How to Do In-and-Outs: Form Guide & Ab Tips

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 onto your spine.

And honestly? That's what makes in-and-outs both effective and risky. When you do them right, the work never lets up. No resting at the top. No relaxing at the bottom. Every phase of the movement requires active control. Do them wrong though, and momentum takes 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.

Muscles Worked

What the references say. The reference exercise libraries file this exercise differently, so this guide lists the muscles that do the work in the app's version. During the "in" phase you pull your knees toward your chest while you curl your chest up to meet them. During the "out" phase you extend your legs and lower your torso under control without letting your feet touch the floor. The continuous tension across both phases is what makes this exercise so demanding compared to a standard crunch, which gets a rest at the bottom of each rep.

Primary movers. The rectus abdominis curls your chest up toward your knees, and the hip flexors pull your knees in toward your chest to meet it. On the way out, both lengthen under control as your legs extend and your torso leans back.

Secondary muscles. The obliques, along the sides of your waist, help the rectus abdominis curl your trunk, and the transverse abdominis, the deepest layer of the abs, braces your trunk so your midsection stays tight through both the "in" and the "out" phase.

Arm assistance. Your hands stay planted behind your hips and share the work: as you tuck in, you push through your palms and your elbows extend partway to help lift your torso, and as you extend out, your elbows bend again. That makes the hands-down version easier than doing it with your arms crossed. Your hips stay on the mat for the whole set. Breathe out as you tuck in, and breathe in as you extend.

Evidence. No EMG study has measured the in-and-out itself. In studies of related movements that flex the hips while the trunk works, the abdominals were highly active in hip-flexion sit-ups, hanging knee-ups and reverse crunches.

How to Do In-and-Outs (Step-by-Step)

1. Set up on your hands. Sit on a mat with your legs together and straight out in front of you, about 10 to 15 degrees above the floor with your feet hovering. Lean your torso back to about 20 to 30 degrees above the floor and support it on your hands, placed flat on the floor behind and a little outside your hips, elbows bent. This is your home base for the entire set. Tip: keep your hips on the mat and your legs together for the whole set.
2. Pull your knees in. Draw your knees toward your chest while you push through your palms so your elbows extend (they can stay a little bent), and bring your chest up toward your knees, so your torso and thighs meet in a tight tuck. You should feel a strong contraction through your entire midsection. Feet stay off the ground. That's the "in" phase. Tip: breathe out as you tuck in. Your hands help lift your torso, but they should not do all the lifting.
3. Extend out with control. Extend your legs straight out and lower your torso back under control, keeping your feet off the floor and your hips on the mat. At the bottom your legs are straight, together and low, about 10 to 15 degrees above the floor with your feet hovering, your torso is back to about 20 to 30 degrees above the floor, and your elbows are bent again. Don't let your lower back collapse into a deep round. This is the "out" phase. And it's where most people lose it. Tip: lower your torso and legs slowly; do not drop back.
4. Reverse direction smoothly. Without pausing, pull your knees back in while bringing your torso forward. The transition should flow continuously without any jerk. Think about bringing your ribcage and pelvis closer together. Not about yanking your legs with momentum. Tip: keep your feet off the floor between reps if you can. If you cannot yet, let your heels touch lightly and build up from there.
5. Breathe and maintain tension. Exhale on the "in" phase as you tuck. Inhale on the "out" phase as your body extends. Keep your feet off the floor between reps. Beginners: aim for 2 to 3 sets of 8 to 12 reps with full control. If your lower back rounds or your form breaks by rep 5, that's your working set for now, and stop if your lower back hurts.

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 core 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. The fix: only extend as far as you can control. The full range takes your legs low, about 10 to 15 degrees above the floor; if your lower back rounds before that, shorten the range for now, and stop if it hurts. 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 you can control 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 the set is done. 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.
• Letting your arms do all the lifting. Pushing through your palms as you tuck in, so your elbows extend, is part of the movement. But if your arms are hauling your torso up while your hip flexors and abs stay passive, the exercise turns into an arm push. Your arms assist; your hip flexors and abs drive the tuck. Once that feels solid, press less through your hands, then try crossing your arms over your chest: that is the harder progression, not the standard form.
• Resting between reps or stopping the legs short. Letting your feet or back settle on the floor between reps gives you a break, and stopping your legs short of straight cuts the range. Extend your legs fully every rep and go straight into the next one. If you need a reset, use the feet-down regression below.

Variations: From Beginner to Advanced

Feet-Down Tuck-Ups (Beginner Regression)

Same movement pattern, but your heels tap the floor between each rep. This gives you a brief reset and cuts the balance demand way down. Focus on bringing your knees and chest together as you draw in, with your hands planted behind you. Once you can knock out 3 sets of 15 with solid control, try keeping your feet off the ground for the entire set.

Bent-Knee In-and-Outs (Intermediate)

Keep your knees bent during the "out" phase instead of straightening your legs, and shorten how far you lean back. Feet stay hovering and your hands stay planted behind you. When you can do 3 sets of 15 with a smooth, controlled tempo and no lower-back rounding, you're ready for the straight-leg version.

