Core Training Science: Why Crunches Are Outdated and What Actually Builds a Functional Core

Ask most people what the core does and they’ll mime a crunch — curling the ribs toward the hips. That answer is backwards.

A strong core resists movement more than it creates it. The best core training is anti-rotation, anti-extension, and anti-lateral-flexion stability work — not high-rep crunches that train a motion you almost never need to produce under load.

This is one of the most settled findings in spine biomechanics, and it has been settled for over a decade. Yet the crunch persists as the default. Let’s fix that.

What Does a Strong Core Actually Do?

The core’s primary job is to keep your spine from moving when everything else is trying to move it. Stiffness, not flexion, is the point.

Picture a sailboat. The mast doesn’t flap in the wind — the sail does. The mast stays rigid so it can transfer the wind’s force into forward motion. Your trunk is the mast. Its job is to stay stiff and channel force between your hips and shoulders, not to bend back and forth like the sail.

Anatomically, the “core” isn’t your six-pack. It’s a canister of roughly a dozen interacting muscle groups: rectus abdominis at the front, the internal and external obliques on the sides, the deep transverse abdominis wrapping around like a belt, the erector spinae and multifidus running up the back, the quadratus lumborum bridging ribs to pelvis, with the diaphragm as the lid on top and the pelvic floor as the base. They work as a unit to pressurize and brace the trunk.

Dr. Stuart McGill, professor emeritus of spine biomechanics at the University of Waterloo, spent a career documenting how this canister works. His central argument: by stiffening the torso, power generated at the hips is transmitted more effectively through a rigid core — proximal stability enables distal force.¹

That points to a distinction most gym programming ignores. Core stability means resisting unwanted motion and controlling a neutral spine. Core strength means producing force or torque through the trunk. Almost everything you do — carrying groceries, sprinting, pressing a barbell overhead, getting out of a chair — is a stability demand, not a flexion demand.

The Canadian Society for Exercise Physiology made the same case in its position stand on core training, led by Dr. David Behm of Memorial University of Newfoundland. The trunk muscles exist primarily to stabilize the spine and transfer load — and training them for that role, rather than chasing isolated abdominal contractions, is what carries over to health and performance.²

So the organizing idea for everything below is simple: train the core to resist the four ways your spine can be forced to move. We’ll get there. First, the crunch.

The Crunch Problem: What the Spinal-Loading Research Actually Shows

Crunches aren’t dangerous in small doses, but repeated end-range spinal flexion is a high-cost, low-return way to train the trunk — and it trains a movement most people never need to produce under load.

Here’s the honest version, because the internet loves to declare exercises “evil.” A few crunches won’t herniate your disc. The problem is using them as your primary core method, day after day, rep after rep.

The mechanism evidence comes from McGill’s lab partner, Dr. Jack Callaghan. In a porcine spine model, Callaghan and McGill subjected motion segments to highly repetitive flexion-extension under modest compressive load. Disc herniation appeared in the posterior and posterior-lateral annulus — driven more by the repeated bending than by the compression itself.³ That’s mechanism evidence in an animal model, not a clinical guarantee that your crunches will herniate a disc. But it tells you which variable to respect: cumulative flexion.

Then there’s the load math. A traditional sit-up imposes roughly 3,300 newtons of compression on the lumbar spine — about 730 pounds.⁴ For context, that figure sits right around NIOSH’s occupational action limit for low-back compression — 3,400 N — the threshold above which repetitive loading is associated with higher injury rates in workers.⁴ You’re hitting that number with each rep of an exercise whose main payoff is fatiguing the rectus abdominis.

To be fair: spinal flexion training has a place. Some athletes and rehab goals genuinely call for trained, tolerant flexion. The issue isn’t that flexion is forbidden — it’s that crunches are a poor default when you can build a far more capable trunk with near-zero flexion load.

Here’s what this means for you: you don’t have to give up “ab work.” You have to swap the pattern.

Stability Beats Reps: The Endurance-Over-Strength Finding

The research consistently links trunk muscular endurance and balanced flexor-to-extensor ratios — not raw strength or rep counts — to back health and force transfer.

This is the finding that should reorganize how you think about progress. It’s not “how many crunches can you do.” It’s “how long can your trunk hold a position before it fails, and are the front, back, and sides balanced.”

