Core of the Matter

Dr. Stephen Flanagan

Welcome to the Flanatomy podcast. I’m your host, Dr. Stephen Flanagan. We’re diving into the wild world of anatomy with a side of crazy stories from my bizarre life. Let’s explore what makes you, you!

Keshia Rayna

And I’m Keshia Rayna, your co-host, keeping it one hundred and making sure Doc doesn’t get lost in his wild tales. I’m locked and loaded with my phone to fact-check and break down the nerdy stuff so y’all can vibe with the science.

Dr. Stephen Flanagan

Alright Keshia, today we’re moving south of the head and neck—down to the trunk. The core real estate of human strength, stability, and vanity.

Keshia Rayna

Vanity? You’re starting spicy already, Professor. You mean the abs, right?

Dr. Stephen Flanagan

Exactly. The anterior trunk muscles are the billboard of the human body. They get way too much attention for how little they do in isolation. But before we hit the abs, we’ve got to talk about the pecs and the serratus—the gateway muscles to the rest of the trunk. These are the muscles King Kong pounds on to show how strong he is. Pectoralis comes from the Latin pectus, meaning “chest.” Major just means “larger.” Straightforward—big chest muscle. It’s a thick, fan-shaped slab that originates from your clavicle, sternum, and the cartilage of your first six ribs, then converges on the humerus. Functionally, it’s the reason you can bench-press, hug a tree, or throw a punch.

Keshia Rayna

So the ultimate “self-hug” muscle. What’s funny is that in cats, it looks totally different, right?

Dr. Stephen Flanagan

Yeah, in cats, the pectoral complex is split into pectoralis major, minor, and profundus layers, and they’re narrower. Cats don’t need broad pecs for bench pressing—they need streamlined musculature for forelimb adduction during running and pouncing. Humans evolved this massive fan because we’re tool-users; we bring our arms forward, not just down.

Keshia Rayna

Translation: cats are stealth hunters, humans are awkward orangutans that made hammers.

Dr. Stephen Flanagan

Pretty much. And when people say “pec day,” they’re recruiting a lot more than just this one muscle—it’s assisted by its smaller sibling.

Dr. Stephen Flanagan

The pectoralis minor lies directly beneath the major. Minor means smaller, of course. It originates from ribs 3 through 5 and inserts on the coracoid process of the scapula. Its main job is to pull the scapula forward and downward—think of reaching into your back pocket or shrugging toward your toes.

Keshia Rayna

So it’s like the “hunch muscle.”

Dr. Stephen Flanagan

Exactly. And it gets tight in people who sit hunched at computers all day, shortening over time and pulling the shoulders into that rolled-forward posture.

Keshia Rayna

As a trainer, I see that constantly. Half my clients don’t need to “train chest”; they need to stretch it. Pec minor’s the secret villain behind neck tension.

Dr. Stephen Flanagan

Next up, the serratus anterior. The word serratus comes from serra, meaning “saw.” Look at the side of a lean torso—you’ll see those saw-tooth projections between the ribs and the scapula. They look like the edge of a circular saw blade. A lot of the gym bros might refer to these as the "fingers" because when well built the serratus anterior will look like the fingers of a giant hand on the lateral aspect of your trunk.

Keshia Rayna

That’s the one that gives boxers that rib-cage definition, right?

Dr. Stephen Flanagan

Exactly—the serratus anterior is also called the “boxer’s muscle.” It protracts the scapula—think of throwing a punch or pushing a door open. Originates from the upper eight ribs and inserts along the medial border of the scapula’s anterior surface.

Keshia Rayna

And when it’s weak, people get that weird winged-scapula thing?

Dr. Stephen Flanagan

Yes—if the long thoracic nerve gets damaged, the scapula literally wings off the rib cage like a door on a broken hinge. I had a student who called it “angel shoulder.” Accurate, but less divine when you can’t do a push-up.

Keshia Rayna

Okay, now the glamour muscles—abs.

Dr. Stephen Flanagan

Here we go. The external abdominal oblique is the most superficial of the lateral abdominal wall muscles. Oblique means “slanting.” Its fibers run downward and medially—like hands in your pockets.

Keshia Rayna

That’s actually how I teach it to clients—“hands in pockets” equals external obliques.

Dr. Stephen Flanagan

Good mnemonic. Functionally, they flex and rotate the trunk. You contract both sides—you crunch forward. One side—you twist. The external oblique also compresses abdominal contents, so it’s a player in respiration and stability.Here’s a fun bit: in anatomy labs, students notice how thin this muscle really is. It’s the overlying fascia that gives the abs their “armor-plated” appearance when lean.

Keshia Rayna

Right, about that—abs are the diva muscles. Everyone wants to see them, but few people earn that ticket.

