹. Its fibres run across the abdomen, like the front of a belt¹.

Anterior lateral view of the trunk showing the rectus abdominis running from the lower ribs to the pubis inside its sheath, crossed by tendinous intersections, with the external oblique on the flank, and an inset of the internal oblique and transversus abdominis beneath. A lower panel shows the quadratus lumborum, psoas major and iliacus on the posterior abdominal wall. — Muscles of the abdomenBetts JG, Young KA, Wise JA, et al. Anatomy and Physiology, Figure 11.16. OpenStax, 2013. Access for free at openstax.org. (source, CC BY 4.0). Re-saved as a more compressed JPEG to reduce file size; same dimensions.

In a dissection of 26 cadavers (bodies donated for study), the fascicles (bundles of muscle fibres) ran level in the upper part⁷. Lower down, they angled more and more downward and toward the midline⁷. Thin partitions inside the muscle split it into regions, and the authors suggest each region may work differently⁷.

Where it attaches

At the back, the muscle reaches the lumbar spine (the lower back) through the thoracolumbar fascia, a sheet of tough connective tissue over the back⁷. In another dissection of 18 cadavers, it reached the tips of the lumbar transverse processes (small bony spurs on each side of a vertebra)⁸. It did so through a middle fascia layer, thicker at those tips⁸.

At the front, it helps build the rectus sheath, the casing of flat tendon sheets around the rectus abdominis⁹. An expert consensus describes the sheath’s back layer as the transversus aponeurosis (a flat sheet of tendon) fused with part of the internal oblique’s⁹. In the upper abdomen, the transversus reaches the sheath as muscle⁹. Lower down it reaches it as aponeurosis instead, because its muscular edge, the linea semilunaris, shifts outward and downward⁹.

At the top, its fibres interlock with the diaphragm (the main breathing muscle) under the lower edge of the rib cage⁹. At the bottom, it attaches to the ilium (the wide upper part of the hip bone) and to the pubis, part of the pelvis¹. With the internal oblique it forms the conjoined tendon, one tendon shared by two muscles, which inserts on the crest of the pubis¹⁰.

Its nerves

The 7th to 11th intercostal nerves, which run between the ribs, supply the side-wall muscles level by level⁹. So does the subcostal nerve, beneath the lowest rib⁹.

Squeezing, breathing and twisting

The transversus abdominis squeezes the abdomen¹. It does this when breathing out forcefully, passing stool or urine, and giving birth¹. In six seated adults, breathing recruited it in preference to the outer abdominal muscles¹¹. With extra carbon dioxide in the air, it became active before either outer muscle could be recorded¹¹. During deliberate efforts, such as speaking or pulling the belly in, it generally worked together with those outer muscles¹¹.

In six men, the authors concluded it appeared to be the abdominal muscle most consistently linked to changes in pressure inside the belly¹². In three people, the diaphragm was stimulated to raise belly pressure with no abdominal or back muscle activity¹³. Spine stiffness rose by 8% to 31% of resting levels, and it rose more when the pressure rose more¹³.

Another study looked at the sacroiliac joint, where the base of the spine meets the pelvis¹⁴. In 13 healthy people lying face-down, tightening the transversus alone reduced joint looseness more than a general abdominal exercise pattern¹⁴. Both reduced it¹⁴.

In six people without spinal pain, it was active during seated trunk rotation¹⁵. Its upper fibres worked in the opposite pattern to its middle and lower fibres¹⁵.

Breakdown

Strains, hernias and bone pulled loose

A side strain is an injury of the internal or external oblique or the transversus abdominis, commonly where they attach to the lower four ribs^16,17,18. In 10 first-class cricket fast bowlers with such an injury, 70% of MRI scans (detailed soft-tissue scans) fitted injury to these muscles near those ribs¹⁷.

An expert consensus calls one part of the transversus aponeurosis the Spigelian fascia⁹. It names defects there as the cause of a Spigelian hernia⁹. The conjoined tendon forms most of the inner back wall of the inguinal canal, a passage through the belly wall at the groin¹⁰.

In three of 18 cadavers, tension on the fascia that carries its attachment broke a lumbar transverse process⁸. The authors suggest the attachment fits a role in controlling single lumbar levels, or in avulsion (pulling-away) fractures of these spurs⁸.

Losing size

In five astronauts across seven six-month missions on the International Space Station, transversus thickness fell by 34.1%¹⁹. It increased again during a 15-day reconditioning programme on Earth¹⁹.

The debate over early activity in low back pain

Feedforward activity is a muscle switching on ahead of a movement, before the movement disturbs the body. Studies disagree on what its feedforward timing means, including in low back pain^3,4.

Studies that describe an early, direction-free stiffener. In 15 people without back pain making fast arm movements while standing, the transversus was always the first trunk muscle to switch on³. Its timing did not depend on the direction of movement, which the authors took to support a role in stiffening the spine³. In another study of 15 people, the transversus switched on before the shoulder muscle whatever the direction of movement²⁰. Timing of the obliques, rectus abdominis and multifidus (a deep muscle running along the spine) varied with direction²⁰.