McGill, working with Childs and Liebenson, built a normative database for three isometric endurance holds — trunk flexor, trunk extensor, and the side bridge — and published clinical target ratios from healthy young adults.⁵ The diagnostic isn’t a single number; it’s the relationship between them. When the side-bridge endurance drops well below the extensor hold, or the flexor-to-extensor balance skews, that imbalance flags a trunk that isn’t equipped to stabilize evenly under load.⁵

The predictive power of trunk endurance shows up in long-run data too. In a classic prospective study of nearly a thousand adults in a Copenhagen suburb, Biering-Sørensen found that weaker isometric back-extensor endurance was a risk indicator for first-time low-back trouble over the following year, particularly in men.⁶ It’s a foundational, older finding — and it’s exactly why the back-extensor endurance test still carries his name. (If back pain is your entry point here, our guide to lower back pain exercises leans on the same stability-and-endurance principles.)

Why does endurance matter more than max strength for most people? Because stability is a sustained demand. A heavy deadlift is over in three seconds, but maintaining a stiff, neutral spine through a full set, a long carry, or a 90-minute match is an endurance problem. And a trunk that stays stiff lets your hips and shoulders produce force — the proximal-stability-for-distal-mobility principle McGill keeps returning to.¹

The Anti-Movement Framework: How the Core Is Actually Built

The cleanest way to build a functional core is to train it to resist the four directions your spine can be forced to move: extension, rotation, lateral flexion, and flexion under load.

Stop thinking in terms of “ab exercises” and start thinking in terms of resisted movement. Here’s the whole system in one table.

Category	What it resists	Sample exercises	Real-world / athletic transfer
Anti-extension	The spine arching backward	Dead bug, plank, hollow hold, ab-wheel rollout	Overhead pressing, sprint posture, staying braced under a bar
Anti-rotation	The torso twisting	Pallof press, bird dog, single-arm carry	Throwing, change of direction, rotational sport, punch resistance
Anti-lateral-flexion	The torso side-bending	Suitcase carry, side plank, single-arm farmer’s carry	Gait, single-leg stability, carrying a load on one side
Anti-flexion / bracing	Collapsing forward under load	Heavy carries, braced squat and deadlift	Every heavy lift you’ll ever do

The EMG evidence backs the choices in that table. A systematic review by Martuscello and colleagues compared core muscle activation across exercise types and found that free-weight, multi-joint lifts and ground-based stability work recruited the deep trunk stabilizers as hard as — and often harder than — isolated abdominal exercises.⁷ The squat and deadlift aren’t just leg movements; they’re some of the most demanding anti-flexion core work you can do.

Escamilla’s EMG lab compared traditional crunches and sit-ups against rollout- and pike-style movements and found the rollout and pike produced the highest activation across the upper and lower rectus abdominis and both obliques — while keeping lumbar paraspinal and hip-flexor activity low.⁸ Translation: the higher-yield movements light up more of the canister with less junk loading on the low back.

For the anti-rotation category specifically, recent work has gotten granular. A 2025 accelerometer study by Juan-Recio and colleagues measured the postural-control challenge of Pallof press variations and showed that standing and tandem-stance versions impose markedly higher lumbopelvic demand than kneeling ones — a clean, objective way to progress an anti-rotation exercise by changing your base of support rather than just adding load.⁹

A note on “functional,” because the word has been abused. Functional here means trained for a real demand pattern — bracing, resisting a twist, holding a stiff trunk under a load. It does not mean doing crunches on a wobble board. Gimmicky balance-circus work usually adds instability without adding the kind of trunk stiffness that transfers.

This is also where programming gets interesting, and where a data-aware coach earns its keep. Mapping anti-movement patterns onto a real training week is a structured decision: if your week already has heavy squats and deadlifts, your trunk is getting hammered with anti-flexion work before you add a single dedicated “ab” set. SensAI reads which heavy lifts already tax the trunk and skips the redundant generic ab day, slotting in the pattern you’re actually missing instead.

Does It Transfer? The Athletic Performance Evidence

Core training improves balance, trunk endurance, and some sport-specific markers — but transfer is specific: train the demand you want to improve, and expect modest, real gains rather than miracles.