Dr. Stephen Flanagan

True. I’m walking around at, let’s just say, “academic” body fat levels—my abs are in witness protection. But they’re under there somewhere, stabilizing every time I pick up a sandbag.

Keshia Rayna

Meanwhile, I’m the one with visible abs, but ask me to carry your 150-pound sandbag, and I’ll pass. Vanity doesn’t equal functional strength.

Dr. Stephen Flanagan

Exactly. Function over form. That brings us to the next layer.

Dr. Stephen Flanagan

Underneath the external oblique is the internal oblique. The fibers run perpendicular—upward and medially, like hands on your chest. Internus means “inner.” It’s a mirror-angled layer providing cross-bracing to the external oblique.

Keshia Rayna

So they’re woven like Kevlar?

Dr. Stephen Flanagan

Perfect analogy. The alternating fiber directions create a crisscross lattice that resists torsion. When you swing a bat or a golf club, both layers fire together. Internal obliques also assist in forced expiration and compress abdominal viscera. That compression stabilizes the spine—critical for lifting.

Keshia Rayna

So when you’re deadlifting or hoisting those sandbags, your obliques are basically your natural weight belt.

Dr. Stephen Flanagan

I think that is a great way to think about the obliques! I always tell students, your core is the biological version of a lifting belt—minus the Velcro and gym selfies.

Dr. Stephen Flanagan

Now for the poster child—rectus abdominis. Rectus means “straight.” Two long parallel muscles running from the pubic crest to the xiphoid process and costal cartilages of ribs 5–7. Those horizontal bands you see? They’re tendinous intersections dividing the muscle into the six—or eight—pack.

Keshia Rayna

Depending on genetics and lighting.

Dr. Stephen Flanagan

Exactly. It flexes the vertebral column, like in a crunch, but its true glory is posture control and maintaining intra-abdominal pressure. Every time you laugh, cough, or lift, it stabilizes the trunk.

Keshia Rayna

And aesthetically, it’s the muscle everyone obsesses over. But functionally, it’s kind of boring, right?

Dr. Stephen Flanagan

It’s honest work. Not glamorous, but essential. And since you brought it up—abs don’t appear because you train them; they appear when subcutaneous fat is low enough. Which, for most humans, means a level of leanness the body doesn’t love to maintain.

Keshia Rayna

So your abs are there, Professor—they’re just shy.

Dr. Stephen Flanagan

They’re introverted. Prefer staying under wraps.

Dr. Stephen Flanagan

Deepest layer: transverse abdominis. Transversus means “across.” Its fibers run horizontally, encircling the abdomen like a corset. Originates from the iliac crest, lumbar fascia, and lower ribs, inserting at the linea alba.

Keshia Rayna

That’s the real “core” muscle trainers talk about, right?

Dr. Stephen Flanagan

Yes. It doesn’t flex your spine; it stabilizes it. Think of it as the internal pressure regulator. When you brace before a lift, that’s your transverse abdominis tightening, pushing abdominal contents inward, stabilizing the lumbar spine.

Keshia Rayna

So when I cue someone to “brace like you’re about to get punched,” that’s transverse abdominis engagement.

Dr. Stephen Flanagan

Exactly. It’s also vital for respiration, supporting the diaphragm during forced exhalation. Weak transverse equals back pain. Strong transverse equals longevity in the gym and at your desk.

Keshia Rayna

So anteriorly, it’s layers on layers—external, internal, transverse—plus rectus running vertically down the front. Together they make this dynamic pressure system.

Dr. Stephen Flanagan

Right. The abdominal wall is both armor and engine. It protects visceral organs while enabling trunk movement and breathing. And if you dissected a cat—strictly in lab, of course—you’d notice those same layers, but thinner and more fused, since quadrupeds rely less on spinal flexion.

Keshia Rayna

Humans traded speed for core control. We stand upright, so our abs evolved into an anti-gravity device.

Dr. Stephen Flanagan

Nicely put. Evolution sculpted us for balance and breath, not just aesthetics.

Keshia Rayna

Speaking of breath, you ready to talk about the deep respiratory muscles? Because I’m still thinking about your “sandbag hobby”—I saw your last post, by the way.

Dr. Stephen Flanagan

Ah, the sandbag saga. Let’s just say my diaphragm got more work than my arms that day. Perfect segue, actually—because when you’re huffing through a strongman carry, that’s when you meet the real MVP of respiration.

Dr. Stephen Flanagan

So, let’s talk respiration. When you’re gasping after a heavy set or hauling a sandbag, your diaphragm and intercostals are in a full symphony.

Keshia Rayna

You mean like when you finish a run and suddenly realize your lungs have opinions?