Straight-Leg In-and-Outs (Advanced)

This is the version described in the step-by-step above, and the one the FitCraft app teaches. Your legs extend fully straight and low at the bottom of every rep, about 10 to 15 degrees above the floor with your feet hovering, while your hands push through the floor to help lift your torso. The longer lever makes your hip flexors and abs work much harder. To make it harder still, press less through your hands, then lift them and cross your arms over your chest.

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 makes the weight harder to decelerate. Start light. Seriously. Even 5 lbs feels like 20 when it's at the end of that lever arm.

When to Avoid or Modify In-and-Outs

In-and-outs are safe and effective for most healthy adults with baseline core strength, but the exercise combines spinal flexion and hip flexion under continuous load, which makes it a poor fit for several conditions. Always consult a qualified healthcare provider or physical therapist before starting or returning to demanding core work, especially in the situations below.

• Acute lower-back pain, disc pathology, or sciatica. The "out" phase loads the lumbar spine in a position prone to rounding under fatigue, which can aggravate disc-related symptoms. Substitute with anti-extension and anti-rotation patterns like dead bugs, bird-dogs, and forearm planks until cleared by a PT.
• First 6 to 8 weeks postpartum or active diastasis recti. Combined trunk-and-hip-flexion movements widen abdominal separation and increase intra-abdominal pressure. Restore deep-core function first with diaphragmatic breathing, transverse abdominis activation, and progressive dead bugs and bird-dogs.
• Recent abdominal surgery (C-section, hernia repair, appendectomy). Get clearance from your surgeon. Most post-surgical protocols start with diaphragmatic breathing, progress to gentle bracing, and only then to loaded core flexion.
• Hernia (umbilical, inguinal, ventral). High intra-abdominal pressure during the "in" phase can worsen the herniation. Talk to your physician about safe core options before attempting this.
• Pregnancy (second and third trimesters). Avoid supine and long V-sit positions; the vena cava compression and flexion-based abdominal work are contraindicated. Use upright or side-lying alternatives.
• Hip-flexor strain or sports hernia. The continuous hip-flexion demand will provoke the symptoms. Substitute with anti-extension patterns until the strain has fully resolved.
• Pelvic-organ prolapse or pelvic-floor dysfunction. The intra-abdominal pressure spikes during the "in" phase can worsen symptoms. Work with a pelvic-floor PT before attempting loaded core flexion.

Related Exercises

If in-and-outs aren't clicking for you yet, or you want complementary core movements to round out a program, these are the most useful neighbors in the same taxonomy:

• Easier regression (same pattern): Reverse crunches teach the pelvic-curl pattern lying flat, without the balance demand. The fastest way to build the ab strength you need for in-and-outs.
• Foundation for spinal bracing: Dead bugs and bird-dogs teach the anti-extension and anti-rotation patterns that protect your lumbar spine when the in-and-out gets hard. Universal foundations; do these even if you can already crank out 30 in-and-outs.
• Same plane, dynamic alternative: Bicycle crunches are another dynamic flexion-plus-rotation movement, performed lying down. Adds the rotational work in-and-outs don't give you.
• Same plane, hip-flexor dominant alternative: Leg raises train a related movement pattern lying flat. Lying leg raises remove the balance challenge while still demanding strong hip-flexor and ab control.
• Isometric anti-extension pair: Forearm planks superset well with in-and-outs to build complementary core endurance under both static and dynamic load.
• Anterior-core advanced peer: Hollow holds are the isometric cousin. If you can hold a hollow for 45 seconds, you have the bracing endurance to handle in-and-outs.

How to Program In-and-Outs

The American College of Sports Medicine's position stand on resistance training (Ratamess et al., 2009) recommends progressive overload through manipulation of volume, intensity, rest, and frequency. The rep and rest ranges below draw on its advice of 8 to 12 repetitions for beginners and, for muscular endurance, light loads for 10 to 25 or more repetitions with rests under 90 seconds. Core work like in-and-outs falls into the dynamic rep-based camp; here's how to dose it across training levels:

Where in your workout. Place in-and-outs at the end of a resistance-training session or as a finisher. Doing them before compound lifts pre-fatigues the core and compromises spinal stability under heavier load. They also work well in a dedicated core circuit paired with anti-extension and anti-rotation exercises, or as part of a "core day" with planks, hollow holds, and bird-dogs.

Form floor over rep targets. The rep ranges above assume clean reps with neutral spine and continuous tension. If your lower back starts rounding at rep 7 of a planned set of 12, that set ends at rep 7. Rounding under load is how this exercise transfers stress to the lumbar discs. Range of motion earns itself over time as your core gets stronger; chasing the rep target with broken form just builds an injury timeline.

In the FitCraft app, the in-and-out is an Advanced exercise, taught as the hand-assisted straight-leg version described above, and the app adjusts your training as you progress. Its interactive 3D demonstration shows the tuck and the extension from any angle, which makes the movement click faster than reading about it.