Let’s be honest about effect sizes, because the honesty is the point. A 2025 meta-analysis in BMC Sports Science, Medicine and Rehabilitation found a large overall effect of core training on general athletic performance — a standardized mean difference of 1.38 (95% CI 0.86 to 1.89) — driven mostly by big gains in core endurance and balance.¹⁰ But the same analysis found core training had almost no effect on power and speed, and the agility effect was negligible and not statistically significant (SMD 0.10, p = 0.69).¹⁰ The trunk gets more durable and your balance improves reliably; the carryover to raw sport output like sprint speed and power is much weaker than the marketing implies.

That matches the older anchor in this literature. Reed, Ford, Myer, and Hewett reviewed isolated versus integrated core stability training and concluded the evidence for a direct line from core work to performance was inconsistent — integrated, task-specific training looked more promising than isolated core drills, but the direct performance link was weaker than the marketing suggests.¹¹

So here’s the framing that keeps you sane: core training is one input, not an injury-prevention magic bullet. Trunk endurance and overall movement competency matter more than any single clever exercise. And because transfer is demand-specific, the pattern you emphasize should match your goal — a rotational athlete should bias toward anti-rotation; an overhead presser or sprinter toward anti-extension. SensAI personalizes which pattern gets the volume based on your sport and goals, rather than handing everyone the same generic ab circuit.

What this means for you in practice: program two to three anti-movement patterns, two to three times a week, and progress them by load, leverage, and time — not by chasing a hundred crunches.

Your Core-Training Blueprint

Build a week around one anti-extension pattern, one anti-rotation pattern, and one anti-lateral-flexion pattern, trained two to three times weekly, plus loaded carries that cover bracing.

Here’s a concrete weekly template you can run:

Anti-extension: dead bug or ab-wheel rollout — 3 sets, controlled, stopping before your low back arches.
Anti-rotation: Pallof press or bird dog — 3 sets per side, resisting the pull, not muscling through it.
Anti-lateral-flexion: suitcase carry or side plank — 3 sets per side, even left and right.
Anti-flexion / bracing: this is largely covered by your heavy squats and deadlifts, where holding a braced, neutral spine under load is the whole game.

Progress by leverage, load, and time — not rep count. One pattern, four levels:

Level	Anti-extension progression
Beginner	Dead bug (back flat, slow limb lowering)
Intermediate	Front plank — short, hard, fully braced holds
Advanced	Ab-wheel rollout from the knees
Elite	Standing ab-wheel rollout

Each step up shortens your leverage or lengthens the lever arm, which raises the difficulty without piling on spinal flexion. That’s the same logic as autoregulating any lift by effort rather than fixed numbers — if you want to dial intensity precisely, our guide to training by RPE and RIR applies directly to core work too.

Bracing deserves its own line. The trunk stabilizes by generating intra-abdominal pressure — a controlled, 360-degree brace, not a hollow suck-in. McGill’s bracing model is the reference here: stiffen the whole canister, breathe behind the brace, and keep the spine neutral.⁴ This is the same skill that protects your spine when you set up for a heavy pull, and it’s why bracing trained in carries carries over to lifting (and to standing tall — see fixing posture and rounded shoulders).

This is where most of the real-world value of a data-aware coach shows up: SensAI programs the right anti-movement patterns into your actual week, factors recovery and wearable data so a hard carry session doesn’t land on top of a heavy lower-body day, and auto-progresses you by leverage and time instead of asking you to count crunches.

Common mistakes to avoid:

Chasing reps. Endurance is the target, but endurance ≠ marathon static holds. Stop the 5-minute plank — short, hard, fully braced holds beat long sloppy ones.
Ignoring breathing and bracing. If you can’t breathe behind your brace, you’re not bracing — you’re holding your breath.
Training only flexion. Crunches alone neglect rotation, side-bending, and bracing — three of the four jobs your trunk actually has.

SensAI flags exactly these patterns — when you’re defaulting to flexion-only work or grinding out marathon plank holds — and nudges you toward the higher-yield movement instead.

