A patient said something recently that’s worth building a whole article around: “My abs are more than what you see in the mirror, right?” Exactly right. The six-pack is one muscle on the front of a system that wraps all the way around you — and the way most people train it (endless crunches) and cue it (suck your belly button to your spine) is, according to the research, close to backwards. Here’s what Dr. Stuart McGill — arguably the most important spine biomechanics researcher of the last several decades — actually found, and why it changes how you should think about your core.
The Core Isn’t the Six-Pack. It’s a Canister.
The visible “washboard” is one muscle: the rectus abdominis. It runs vertically down the front of your abdomen, and its main job is to flex your trunk — curling your ribcage toward your pelvis. That’s it. It’s one panel of a much larger structure.
Think of your core as a canister — a pressurized cylinder that surrounds and protects your spine from every direction:
The Core, All the Way Around
Your primary breathing muscle also forms the ceiling of the canister and helps generate internal pressure.
The base of the canister — it has to coordinate with the diaphragm to contain pressure.
The wraparound walls. The obliques and deep transverse abdominis matter far more for stability than the show-muscle rectus.
The deep and long muscles running along the spine that complete the cylinder.
Train only the front window of that canister with crunches and you’ve ignored most of the structure that actually protects your spine. Which leads to the single most important distinction in this whole topic.
Core Strength Is Not Core Stability
These get used interchangeably, and they’re not the same thing. Core strength is how much force your abdominal muscles can produce — how hard you can crunch. Core stability is your ability to resist unwanted movement of the spine while force flows through your body. One is about creating motion. The other — the one that protects your back and transfers power — is about preventing it.
McGill’s central insight is that the spine is like a flexible rod. On its own, it buckles under surprisingly little load. The job of the muscular canister is to stiffen that rod so it can bear load and transfer force without buckling. Stability comes from stiffness, and stiffness comes from the whole canister contracting together — not from one strong muscle.[1]
A six-pack is a strength display.
A stable core is a stiffness system.
The Big One: Bracing vs. Sucking In
Here’s where McGill overturned a widely taught cue. For years, people were told to stabilize their spine by hollowing — drawing the belly button in toward the spine to isolate the deep transverse abdominis. It sounds sophisticated. The biomechanics don’t support it as a stability strategy.
McGill’s alternative is bracing: a co-contraction of the entire abdominal wall at once — like the tension you’d create bracing to take a punch to the stomach. Not sucking in, not pushing out. A 360-degree stiffening all the way around the canister.
Bracing
Circumferential co-contraction
The whole abdominal wall tightens together, creating stiffness in every direction. McGill’s research measured significantly greater spinal stability with bracing.
Hollowing
Drawing the belly inward
Pulling the belly button toward the spine narrows the muscular base of support and, in stability terms, produces a less stable spine — the opposite of the intent.
Picture a ship’s mast held upright by guy-wires. The wires stabilize the mast best when they anchor to a wide base. Your obliques and abdominal wall are those guy-wires for your spine. When you brace, the wires anchor wide and stabilize in all directions. When you hollow, you pull that base inward — narrowing it and reducing its ability to stabilize. Studies comparing the two found bracing produced significantly greater spine stiffness and stability than hollowing; one analysis concluded there was no mechanical rationale for using hollowing to enhance stability.[2],[3]
Sources: Grenier & McGill, Arch Phys Med Rehabil 2007; Stanton & Kawchuk, Spine 2008.This isn’t 100% one-sided. Some research suggests hollowing-style activation has a place in specific low-load motor-control and rehabilitation contexts, and at least one study found a hollowing strategy performed well in certain hopping tasks. The fair summary: for general spinal stability under meaningful load — lifting, athletics, protecting a cranky back — bracing has the stronger evidence and is the better default. “Suck your belly in to protect your back” is the cue that hasn’t aged well.[4]
The Pressure System: Your Built-In Weightlifting Belt
Bracing does something beyond muscular stiffness — it pressurizes the canister. When the diaphragm descends and the abdominal wall tensions against it, you generate intra-abdominal pressure (IAP): a column of pressure inside your trunk that supports the spine from the inside out. It’s the same reason a weightlifting belt works — except you’re building the belt out of your own tissue.
This is where breathing enters, and where the cue “suck in” fails most obviously — you can’t suck your belly in and generate proper IAP at the same time. Instead, the goal is diaphragmatic breathing: breathing into your belly and lower ribs (a 360-degree expansion) rather than shrugging air into your upper chest. That breath sets the pressure; the brace holds it.
