External oblique

Key points

The external oblique is the outer layer of the three flat muscles on each side of the abdomen¹.
It is named as a main mover for twisting the trunk and bending it to the side¹.
In Major League Baseball from 1991 to 2010, at least 92% of abdominal muscle strains were oblique strains or strains of the muscles between the ribs².
The side-strain counts and scans cited here do not separate the external oblique from the internal oblique, the middle of the three flat side muscles^2,3,4,5.
In one trial, the 40 people who did four weeks of rotation training had a thicker external oblique afterward; the published summary gives no comparison with the untrained group⁶. In one small study of five astronauts, the side and front abdominal muscles together, this one included, thinned over six months in space⁷.

Function

Where it lies and what it attaches to

On the flanks, the wall of the abdomen has three layers of muscle¹. The external oblique is the outer layer, the internal oblique is the middle one, and the transversus abdominis is the deepest¹. The external oblique’s fibres run down and toward the midline, like fingers slid into trouser pockets¹. The internal oblique’s fibres run at right angles to them, up and toward the midline¹.

The muscle starts on the lower eight ribs³. Below, it attaches to the front half of the outer edge of the iliac crest, the top rim of the hip bone³. Toward the midline, it ends in the linea alba, a band of tissue running down the centre of the abdomen³.

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.

It reaches the linea alba through an aponeurosis, a broad flat sheet of tendon⁸. This sheet passes in front of the rectus abdominis, the long muscle down the front of the abdomen⁹. In the upper abdomen, it is the main part of the front wall of the rectus sheath, the casing around that muscle⁸. Some centimetres below the navel, the internal oblique’s sheet also passes in front of the rectus abdominis⁹. The transversus abdominis’s sheet does the same from a level that varies⁹.

The intercostal nerves, which run between the ribs, supply the external oblique and the other abdominal wall muscles¹⁰. These nerves also carry feeling from the matching strips of skin¹⁰. In 40 dissected half-walls of the abdomen, the tenth intercostal nerve sent a median (middle value) of nine branches to this muscle¹¹.

How it twists and bends the trunk

OpenStax, an anatomy textbook, names both obliques as main movers for twisting at the waist¹. It also names them for lateral flexion, bending the spine to the side¹. In 24 healthy young men turning the trunk at set speeds on a machine, the external oblique opposite the turn was a main driver¹². The external oblique on the same side as the turn acted as an antagonist, working against the movement¹². Of the three turning muscles recorded, the external oblique showed the most electrical activity¹².

How held positions and lifts load it

Several exercise studies have recorded external oblique activity^16,17. In one small study of 13 people with low back pain, a braced front plank reached a median of 77% of the muscle’s session peak¹⁵. In 17 healthy people, a side plank with the upper body raised reached about 78% of maximum¹³. In 25 people, the rotational side-bridge reached 62.8% of maximum, the most of four side-bridge exercises tested¹⁴. The authors of a 30-person study concluded that the side bridge could be used to strengthen this muscle¹⁸.

Each row below compares two versions of a task within one study^19,20:

Task	External oblique activity
Prone exercises, including the plank on the toes, against a crunch	About 10 to 100% more in the prone exercises¹⁶
Long-lever plank with pelvis tucked, against a plain plank	Higher¹⁹
Plank with feet raised, against feet on the ground	Higher²¹
Plank with belly drawn in, against a plain plank	Higher²²
Side plank, weight-bearing side against the other side	Higher on the weight-bearing side, in one study of 25 people¹⁴
Farmer’s carry, against holding the same weights still	Higher on both sides¹⁷
One-hand suitcase carry, against holding it still	Higher on the loaded side, not on the other¹⁷
One-arm rows, against two-arm rows	Higher²⁰
Seated two-arm dumbbell press, against standing	About 58% lower when seated²³
Deadlift with a lifting belt, against without	Lower with the belt²⁴

In a held bridge, rectus abdominis activity was lower than oblique activity²⁵. Squeezing the knees together during a bridge raised external oblique activity²⁶. In 30 adults, no difference in external oblique activity was found between a held V-sit and a plank²⁷.

Breakdown

What a side strain is

A side strain is damage to one of three flat side muscles: the external oblique, the internal oblique or the transversus abdominis^3,4,5. It commonly occurs where these muscles attach to the lower four ribs^3,4,5. The authors of a report on four cricketers describe it as rare where the muscles attach to the pelvis³. A narrative review, a summary without a set search method, linked side strain mostly with cricket and baseball⁵. It usually came on suddenly, mostly on the side opposite the dominant arm⁵.

The side-strain counts and scans below do not separate the external oblique from the internal oblique^2,3,4,5. The narrative review above does name one muscle: it found that scans typically show the internal oblique torn away from the lower ribs⁵. In ten cricket fast bowlers, 70% of MRI scans, a type of detailed body scan, fitted injury to any of the three flat muscles⁴. The baseball counts group the two obliques with the muscles between the ribs².