Frequently Asked Questions

Are crunches bad for you? Not inherently. A handful of crunches won’t hurt a healthy back. The problem is repeated, high-volume end-range flexion as your primary core method — it loads the lumbar spine heavily for a relatively small training return, and the mechanism research links cumulative flexion to disc injury in animal models.³⁴

Do crunches build a strong core? Partly. They do train the rectus abdominis, so you’ll feel them. But they neglect the trunk’s main jobs — resisting rotation, side-bending, and forward collapse under load — which is where real-world and athletic strength actually lives.⁷

What’s the difference between core stability and core strength? Core stability is the ability to resist unwanted spinal motion and hold a neutral spine; core strength is the ability to produce force or torque through the trunk. Most daily and athletic demands are stability demands, which is why endurance and anti-movement work matter more than max flexion strength for most people.¹⁵

What are the best anti-rotation exercises? The Pallof press, bird dog, and single-arm loaded carries. Progress the Pallof press by moving from kneeling to standing to a tandem stance — research shows the standing and tandem variations impose substantially higher trunk-control demand.⁹

How often should I train my core? Two to three times a week, integrated into your existing program rather than tacked on as a separate “ab day.” Cover anti-extension, anti-rotation, and anti-lateral-flexion across the week, and let your heavy squats and deadlifts handle much of the bracing.¹⁰

References

McGill, Stuart M. “Core Training: Evidence Translating to Better Performance and Injury Prevention.” Strength & Conditioning Journal, 2010, 32(3):33-46. https://journals.lww.com/nsca-scj/fulltext/2010/06000/core_training__evidence_translating_to_better.4.aspx ↩ ↩² ↩³
Behm, David G., Eric J. Drinkwater, Jeffrey M. Willardson, and Patrick M. Cowley. “The use of instability to train the core musculature.” Applied Physiology, Nutrition, and Metabolism, 2010, 35(1):91-108. https://pubmed.ncbi.nlm.nih.gov/20130672/ ↩
Callaghan, Jack P., and Stuart M. McGill. “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. https://pubmed.ncbi.nlm.nih.gov/11114441/ ↩ ↩²
McGill, Stuart M. “Low Back Disorders: Evidence-Based Prevention and Rehabilitation.” 3rd ed., Human Kinetics, 2015. (Sit-up lumbar compression ~3,300 N; abdominal bracing model. NIOSH low-back compression action limit of 3,400 N per NIOSH 1981 Work Practices Guide for Manual Lifting.) ↩ ↩² ↩³ ↩⁴
McGill, Stuart M., A. Childs, and Craig Liebenson. “Endurance times for low back stabilization exercises: clinical targets for testing and training from a normal database.” Archives of Physical Medicine and Rehabilitation, 1999, 80(8):941-944. https://pubmed.ncbi.nlm.nih.gov/10453772/ ↩ ↩² ↩³
Biering-Sørensen, Fin. “Physical measurements as risk indicators for low-back trouble over a one-year period.” Spine, 1984, 9(2):106-119. https://journals.lww.com/spinejournal/abstract/1984/03000/physical_measurements_as_risk_indicators_for.2.aspx ↩
Martuscello, Jason M., James L. Nuzzo, Candi D. Ashley, Bill I. Campbell, John J. Orriola, and John M. Mayer. “Systematic review of core muscle activity during physical fitness exercises.” Journal of Strength and Conditioning Research, 2013, 27(6):1684-1698. https://pubmed.ncbi.nlm.nih.gov/23542879/ ↩ ↩²
Escamilla, Rafael F., Clare Lewis, Duncan Bell, Gwen Bramblet, Jason Daffron, Steve Lambert, Amanda Pecson, Rodney Imamura, Lonnie Paulos, and James R. Andrews. “Core muscle activation during Swiss ball and traditional abdominal exercises.” Journal of Orthopaedic & Sports Physical Therapy, 2010, 40(5):265-276. https://pubmed.ncbi.nlm.nih.gov/20436242/ ↩
Juan-Recio, Casto, Amaya Prat-Luri, Heidy Rondón-Espinosa, David Barbado, and Francisco J. Vera-Garcia. “Effect of Body Position and Support Surface on the Postural Control Challenge During the Pallof Press Exercise: A Smartphone Accelerometer-Based Study.” Medicina (Kaunas), 2025, 61(2):312. https://pmc.ncbi.nlm.nih.gov/articles/PMC11857476/ ↩ ↩²
“The impact of core training on overall athletic performance in different sports: a comprehensive meta-analysis.” BMC Sports Science, Medicine and Rehabilitation, 2025. https://link.springer.com/article/10.1186/s13102-025-01159-6 ↩ ↩² ↩³
Reed, Casey A., Kevin R. Ford, Gregory D. Myer, and Timothy E. Hewett. “The effects of isolated and integrated ‘core stability’ training on athletic performance measures: a systematic review.” Sports Medicine, 2012, 42(8):697-706. https://pubmed.ncbi.nlm.nih.gov/22784233/ ↩