In 15 people with low back pain and 15 matched controls, the transversus switched on significantly later in those with pain, for every arm direction³. With leg movements, it was delayed in every direction, while delays in other trunk muscles appeared only in some directions²¹. The authors read the delay as a sign of a problem in controlling the muscles³. They suggested it makes the muscles less efficient at steadying the spine³. A systematic review of 27 studies, pooling results where they could be combined, found delayed muscle onsets in chronic (long-lasting) low back pain².

Studies that describe a side-specific, direction-specific muscle. In six healthy people making eight kinds of arm movement, feedforward transversus activity was not equal on the two sides²². It varied with the direction of the disturbance, tracking the twisting force the arm put on the trunk²². A laboratory study with a repeated case study also found its early activity depended on arm direction and differed between sides⁴. The authors said its role as a two-sided stabiliser needs revising⁴.

In another laboratory study, the transversus did not usually contract on both sides at once; one of six people did⁵. The authors said this does not fit the idea of a unique two-sided stiffening role⁵. The same review that found delays calls their functional relevance unknown². It found no conclusive differences in movement, or in centre of pressure, where body weight is centred under the feet². For acute (recent) low back pain, it found only two studies, with conflicting results².

Recovery

How the muscle changes with exercise

A systematic review of 14 studies covered 395 healthy people²⁴. The greatest gains in side-wall muscle thickness on ultrasound came with local abdominal exercises²⁴. They also came with programmes of four weeks or more, three sessions a week, and sessions of 30 minutes or longer²⁴.

Hollowing and bracing

A scoping review, which maps the research on a topic, describes hollowing as gently drawing in the lower belly below the navel²⁵. The upper belly, back and pelvis stay still²⁵.

It describes bracing as tightening all four abdominal muscles and pushing the waist out, without drawing in²⁵. Covering 22 randomised controlled trials, it reports that hollowing is often linked to selective transversus activity²⁵. Bracing works the trunk muscles together more broadly²⁵. Few trials compared the two directly, leaving open whether they work equally well²⁵.

Studies in people with chronic low back pain

Motor control training is exercise that trains how the deep trunk muscles are switched on and coordinated. A Cochrane review of 29 trials covered 2,431 people with chronic non-specific low back pain²⁶. Low to high quality evidence showed motor control exercise was not clinically more effective than other exercise²⁶. Against minimal intervention, low to moderate quality evidence showed it improved pain, with medium effect sizes (a standard measure of how large a difference is)²⁶.

In chronic low back pain, a meta-analysis found a greater transversus contraction ratio after motor control training than other approaches²⁷. The standardised mean difference, the gap on a common scale, was 0.93 (95% confidence interval −0.0 to 1.85)²⁷. It found no difference in resting thickness in programmes shorter than 12 weeks²⁷.

In a randomised trial of 109 people with chronic low back pain, deep abdominal muscle timing in fast arm raises barely changed, if at all²⁸. This held after 8 weeks of low-load core stability, high-load sling or general exercise²⁸. Changes in timing were not linked to changes in pain²⁸.

A review of 15 studies looked at non-surgical programmes⁶. It found strong evidence that changes in contraction thickness or feedforward activity were unrelated to changes in pain or disability⁶. A review included five cohort studies, which follow a group over time²⁹. It found limited evidence that transversus contraction thickness and early switch-on at the start were unrelated to later pain after exercise programmes²⁹.

Connections

The other abdominal muscles

During trunk rotation, the internal oblique, transversus and latissimus dorsi are described as acting on the same side³⁰. The external oblique, rectus abdominis and multifidus act on the opposite side³⁰. During repeated arm raises, the transversus and internal oblique opposite the moving arm switched on earlier than on the other side⁴.

The back muscles and the legs

Along the edge of the sheath around the back muscles, such as the erector spinae, a seam forms where it meets the transversus aponeurosis³¹. A review describes it as placed to spread tension from nearby muscles into the thoracolumbar fascia³¹.

One laboratory study recorded six pairs of trunk and leg muscles during arm movements⁵. With one-sided arm movements, early activity, transversus included, usually followed a diagonal pattern that depended on direction⁵. It included a twisting motion that passed energy up from the ground⁵.

Exercises that load it

Studies of the bird dog report these findings^32,33,34:

A systematic review reports that the bird dog produced greater transversus activity than internal oblique or multifidus activity³².
In 15 healthy men, bracing and hollowing each raised shear modulus (a measure of tissue stiffness from ultrasound) significantly in the transversus alone³³.
In 44 healthy young adults, the bird dog and side plank with hollowing gave the most relative thickening of the transversus, internal oblique and multifidus³⁴.

Related exercise pages: bird dog and side plank³⁴.