How common it is in baseball

Across the 1991 to 2010 Major League Baseball seasons, 393 abdominal muscle strains made up 5% of injuries². At least 92% of these were oblique or intercostal strains². A later study counted 996 oblique injuries that cost playing time in the major and minor leagues from 2011 to 2015²⁸. Most happened while batting (46%) or pitching (35%)²⁸. Its authors describe sudden sharp pain on the side of the abdomen after a forceful twist²⁸.

Which side it affects

In these studies, the strains tended to fall on one side^2,4,28. The 1991 to 2010 baseball rows cover abdominal muscle strains in general²:

Group	Side most affected
Baseball pitchers, 1991 to 2010	Opposite the throwing arm, in 78.1%²
Baseball position players, 1991 to 2010	Opposite the batting side, in 70.3%²
Baseball players, 2011 to 2015	The leading side, in 77%²⁸
Cricket fast bowlers, one report of ten	The side of the non-bowling arm, the authors state⁴

In those ten bowlers, the most consistent signs were a tender spot and pain when bending against resistance toward the sore side⁴. For six of the ten, it was a repeat injury⁴.

Recovery

How long players are out

In these studies, average time away from sport after these strains ranged from about 22 to 39 days^28,29. Like the injury counts, these figures cover abdominal strains in general, not the external oblique alone^{2,3,4,5,28,29}. The 1991 to 2010 baseball figures also include strains of the muscles between the ribs².

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

Group	Average days out
Baseball players, 2011 to 2015	22.2 days²⁸
Baseball position players, 1991 to 2010	26.7 days²
Baseball pitchers, 1991 to 2010	35.4 days²
Cricket fast bowlers	39 days²⁹

In the later baseball study, pitchers lost five more days per injury than batters²⁸. Leading-side injuries took five days longer than trailing-side ones²⁸. Among 57 cricket fast bowlers, 44% needed more than six weeks²⁹. Bowlers whose MRI showed a muscle tear were eight times as likely to need that long²⁹. Repeat injury rates were 12.1% in the earlier baseball study and 8.15% in the later one^2,28.

How it adapts to training

In a randomised trial, 40 people did four weeks of rotation exercises⁶. Their external oblique grew thicker on ultrasound, a scan made with sound waves, compared with their own first scan⁶. The published summary gives no comparison with the untrained group⁶. In a small one-time comparison, 16 female rugby players had a thicker right external oblique than 16 women who did not play sport³⁰.

In one study, a one-sided sport went with more muscle on one side³¹. On MRI, professional tennis players had 18% more oblique and transversus abdominis muscle on the non-dominant side³¹. In soccer players and inactive people, the two sides did not differ significantly³¹. In people with long-lasting low back pain, a meta-analysis found no difference in resting external oblique thickness between motor control training and other approaches³².

What happens when load is taken away

Five astronauts spent six months on the International Space Station across seven missions⁷. The combined thickness of their side and front abdominal muscles, including the external oblique, fell by 16.2%⁷. It rose again during a 15-day reconditioning programme back on Earth⁷.

Connections

In trunk rotation, the external oblique is described as working on the side opposite the turn³³. The rectus abdominis and the lumbar multifidus, a deep lower-spine muscle, are described as doing the same³³. The latissimus dorsi, the broad muscle of the back, is described as working on the same side³³. So are the internal oblique and transversus abdominis³³. Whether the latissimus dorsi helps rotate the spine is disputed³⁴.

In ten healthy people, the muscles that bend the trunk forward and those that straighten it worked at the same time around an upright spine³⁵. The external oblique was one of six right-side muscles recorded, and this shared work rose with 32 kg added to the torso³⁵. A model explained it as keeping the lower spine mechanically stable³⁵.

Where it meets other structures

The external oblique joins or overlaps several nearby structures^36,37:

Latissimus dorsi: in nine dissected bodies, it merged with the external oblique at an average level of about the 9th to 11th rib³⁶.
Serratus anterior: this side-of-chest muscle interlocks with the external oblique’s start on the 5th to 9th ribs³⁷.
Pectoralis major: part of this chest muscle arises from the external oblique’s aponeurosis³⁸.
Thoracolumbar fascia: the external oblique’s covering ran straight into this sheet over the lower back in most bodies and scans studied³⁹.

In the rest, the external oblique’s covering first joined the latissimus dorsi’s covering: two of ten bodies and four of 27 scans³⁹.

Exercises that load it

Studies have recorded the muscle working in the plank, side plank, farmer’s carry and suitcase carry^13,16,17. They also recorded it in one-arm rows, standing presses, the deadlift and the held bridge^20,23,24,26. Exercise pages that load it: barbell row, deadlift, farmer’s carry, glute bridge hold, hollow hold, overhead press, plank, side plank.