Internal oblique

Function

Where it lies and how its fibres run

In the classic account, the side and front abdominal wall lies in layers¹. The internal oblique sits between the external oblique outside and the transversus abdominis inside¹. Its fibres run at right angles to those of the external oblique, up and toward the midline².

A study of 26 embalmed cadavers (bodies donated for study) looked more closely⁷. There, the upper and middle fascicles (bundles of muscle fibres lying side by side) ran up and toward the midline⁷. The lower fascicles ran down and toward the midline⁷. In its lower and middle regions, the muscle could be split into two distinct layers⁷.

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.

What it attaches to

A baseball injury study describes the internal oblique as starting from the iliac crest, the curved top rim of the hip bone⁴. It also starts from the inguinal ligament, a band across the groin⁴. A third start is the thoracolumbar fascia, a sheet of tough connective tissue over the lower and middle back⁴. It ends on the lower ribs and nearby tissue, including the rectus sheath, the casing of flat tendon sheets around the rectus abdominis⁴. Both obliques and the transversus abdominis attach to the ilium, the wide upper part of the hip bone in the pelvis².

Toward the front, the muscle helps build casings and walls^8,9:

The rectus sheath around the rectus abdominis is made from the coverings of the three flat side muscles⁸.
Its aponeurosis (a broad, flat sheet of tendon), as usually described, splits to pass in front of and behind the rectus muscles¹⁰.
The conjoint tendon forms from the sheet the internal oblique shares with the transversus abdominis⁹. Its fibres attach to the pubic crest, the ridge along the top of the pubic bone⁹. It makes up most of the inner part of the back wall of the inguinal canal, a passage through the groin⁹.

Its nerves

An expert anatomy consensus describes the side abdominal wall muscles as supplied by the 7th to 11th intercostal nerves, which run between the ribs¹¹. The subcostal nerve, which runs beneath the lowest rib, supplies them too¹¹. In 40 dissected half-walls of the abdomen, the 10th intercostal nerve was consistently found between these two muscles¹². It sent a median (middle value) of nine branches to the internal oblique¹².

How it twists the trunk

OpenStax, an anatomy textbook, names both obliques as prime movers, the main muscles, for twisting at the waist². It names them for bending the spine to the side too². One trunk-rotation study cites earlier work: turning toward the same side, activity reached 58% of maximum voluntary contraction, the hardest possible effort¹³. Turning the other way, it was 16%¹³.

In 25 volunteers twisting as hard as they could, the internal oblique reached 55% of the maximum it showed in non-twisting efforts¹⁴. Opposing trunk muscles worked together to a large degree, known as co-contraction¹⁴. The author took this to suggest that holding the lower-back joints steady matters more in twisting than making a large turning force¹⁴.

Pressure and timing

Six men did repeated, varied breath-hold strains¹⁵. Peak pressure inside the abdomen tracked the activity of the internal oblique and the other abdominal muscles measured, though not the rectus abdominis¹⁵. The same men also pushed the trunk forward while holding a maximal breath-hold strain¹⁵. Internal oblique activity rose markedly¹⁵. Pushing backward, it fell, unlike the transversus abdominis¹⁵.

Studies of when the muscle switches on give different pictures for different tasks^16,17:

In 15 healthy people moving the hip, trunk muscles switched on before the leg’s main mover¹⁶. The obliques’ reaction time was consistent whichever way the hip moved¹⁶.
In 15 people making fast arm movements while standing, the obliques’ start time varied with the direction of movement¹⁷.
In 11 people without disabling low back pain making fast arm movements, the middle and lower regions switched on at similar times¹⁸. In the six who also moved the arm while sitting, the abdominal muscles switched on later sitting than standing¹⁸.

Breakdown

Where side strains tear it

An MRI study of athletes with side strain found the internal oblique torn away from the underside of one of the lower four ribs³. The tear could also be from one of their costal cartilages, the strips of cartilage that join ribs to the breastbone³. A narrative review of side strain in sport found that imaging typically shows the internal oblique torn from the lower ribs¹⁹. A study of one professional baseball team also found most strains at the lower ribs⁴. MRI, a scan that shows soft tissue in detail, can show where the tear is and how severe it is³.

In a case series (a report on several patients) of four cricketers, the authors conclude side strain comes from eccentric loading of the torso²⁰. That is load while the muscle lengthens²⁰. They describe it as rare where these muscles attach to the pelvis²⁰. In those four cricketers, the injuries involved only the internal oblique’s attachment to the iliac crest²⁰. In one, MRI showed a tear right through the front half of that attachment, 4 cm long²⁰.

A case report (a report on a single patient) describes a professional tennis player with an internal oblique strain²¹. It came from an unbalanced twist of the trunk while the muscle lengthened, and MRI confirmed it²¹.

