Your core isn't just a six-pack. It’s a 3D corset of muscle that stabilizes your spine, rotates your torso, and keeps you from folding like a lawn chair when you pick up a heavy grocery bag. Most people spend their lives doing crunches, which is fine, I guess, if you only care about moving in one direction. But if you want real-world strength, you need to rotate. That is where the side plank with reach under comes in. It’s a beast of a move. It’s also one of the most butchered exercises in the gym.
I’ve seen it a thousand times. Someone gets into a shaky side plank, starts flailing their top arm around like they’re trying to swat a fly, and calls it "oblique work." It’s not.
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To get the most out of the side plank with reach under, you have to understand that the "reach" isn't about the arm. It’s about the ribcage. When you thread that needle, you are forcing your internal and external obliques to decelerate your body weight against gravity. It’s basically a masterclass in spinal stability under rotational load.
The Anatomy of the Rotation
Think about the muscles actually firing here. You’ve got the transversus abdominis—your deepest abdominal layer—acting like a weight belt. Then you have the gluteus medius on your bottom leg, which is screaming because it’s trying to keep your hips from sagging toward the floor. When you add the reach under, the serratus anterior (that finger-like muscle on your ribs) kicks in to stabilize your shoulder blade.
It’s a kinetic chain party.
Dr. Stuart McGill, a world-renowned spine biomechanics expert, often talks about the importance of "proximal stiffness for distal mobility." Essentially, your core needs to be a rock so your limbs can move freely. The side plank with reach under takes this concept and adds a twist. Literally. By rotating the thoracic spine while keeping the lumbar spine and pelvis relatively neutral, you’re building the kind of resilience that prevents lower back blowouts during sports or daily life.
Setting Up Without Wrecking Your Shoulder
Stop rushing.
The biggest mistake is the elbow placement. If your elbow is too far out, you’re putting a massive amount of shear force on the rotator cuff. You want that elbow directly under the shoulder. Bone on bone support is your friend here.
- Lie on your side with your legs stacked. If stacking is too hard, stagger your feet. Put the top foot in front. It gives you a wider base of support and honestly, most people find it much more comfortable for their ankles.
- Push the floor away. Don't just "rest" on your arm. Actively drive your forearm into the mat. This engages the serratus and protects the shoulder joint.
- Bridge those hips up. Your body should be a straight line from your head to your heels. No "pike" in the hips and no sagging.
Now, the reach. Reach your top arm toward the ceiling. As you bring it down, don't just move the arm. Rotate your entire chest toward the floor. Imagine there is a tiny, fragile object under your armpit and you’re trying to reach past it without crushing it.
The rotation should come from your middle back (thoracic spine), not your low back. If you feel a "tweak" in your lumbar spine, you’ve gone too far or lost tension. Tighten your glutes. Seriously. Squeezing your butt is the fastest way to stabilize your pelvis during this move.
Why Your Obliques Are Ignoring You
If you don't feel this in your side-abs, you're probably "cheating" with your hips. People love to let their hips drop as they reach under because it makes the range of motion feel bigger. It’s a lie.
True core strength is about resisting that drop.
There was a study published in the Journal of Strength and Conditioning Research that looked at EMG activity during various plank iterations. The side plank consistently showed higher activation of the internal obliques compared to traditional planks. When you add the reach, you’re hitting those fibers in a way that static holding just can’t match. You’re teaching the muscles to fire while the geometry of the load is changing.
Variations for the Rest of Us
Maybe your balance is trash today. Or maybe you've had a shoulder injury.
You can do this from your knees. It’s not "cheating"; it’s a regression. Shortening the lever arm makes it easier to focus on the rotational quality without your whole body shaking like an earthquake.
On the flip side, if you're a freak of nature and this is too easy, grab a light dumbbell. Emphasis on light. A 3lb or 5lb weight is plenty. When you add a weight to the reaching arm, the centrifugal force wants to pull you off balance. Your core has to work double time to keep you from face-planting.
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Common Myths About "Spot Reduction"
Let’s get this out of the way: the side plank with reach under will not melt the fat off your "love handles."
Physiology doesn't work that way. You can't choose where your body burns fat by exercising the muscle underneath it. What this move will do is build the muscle density in that area so that when your body fat levels drop through diet and overall activity, you have a defined, strong-looking midsection. Plus, having strong obliques makes your waist look tighter because those muscles act like a natural corset.
Integration Into Your Routine
Don't do these at the very start of a workout when you need all your "pop" for heavy squats or sprints. But don't do them at the very end when you're so exhausted your form is garbage.
The sweet spot is usually after your main lifts but before your "finisher."
I like to prescribe these in sets of 8 to 12 controlled reps per side. Forget the timer. If you’re doing a side plank with reach under for a minute, you’re probably moving too fast and using momentum. Slow it down. Take three seconds to reach under, a one-second pause at the peak of the rotation, and two seconds to return to the top.
Real-World Benefits
Think about swinging a golf club, a tennis racket, or even just reaching into the backseat of the car to grab a bag of groceries. Those are all rotational movements. Most back injuries happen in the "transverse plane"—which is just a fancy way of saying "while twisting." By mastering the side plank with reach under, you are bulletproofing your spine against those awkward, everyday movements.
It’s functional. It’s hard. It’s effective.
Actionable Next Steps to Master the Move
- Check your mirror: Perform the move facing a mirror to ensure your hips aren't sagging or piking back as you reach.
- Film yourself: Use your phone to record a set from the side. You'll likely see that your "straight line" is actually a bit of a curve. Fix it by engaging your glutes harder.
- Breath work: Exhale deeply as you reach under. This helps engage the deep core muscles (transversus abdominis) and allows for a deeper, safer rotation of the ribcage.
- Frequency: Add this to your routine 2-3 times a week. Start with 2 sets of 10 reps per side, focusing entirely on the quality of the rotation rather than the speed of the arm.
- Progress slowly: Once you can do 3 sets of 12 with perfect form, try lifting the top leg into a "star" position while holding the side plank, then return it to the stack before performing the reach under.