The in-and-out exercise looks simple enough. Sit on the floor, pull your knees in, push them back out. Repeat. But there's a reason most people's lower backs start screaming by rep six. This exercise demands constant core engagement with zero downtime between reps, and the moment your form breaks down, the load shifts from your abs to your spine.
And honestly? That's what makes in-and-outs both effective and risky. When you do them right, your rectus abdominis never gets a break. No resting at the top. No relaxing at the bottom. Every phase of the movement requires active stabilization, which is why EMG research consistently ranks exercises involving simultaneous trunk and hip flexion among the highest for abdominal activation (Martuscello et al., 2013). Do them wrong, though, and your hip flexors take over, your lower back rounds, and you end up doing a sloppy rocking motion that builds nothing except frustration.
So this guide covers the real technique for in-and-outs, the form errors that turn it into a back exercise, and how to scale it whether you're working up to your first clean rep or looking for ways to make it brutal.
Quick Facts
| Primary Muscles | Rectus abdominis (full length) |
| Secondary Muscles | Transverse abdominis, obliques, hip flexors (iliopsoas, rectus femoris), quadriceps |
| Equipment | None (bodyweight only, mat optional) |
| Difficulty | Advanced |
| Movement Type | Compound · Trunk flexion + hip flexion |
| Category | Core / Strength |
| Good For | Core endurance, rectus abdominis development, hip flexor strength, V-sit progression |
How to Do In-and-Outs (Step-by-Step)
- Find your balance point. Sit on the floor with your knees bent and feet flat. Place your hands lightly behind your hips for support (or hold them out front for more difficulty). Lean back about 30 degrees until you feel your abs kick in, then lift your feet off the ground. Knees stay bent at roughly 90 degrees. This balanced V-sit position is your home base for the entire set.
- Pull your knees in. Contract your abs to draw your knees toward your chest. At the same time, bring your torso slightly more upright so your upper body and lower body are compressing toward each other. You should feel a strong contraction through your entire midsection. Feet stay off the ground. That's the "in" phase.
- Extend out with control. Extend your legs forward (keep them hovering above the floor) while leaning your torso back. Your body opens up into a wide, shallow V shape. Keep your chest open and your spine as neutral as possible. Don't let your lower back collapse into a deep round. This is the "out" phase. And it's where most people lose it.
- Reverse direction smoothly. Without pausing, pull your knees back in while bringing your torso forward. The transition should be smooth, not a jerk. Think about your abs shortening to bring your ribcage and pelvis closer together. Not about yanking your legs with momentum.
- Breathe and maintain tension. Exhale on the "in" phase when your abs are crunching. Inhale on the "out" phase as your body extends. Your feet never touch the floor between reps. Beginners: aim for 3 sets of 8-10 reps with full control. If your lower back rounds or your form breaks by rep 5, that's your working set for now.
Coach Ty's Tips: In-and-Outs
These cues come straight from Coach Ty, FitCraft's 3D AI coach. They're the exact form errors Ty flags when watching your in-and-outs in real time:
- Keep your chest proud. The second your shoulders round forward, your spine follows. And then you're not doing an ab exercise anymore. You're straining your lower back. Think about keeping your collarbone wide and your sternum lifted, even as you lean back during the "out" phase. If you can't hold that posture, shorten your range of motion until you can.
- Feet never touch. The moment your heels hit the floor, your abs get a vacation. That's the whole point of in-and-outs: continuous tension. Even in the fully extended position, your feet should hover at least two inches off the ground. If that's too hard right now, bend your knees more during the "out" phase to shorten the lever.
- Drive with your abs, not your hip flexors. Look, this is the most common mistake by far. People pull their knees in using their hip flexors and let their abs go along for the ride. The cue that fixes it: think about curling your pelvis toward your ribcage as you draw in. Not just bending at the hips. You should feel the contraction deep in your abs, not in the front of your thighs.
- Slow down the "out" phase. The extension is where people lose control. They let gravity yank their legs down and their torso back. But the eccentric phase is where a huge chunk of the muscle-building stimulus happens. Take a full 2 seconds to extend out. If you rush it, you're training momentum. Not muscle.
Common Mistakes to Avoid
In-and-outs have a tiny margin for error. The balance demand is high, the range of motion is long, and there's no rest position to reset in. Here are the mistakes that turn this from one of the best ab exercises into one of the worst:
- Rounding the lower back. This is the big one. When you extend your legs out and lean back, your lower back wants to round into flexion. That puts compressive load on your lumbar discs instead of your abs. The fix: only extend as far as you can while keeping a neutral spine. For most people starting out, that means their legs extend to about a 45-degree angle. Not parallel to the floor. Range of motion earns itself over time as your core gets stronger.
- Using momentum to rock back and forth. If you're rocking like a boat in rough water, momentum is doing the work. Slow the tempo way down. Each "in" and "out" phase should take at least one full second. Can't do it slowly? Reduce reps or switch to the feet-down regression until your abs can handle the load without cheating.
- Holding your breath. Bracing matters, sure. But holding your breath through a high-rep set of in-and-outs will gas you before your abs give out. Exhale on the crunch (in), inhale on the extension (out). Rhythmic breathing keeps you going and helps maintain intra-abdominal pressure without spiking blood pressure.