Dr. Stephen Flanagan

Exactly. The diaphragm is the conductor of that orchestra. Etymology check: dia- means “through,” and phragma means “fence” or “partition.” It’s literally the wall between the thoracic and abdominal cavities. Dome-shaped, innervated by the phrenic nerve—“C3, 4, 5 keep the diaphragm alive.”When it contracts, it flattens, enlarging the thoracic cavity and pulling air in. That’s inspiration.

Keshia Rayna

And expiration is just the diaphragm relaxing back into that dome, shrinking the space, and pushing air out?

Dr. Stephen Flanagan

Passive expiration, yes. But once you start exerting yourself—say, you’re lifting a 150-pound sandbag and trying not to swear on camera—you recruit help.

Keshia Rayna

That’s your famous “sandbag circuit,” right?

Dr. Stephen Flanagan

Indeed. Pick up a sandbag, carry it 50 feet, shoulder it, drop it, and repeat until your vision goes sepia. First lift: diaphragm does the work. Second lift: internal intercostals kick in. By the third, your external intercostals, scalenes, and even your sternocleidomastoid join the party just to get air moving.

Keshia Rayna

So your trunk becomes one giant bellows.

Dr. Stephen Flanagan

Exactly. Intercostals—the muscles between the ribs—literally mean “between the ribs.” There are external and internal sets.The external intercostals run obliquely downward and forward, like the external oblique, and they elevate the ribs—inhale. The internal intercostals run perpendicular, downward and backward, depressing the ribs—exhale during forceful breathing.

Keshia Rayna

So, it’s like a layered accordion expanding and compressing.

Dr. Stephen Flanagan

Perfect analogy. And for students listening: these small muscles are vital in stabilizing the ribcage too. You wouldn’t believe how much proprioceptive feedback they give during deep breathing.

Keshia Rayna

I’ve noticed during intense workouts, people start using neck muscles to breathe—those scalenes and sternocleidomastoids pop out like cables.

Dr. Stephen Flanagan

Exactly. Accessory breathing muscles. When your diaphragm tires, you compensate. Chronic stress, bad posture, or tight pec minors make that your default, which is why so many folks live in a state of low-grade breathlessness.

Keshia Rayna

That’s actually why I teach diaphragmatic breathing before any heavy lifting. If your breath pattern’s off, your trunk stability’s shot.

Dr. Stephen Flanagan

Absolutely. Breath is biomechanics. It’s why singers, powerlifters, and yogis all end up preaching the same gospel: learn to use your diaphragm.

Keshia Rayna

And the cool cross-species tie-in—some of these respiratory muscles correspond to prime cuts in butchery, right?

Dr. Stephen Flanagan

Ha, yes. In cows, the diaphragm is the skirt steak; intercostals contribute to short ribs. Morbid, but instructive. Just remember, the next time you’re grilling, you’re basically looking at the architecture of breath.

Keshia Rayna

Educational and mildly disturbing. My favorite combo.

Dr. Stephen Flanagan

Alright, spin around. Posterior trunk—where real power hides. We’ll start with the deltoid, though technically it bridges the shoulder and upper trunk.

Keshia Rayna

Etymology?

Dr. Stephen Flanagan

From the Greek letter delta, shaped like a triangle. In humans, it’s one big muscle; in cats, it’s three—acromiodeltoid, spinodeltoid, clavodeltoid. Function: abduction, flexion, extension of the arm depending on which fibers you use.

Keshia Rayna

So, kind of like a 3-in-1 shoulder engine.

Dr. Stephen Flanagan

Exactly. Below it, teres major—Latin for “round and large.” Originates on the inferior scapula, inserts on the humerus. It adducts and medially rotates the arm, a helper to the latissimus dorsi.

Keshia Rayna

And speaking of lats—your specialty.

Dr. Stephen Flanagan

Ah yes, my pride and posterior joy. Latissimus dorsi means “broadest of the back.” Huge, fan-shaped, originating from the thoracolumbar fascia, iliac crest, and lower ribs, inserting on the humerus. It’s the muscle of pull-ups, swimming strokes, and heroic cape poses.

Keshia Rayna

You do pull-ups impressively for a “bulky academic.”

Dr. Stephen Flanagan

That’s generous. The lats are the wingspan of the human body. Evolutionarily, they connect upper limb to spine, giving us that powerful adduction and extension needed for climbing and, nowadays, hauling groceries.

Keshia Rayna

And above them sits the trapezius.

Dr. Stephen Flanagan

Right—the trapezoid-shaped muscle from occipital bone down to T12. It’s divided functionally: upper fibers elevate and rotate the scapula, middle fibers retract, lower fibers depress. It’s like three muscles in one.

Keshia Rayna

The trap game. People train it for that “yoked” look, but it’s also crucial for posture.

Dr. Stephen Flanagan

Yes. Without balanced trap strength, you end up with rounded shoulders and neck strain. Interestingly, the trapezius and latissimus meet near the midline in a beautiful diamond of connective tissue—the thoracolumbar fascia. A reminder that the back works as an integrated sheet.

