You’ve probably seen it. Maybe back in the P90X days with Tony Horton yelling about "Ab Ripper X," or perhaps you just saw someone at the gym frantically tucking their knees while balanced on their glutes. It’s the in n out exercise. It looks simple—kinda like a human accordion—but if you’ve ever actually tried to knock out fifty reps, you know it burns in a way that regular crunches just can’t touch.
It’s basic. It’s effective. Honestly, it’s one of those moves that reminds you that you don't need a $3,000 cable machine to build a strong midsection.
The beauty of the in n out exercise is the sheer mechanical efficiency of it. You’re essentially using your legs as a heavy lever. By extending them out and pulling them back in, you’re forcing the rectus abdominis and the deep-seated iliopsoas (the primary hip flexor) to work in tandem. Most people think they're just "doing abs," but you're actually training the entire anterior chain to stabilize your trunk while your lower extremities move through space. That's a functional skill.
Breaking Down the Mechanics of the In N Out Exercise
To do it right, you sit on the floor. Hands can be behind your hips for support, or if you’re feeling like a masochist, you can lift them off the ground entirely. Lean back slightly. Lift your feet. Now, pull your knees toward your chest while simultaneously bringing your upper body forward to meet them. Then, extend everything back out without letting your feet or shoulders touch the mat.
That’s one. Now do it again. And again.
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The tension never leaves. That’s the "secret sauce" here. Unlike a sit-up where you might rest at the bottom or the top, the in n out exercise keeps your core under constant tension (TUT). According to various kinesiology studies regarding isometric and isotonic contractions, maintaining this "V-hold" position while adding the dynamic "crunching" motion maximizes motor unit recruitment in the lower fibers of the abs.
Why Your Hip Flexors Are Screaming
Let’s be real for a second: most people complain that they feel this more in their hips than their stomach. You aren't doing it wrong, necessarily. The hip flexors are naturally heavily involved in any movement where the femur is pulled toward the pelvis.
If your hips are taking over entirely, it’s usually because your core isn't "bracing" properly. Think about pulling your belly button toward your spine before you even start the movement. If you can’t keep your lower back from arching excessively, you’re losing the battle. Lower back pain during the in n out exercise is a giant red flag that your transverse abdominis has checked out of the building.
Variations That Actually Make a Difference
You don't have to stick to the standard version. In fact, you probably shouldn't if you want to keep seeing progress.
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The "Hands-Free" Version
Take your hands off the floor. Reach them out to your sides or keep them tucked by your chest. This immediately removes your "kickstand" and forces your obliques to fire just to keep you from tipping over like a tree. It’s significantly harder. Your stabilizer muscles have to work overtime to manage the shifting center of gravity.
Weighted In N Outs
Grab a small dumbbell. Hold it between your feet. It sounds ridiculous until you try it. Suddenly, those "lever arms" we talked about are twice as heavy. This turns a high-rep endurance move into a genuine strength-building exercise for the lower abdominal region.
The Cyclical Variation
Instead of a straight in-and-out motion, some trainers suggest a "bicycle" hybrid. You’re still doing the core movement, but you’re adding a rotational element. This brings the internal and external obliques into the mix. It’s great for athletes who need rotational power—think golfers, baseball players, or anyone who needs to move explosively in more than one plane of motion.
The Science of Core Stability and Recovery
We often treat the core like it’s invincible, but it’s a muscle group like any other. The in n out exercise targets the fast-twitch muscle fibers if done explosively, but most of us use it for "metabolic stress"—that burning sensation that comes from high reps.
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Research from institutions like the American Council on Exercise (ACE) has consistently shown that "crunch-style" movements are effective, but they work best when paired with "bracing" movements like planks. The in n out exercise is sort of the bridge between the two. You’re bracing to stay balanced on your tailbone, but you’re crunching to get the muscular contraction.
Common Pitfalls to Avoid
- Rushing the rep: Speed is the enemy of the abs. If you’re using momentum, you’re just swinging your legs. Slow it down. Feel the burn.
- The "Turtle" Neck: Don’t tuck your chin so hard into your chest. Keep a neutral spine. You want your abs to do the work, not your neck muscles.
- Holding Your Breath: This is a big one. People forget to breathe. Exhale as you bring your knees in (the exertion phase) and inhale as you extend. This helps manage intra-abdominal pressure.
Where This Fits Into Your Routine
Don't make this your entire workout. That’s a mistake. The in n out exercise works best as a "finisher" or as part of a circuit.
I’ve seen some people do these every single day. Honestly? That's overkill. Your abs need rest. If you're hitting them with high intensity, three times a week is plenty. Mix it up with some hanging leg raises or some heavy compound lifts like deadlifts and squats, which require a different kind of core stability altogether.
There's a reason the in n out exercise has survived every fitness trend from the 80s to now. It doesn't require a gym membership. It doesn't require "bio-hacking" gadgets. It just requires you to sit on the floor and put in the work.
If you're looking for a way to sharpen your midsection, stop overcomplicating things. Go back to the basics. The results usually follow the effort, not the complexity of the equipment.
Actionable Next Steps
Start by integrating the in n out exercise at the end of your next two workouts. Don't go for a specific number of reps; go for quality for 45 seconds. Keep your hands on the floor for the first week to master the form and protect your lower back. Once you can do 45 seconds with perfect control, lift your hands two inches off the mat. Focus on the "hollow body" position during the extension phase—this is where the real strength is built. If your back arches, shorten the range of motion. Success with this move isn't about how far your legs go out, but how well you control the weight of your own body.