- Gripping the floor too hard with your hands. If you're pressing your hands into the floor behind you and using your arms to prop yourself up, your abs aren't doing the full job. Your hands should be lightly touching the floor for balance. Not bearing weight. Actually, here's a good test: try holding your arms in front of you or crossing them over your chest. If the exercise suddenly feels twice as hard, your hands were doing too much.
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Variations: From Beginner to Advanced
Feet-Down Tuck-Ups (Beginner Regression)
Same movement pattern, but your feet tap the floor between each rep. This gives your abs a brief reset and cuts the balance demand way down. Focus on the pelvic curl as you draw your knees in, keeping your chest lifted throughout. Once you can knock out 3 sets of 15 with solid control, try keeping your feet off the ground for the entire set.
Standard In-and-Outs (Intermediate)
This is the version described in the step-by-step above. Knees bend during the "out" phase, feet stay hovering, hands lightly on the floor for balance. When you can do 3 sets of 15 with a smooth, controlled tempo and no lower-back rounding, you're ready to level up.
Straight-Leg In-and-Outs (Advanced)
Instead of bending your knees during the "out" phase, keep your legs fully extended. This dramatically increases the lever arm and makes your abs work way harder to control the movement. Fair warning: your rep count will drop by at least half. Your legs feel so much heavier in this position. Keep a slight bend in your knees to protect your hamstrings, but the legs should be mostly straight.
Weighted In-and-Outs (Advanced)
Hold a light dumbbell (5-15 lbs) between your feet or wear ankle weights. The added resistance increases the load during the "out" phase and forces your abs to work harder to decelerate the weight. Start light. Seriously. Even 5 lbs feels like 20 when it's at the end of that lever arm.
Alternative Exercises
If in-and-outs aren't clicking for you yet, these alternatives hit similar muscles:
- Reverse crunches: A supine exercise that teaches the pelvic curl pattern without the balance demand. Great for building the lower-ab strength you'll need for in-and-outs.
- Bicycle crunches: Another dynamic ab exercise with continuous movement, but performed lying down. Adds the rotational work that in-and-outs don't give you.
- Leg raises: Similar muscles, different movement pattern. Lying leg raises remove the balance challenge while still demanding strong hip flexor and lower-ab control.
Programming Tips
Here's how to fit in-and-outs into your training:
- Beginners (working up to it): Start with feet-down tuck-ups, 3 sets of 12-15 reps. Pair with dead bugs and forearm planks to build the baseline core stability you need. Place at the end of your workout.
- Intermediate: 3 sets of 10-15 reps of standard in-and-outs. Pair with an anti-rotation exercise like Pallof presses for balanced core training. Rest 45-60 seconds between sets.
- Advanced: 3-4 sets of 15-20 reps, or 3 sets of 10-12 with straight legs or added weight. Superset with planks or hollow holds for a brutal core circuit. Keep total weekly core volume around 12-18 direct sets.
- Frequency: 2-3 times per week with at least 48 hours between sessions. In-and-outs are demanding on the hip flexors as well as the abs, so recovery isn't optional here.
FitCraft's AI coach Ty programs in-and-outs into your personalized plan based on your assessment results. Ty's 3D demonstrations show you the exact balance point, the pelvic curl, and the extension from multiple angles. Honestly, it makes the movement click way faster than reading about it. And the app tracks your rep quality and progresses you from regressions to full in-and-outs as your core gets stronger.
Frequently Asked Questions
What muscles do in-and-outs work?
In-and-outs primarily target the rectus abdominis across its full length, with strong activation of the transverse abdominis for stabilization, the obliques for anti-rotation, and the hip flexors (iliopsoas and rectus femoris) for leg movement. Your lower back and quadriceps also work as secondary stabilizers during the extended "out" phase.
Are in-and-outs good for losing belly fat?
No single exercise spot-reduces belly fat. But in-and-outs are a high-intensity core exercise that burns more calories per rep than static exercises like planks, because they demand continuous movement and stabilization. Combined with a caloric deficit and consistent training, stronger abs become visible as overall body fat drops.
How many in-and-outs should I do?
Beginners should start with 3 sets of 8-10 reps, focusing on control rather than speed. Intermediate trainees can aim for 3-4 sets of 12-15 reps. Advanced athletes can do 4 sets of 15-20 reps or add resistance by holding a light dumbbell between the feet. Quality always beats quantity with this one.
Can beginners do in-and-outs?
Honestly, they're advanced. You need solid abdominal strength and hip flexor endurance before attempting them. Beginners should build up by mastering dead bugs, reverse crunches, and tuck-ups first. Once you can hold a V-sit for 20 seconds, you probably have the baseline strength to try in-and-outs with reduced range of motion.
What is the difference between in-and-outs and V-ups?
In-and-outs are performed from a seated balance point where you alternate between pulling the knees in and extending the legs out, with your feet never touching the ground. V-ups start flat on the floor and involve lifting the torso and straight legs to meet in the middle. V-ups demand more hamstring flexibility and generate a harder peak contraction, while in-and-outs maintain constant tension and lean more endurance-focused.