Keshia Rayna

Then there’s the supporting cast: levator scapulae, rhomboid major and minor, splenius capitis, erector spinae, and of course, the rotator cuff.

Dr. Stephen Flanagan

Let’s break those down. Levator scapulae—Latin “raiser of the scapula.” Originates from C1–C4 vertebrae, inserts on the scapula’s superior angle. Elevates and rotates the scapula downward.The rhomboids—so named for their rhombus shape—sit deep to the trapezius, retracting the scapula like you’re pinching a pencil between your shoulder blades.

Keshia Rayna

I cue that in posture training all the time. “Pinch an imaginary credit card.”

Dr. Stephen Flanagan

Then splenius capitis: splenion means “bandage,” caput means “head.” Looks like a wrap from the upper thoracic spine to the skull. When both sides contract, you extend your head; one side—rotation.

Keshia Rayna

So when students nod off in class, that’s eccentric control of their splenius capitis failing.

Dr. Stephen Flanagan

Ha! Precisely. Then deep beneath lies the erector spinae group—iliocostalis, longissimus, spinalis. It’s the backbone of posture—literally. Extends and laterally flexes the spine. Chronic weakness here equals back pain city.

Keshia Rayna

And the rotator cuff quartet—supraspinatus, infraspinatus, subscapularis, teres minor.

Dr. Stephen Flanagan

Exactly. Together, they stabilize the humeral head in the glenoid cavity. Supraspinatus initiates abduction, infraspinatus and teres minor externally rotate, subscapularis internally rotates. They’re small but mighty—overlooked until injured.

Keshia Rayna

As every gym bro learns the hard way.

Dr. Stephen Flanagan

Indeed. And in cats, you can see how these posterior muscles form sleek, layered sheets enabling fluid locomotion. Humans adapted them for endurance and manipulation rather than sprinting and pouncing.

Keshia Rayna

So, front and back together make the trunk a literal powerhouse. What fascinates me is how breathing and movement are intertwined—how your abs assist your lungs, and your back supports both.

Dr. Stephen Flanagan

Right. The trunk is a closed cylinder of pressure and motion. The diaphragm is the lid, the pelvic floor the base, abdominal walls the front, and erector spinae the back. Everything you do—lifting, laughing, singing—depends on this pressure regulation.

Keshia Rayna

And dysfunction anywhere in that system shows up everywhere—weak transverse abdominis, tight hip flexors, stiff thoracic spine.

Dr. Stephen Flanagan

Exactly. That’s why understanding anatomy isn’t just memorizing origins and insertions—it’s seeing how form follows function.

Keshia Rayna

And how personality sneaks in. Abs are the extroverts—showy and social media friendly. The back muscles are introverts—quietly holding everything together.

Dr. Stephen Flanagan

Ha! I like that. And the diaphragm? The therapist. Regulates everyone’s emotions through breathing.

Keshia Rayna

So true. You can tell how someone’s day is going by their posture and breathing.

Dr. Stephen Flanagan

The somatic window to the psyche. You can’t fake relaxed intercostals.

Keshia Rayna

If that’s not a t-shirt quote, it should be.

Dr. Stephen Flanagan

Final takeaway for the students: the trunk muscles are a community. The anterior group flexes and compresses; the posterior group extends and stabilizes; the respiratory group inflates the whole system with life.

Keshia Rayna

And evolution tuned that community from quadruped locomotion to upright balance. Cats and humans share the blueprint, but we built skyscrapers on theirs.

Dr. Stephen Flanagan

Exactly. Next time you breathe deep or sit tall, thank your trunk. It’s doing graduate-level biomechanics just to keep you from collapsing.

Keshia Rayna

I’ll never take a sit-up—or a breath—for granted again.

Dr. Stephen Flanagan

Nor will I take my next sandbag session lightly. I’ll be hearing the Latin roots of every muscle screaming back at me.

Keshia Rayna

You always say pain is the best mnemonic.

Dr. Stephen Flanagan

It is. Pectoralis major? Burning. Rectus abdominis? On fire. Diaphragm? Cramping. Education through sensation.

Keshia Rayna

Alright, professor, before you hyperventilate on-air, we’ll call it there.

Dr. Stephen Flanagan

Fair. That’s the trunk—front, back, and breath. Respect it, train it, and remember: anatomy isn’t just about parts—it’s about the stories your body tells when it moves.

Keshia Rayna

Beautifully said. See you all next time on Flanatomy.

Dr. Stephen Flanagan

Where the jokes are corny, the Latin is correct, and the body is always fascinating.

Dr. Stephen Flanagan

I am thinking we should grill up some skirt steak with chimichurri sauce, what do you think?

Keshia Rayna

Sounds great Doc! Let's eat. Bye everybody.