McGill’s research established that spinal stability comes from trunk-muscle co-contraction, and his own work with colleagues examined how the diaphragm and breathing interact with spine stability — so intra-abdominal pressure is very much part of this picture. The detailed breathing mechanics — rib positioning, diaphragm-to-pelvic-floor coordination, the 360-degree expansion cue — were refined largely through the Dynamic Neuromuscular Stabilization (DNS) approach out of the Prague School. In practice, the modern approach we use combines both: McGill’s bracing for stiffness, and DNS-style breathing to drive the pressure that supports it. Neither is “suck your belly button to your spine.”[5]
Why This Matters: Stiff Center, Athletic Limbs
Here’s the principle that ties it all together, and it’s the reason a strong-looking six-pack and a genuinely functional core aren’t the same thing. McGill’s phrase is: proximal stiffness enables distal mobility and athleticism.
Try flicking your finger back and forth quickly. To do it, your wrist has to stiffen — otherwise your whole hand just flops along and the finger has nothing to move against. Same with walking: your pelvis has to stiffen to your spine, or your hip would drop every time a leg swings forward. Your limbs can only produce force and speed against a stable base. A soft, unstable core leaks power and forces the spine to absorb loads it shouldn’t.[6]
For a golfer, a pitcher, a runner, a lifter — the stable core isn’t the thing that looks good at the pool. It’s the thing that lets you transfer force from the ground, through your trunk, and out into a club, a ball, or a stride without bleeding energy or grinding your lumbar discs.
So About Those Sit-Ups…
Now the part you’ve been waiting for. If the core’s real job is to resist movement and transfer force, then the classic sit-up — which trains the spine to repeatedly flex under load — is training the opposite of what you want. And the mechanics are worse than most people realize.
Most of a sit-up isn’t even an ab exercise
Your abs (the rectus abdominis) do their work in roughly the first 30 degrees of a sit-up — the initial curl that lifts your shoulder blades and ribs toward your pelvis. Once your trunk leaves the floor and you’re hauling yourself up toward your knees, that’s no longer abdominal work. It’s driven by your hip flexors — primarily the psoas, which attaches directly onto your lumbar vertebrae. So the majority of the movement isn’t training your abs at all; it’s hip-flexor work that pulls directly on your lower spine.
And that flexion under load adds up
The US National Institute for Occupational Safety and Health (NIOSH) set an action limit for low-back compression at 3,300 newtons — repetitive loading above that level is associated with higher injury rates in workers. McGill’s lab measured a straight-leg sit-up at roughly 3,500 newtons of spinal compression — above that limit, on every rep. Separately, his research showed that repeated spinal flexion, even under modest load, can gradually contribute to disc herniation as the repeated bending works the disc’s nucleus backward through its layers.[1],[7]
Sources: McGill, Low Back Disorders; Callaghan & McGill, Clinical Biomechanics 2001.Honesty check, because this claim gets wildly overstated online. McGill himself has pushed back on being misquoted about a fixed “number of bends” your spine gets — it’s a variable, not a countdown, and it depends on your individual disc shape, spine thickness, and loading history. Some researchers argue the spine is more robust and adaptable than the strictest “never flex” reading suggests, and they’re not wrong. The honest takeaway isn’t “sit-ups are evil.” It’s that they’re a poor risk-to-reward trade: high spinal load, mostly hip-flexor work, training a flexion pattern you don’t want — when far better options exist that build real stability without the downside.[8]
The Reframe: You’re Already Doing Core Work
Here’s the liberating part. If you brace properly, you don’t need to isolate your core — because nearly every compound movement becomes a core exercise on its own.
A heavy squat, a deadlift, an overhead press, a loaded carry, even a proper push-up — every one of these demands that your core resist being bent, extended, or twisted while load passes through it. That’s exactly what core stability is: anti-flexion, anti-extension, anti-rotation. When you brace and breathe correctly under a barbell, your core is working as hard as it ever needs to — as a stabilizer, in the role it’s actually built for.
Brace correctly under a heavy load,
and the lift is the ab workout.
This is why you’ll rarely catch a strong, resilient athlete grinding out hundreds of crunches. The stability demand is already baked into the way they lift and move. Chase the mirror with isolation crunches and you get a display muscle; train the whole canister to brace and transfer force, and you get a core that actually does its job — and often looks the part anyway.