Strains in baseball

Across the major and minor leagues from 2011 to 2021, there were 1,928 oblique injuries that cost playing time²². The number per season rose significantly²². These counts are for oblique injuries, not the internal oblique alone²².

One Japanese professional team of 188 players was followed for a decade⁴. Of these, 23 players (12.2%) had an internal oblique strain confirmed by MRI⁴. Fielders had it significantly more often than pitchers⁴:

Group	Share with an internal oblique strain
Fielders	24.7%⁴
Pitchers	7.5%⁴

In that team, most strains were where the muscle attaches to the lower ribs⁴. They were more common on the side opposite the batting or throwing side, but happened on either side⁴. The authors state that swinging a bat turns the trunk toward that opposite side, which asks a lot of the internal oblique there⁴. Five of the 23 players (21.7%) had another strain, on average 36.5 months after the first⁴. Most second strains were on the side opposite the first⁴.

Thinning and weak spots

A systematic review, which gathers and sums up the research on a question, looked at trunk muscles in adults aged 50 and over²³. The internal oblique, rectus abdominis and external oblique were 36% to 48% thinner in older than in younger adults²³.

Recovery

Time away from sport

Average time out after a strain was measured in two baseball studies^4,22.

Average days out (days). Bars start at zero; the table below gives each value with its source.

Group	Average days out
Major and minor leagues, batting injuries	22 days²²
One Japanese team, internal oblique strains	27.7 days⁴
Major and minor leagues, pitching injuries	29 days²²

How it adapts to training

In a randomised trial of 73 people aged 18 to 45, 40 did 16 sessions of rotation exercises over four weeks²⁵. Their internal oblique grew thicker on ultrasound, a scan made with sound waves, compared with their own first scan²⁵. The published summary gives no comparison with the untrained group²⁵. In a one-time comparison, 16 female rugby players had a thicker right internal oblique than 16 women who did not play sport⁵.

An MRI study looked at professional tennis and soccer players²⁶. The tennis players’ extra oblique and transversus abdominis muscle was confined to the upper segments of the trunk²⁶. In the soccer players, it was similar along the trunk²⁶.

A meta-analysis looked at people with long-lasting low back pain²⁷. It found no difference in resting thickness between motor control training and other approaches²⁷. Motor control training trains how the deep trunk muscles are switched on and coordinated.

What happens without load

Five astronauts spent six months on the International Space Station, across seven missions⁶. Between the scans before and after flight, their internal oblique thinned by 15.4% on ultrasound⁶.

Connections

The internal oblique is described as working with the transversus abdominis and latissimus dorsi on the side the trunk turns toward¹³. The external oblique and rectus abdominis are described as working on the opposite side¹³. In 24 healthy young men turning the trunk on a machine, the external oblique showed more activity than the internal oblique or latissimus dorsi²⁸.

Different ways of tightening the belly have been studied^29,30,31:

Bracing is described as tightening the rectus abdominis, both obliques and the transversus abdominis together, pushing the waist out²⁹.
Hollowing, drawing the belly button in toward the spine, thickened the internal oblique more than bracing in 44 healthy young adults³⁰.
Posterior pelvic tilt, tucking the tailbone under, made the middle internal oblique the most active muscle in one study; bracing mainly used the external oblique³¹.

In the bird dog, 29 healthy volunteers showed the most activity in the internal oblique on the side of the raised arm³². The erector spinae, the long muscles beside the spine that straighten the back, peaked on the other side³².

Exercises that load it

A systematic review of healthy adults found the highest internal oblique activity in core stability exercises³³. Many of the results below use EMG (electromyography, which picks up a muscle’s electrical signals)³³; others measure thickness on ultrasound³⁰. They compare activity with MVIC, a muscle’s hardest effort while the joint stays still³³. Each row below comes from one study or one review^33,34:

Task	What was found
Front plank, shoulder blades drawn together and pelvis tucked (review)	About 120% of MVIC, the highest of the core stability exercises³³
Plank with the belly drawn in, 20 people	More activity than a plain plank³⁴
Plank with feet raised, 24 young adults	More activity than feet on the ground³⁵
Side plank (one study, in a review)	About 26% of maximum³³
Side plank with a hard breath out (one study, in a review)	About 55% of maximum, against about 18% in the ordinary side plank³³
Side plank, bird dog and beast crawl, 44 young adults	Largest thickness gain in the side plank, then the bird dog, then the beast crawl³⁰
Full bird dog against one arm raised, 30 women	More activity in the full position, except the right internal oblique³⁶
Bridge with one leg lifted, 15 adults	More activity on both sides than the standard bridge³⁷
Kettlebell swing with a forceful finish (review of three studies)	About 81% of MVIC, the highest of the free-weight exercises³³

The hollow hold page covers related holds such as the V-sit³⁸. In 30 adults, internal oblique activity did not differ detectably between a V-sit and a plank³⁸.