Where to Actually Start
You don’t need a complicated program. You need to train the canister to stiffen and to resist movement — not to crunch. A few evidence-based starting points:
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Learn to Brace and Breathe
Before anything else: practice a 360-degree brace with diaphragmatic breathing. Tension the whole wall like you’re about to take a light punch, while still breathing into your lower ribs and belly — not sucking in.
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The McGill Big 3
McGill’s own foundation: the modified curl-up, the side plank (side bridge), and the bird dog. All three build stability and endurance without loading the spine into flexion. They train the canister to hold, not to crunch.
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Loaded Carries
Pick up something heavy and walk with it, braced. Farmer’s carries and suitcase carries are among the most honest anti-movement core exercises there are — and they carry straight over into real life and sport.
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Brace Everything Else
Once bracing is second nature, your squats, deadlifts, presses, and carries become core training by default. The core work is in how you move under load, not a separate set of crunches.
If you have current back pain, the Big 3 and bracing are generally safe and useful — but the cause of your pain should be assessed first, because the right starting point depends on what’s actually driving it. McGill’s own approach begins with identifying and removing the movements that trigger your pain, not just adding exercises on top. That’s a big part of what a proper assessment sorts out.
The Bottom Line
Your abs really are more than the mirror. The core is a full canister — diaphragm, pelvic floor, the entire abdominal wall, and the deep back muscles — and its real job is stability: resisting unwanted spinal movement and transferring force. The research says stiffness comes from bracing, not sucking in; that intra-abdominal pressure is your built-in weightlifting belt; that sit-ups are a poor trade of high spinal load for mostly hip-flexor work; and that a properly braced compound lift is itself a core workout.
Stop training your core for the mirror. Train it for the job it actually has — holding your spine steady so the rest of you can move well.
Want to Learn to Brace and Move Correctly?
We teach bracing, breathing, and the Big 3 as part of a plan built around your assessment — and we find what’s actually limiting your core or driving your back pain first. Let’s build a core that does its real job.
Book an Assessment ?References
- McGill SM. Low Back Disorders: Evidence-Based Prevention and Rehabilitation. Human Kinetics. (Spine stability via muscular stiffness; guy-wire model; sit-up spinal compression vs. NIOSH 3,300 N action limit; the Big 3.)
- Grenier SG, McGill SM. Quantification of lumbar stability by using 2 different abdominal activation strategies. Archives of Physical Medicine and Rehabilitation. 2007;88(1):54-62. (Bracing produced greater spine stability than hollowing; no mechanical rationale found for hollowing.) PubMed
- Stanton T, Kawchuk G. The effect of abdominal stabilization contractions on posteroanterior spinal stiffness. Spine. 2008;33(6):694-701. (Abdominal brace produced significantly greater posteroanterior spinal stiffness than hollowing.) PubMed
- Comparative studies of hollowing vs. bracing noting task-dependent findings, including Dupeyron et al. on hopping tasks; scoping reviews on abdominal hollowing vs. bracing for trunk stability and rehabilitation. (Basis for the honest caveat that hollowing may have niche low-load/motor-control applications.) PMC
- Wang S, McGill SM. Links between the mechanics of ventilation and spine stability. Journal of Applied Biomechanics. 2008;24(2):166-174. (Diaphragm/ventilation and intra-abdominal pressure relationship to spine stiffness.) On breathing mechanics and IAP coaching, see also Dynamic Neuromuscular Stabilization (Kolar / Prague School) literature. PubMed
- McGill SM. Distal mobility requires proximal stability; the linked-system model of core stiffness and force transfer. (Summarized across McGill’s writing and lectures; “proximal stiffness enables distal athleticism.”)
- Callaghan JP, McGill SM. Intervertebral disc herniation: studies on a porcine model exposed to highly repetitive flexion/extension motion with compressive force. Clinical Biomechanics. 2001;16(1):28-37. (Repeated flexion with modest compression produced disc herniation.) PubMed
- McGill SM, on the “fatigue life” of discs being a variable rather than a fixed number of cycles (backfitpro.com); and countervailing perspectives on spinal flexion robustness (e.g., Lehman; Somerset). Included for balanced representation of the ongoing debate. backfitpro.com
This article is for general educational purposes and does not constitute medical advice. If you have back pain, consult a qualified provider before beginning new core exercises — the right starting point depends on the cause of your pain.
