Seated In and Out

What the seated in and out actually trains

A seated in and out is an abdominal exercise in which you sit on the end of a bench, lean back slightly, and repeatedly draw your knees in towards your chest before extending the legs out again without letting the feet touch the floor.

Two things are happening, and they are not equal.

The larger part is hip flexion. The psoas major, iliacus and rectus femoris pull the thighs towards the trunk on the way in and control them on the way out, and they do most of the visible work. This is not a flaw to be coached away. Hip flexors flex hips, and any exercise that moves your legs relative to your torso will involve them heavily.

The smaller part is the abdominal contribution. The rectus abdominis and the obliques hold your torso at a fixed angle against the pull of your legs, which is anti-extension work, and they shorten a little if you curl the pelvis under at the end of the tuck. That pelvic tuck is the only genuine spinal flexion in the movement, and most people never add it.

So this is a hip flexor exercise with an abdominal component, performed against bodyweight, through a moderate range.

That is a fair description rather than a criticism. It is convenient, it needs nothing but a bench, and it is a reasonable entry point. What it is not is a serious way to build abdominal muscle, because there is no practical way to make it much harder. Once you can do twenty clean reps, the exercise has given you everything it has.

Setting up and executing

Sit on the end of a flat bench and grip the edges beside your hips.

Lean the torso back to roughly forty five degrees and lift the feet off the floor with the knees bent.

Brace the abs and fix that torso angle. It should not change for the rest of the set.

Draw the knees in towards the chest, curling the pelvis slightly under as they arrive. Pause for a moment.

Extend the legs out until they are nearly straight, keeping the feet clear of the floor, then reverse under control.

Slow is the point. You have no load to add, so tempo is the only variable you own.

The mistakes that cost you the most

Rocking the torso is what most people do without noticing. Leaning back as the legs extend and forward as they tuck creates a see saw in which the two halves of your body counterbalance each other and momentum carries the reps. The torso angle should be fixed from the first rep to the last, and holding it still is most of the abdominal work in the exercise.

Short, fast reps are the second habit. A quick partial tuck through the middle of the range does very little. Knees fully in, legs fully out.

Letting the feet touch down between reps resets the tension and turns a set into a series of separate repetitions. Keep them clear of the floor for the whole set.

Never curling the pelvis leaves the exercise almost entirely to your hip flexors. A small posterior tilt at the end of the tuck is what brings the rectus abdominis into it.

Doing sets of fifty is the strategic error. High repetition unloaded work is the least effective way to train abdominals, and it is the default everyone falls into precisely because the exercise cannot be loaded.

Sets, reps and rest by goal

Three sets of twelve to twenty is a sensible range for muscle gain, with forty five seconds rest. For a strength focus, three sets of ten to fifteen with slower tempo and a minute. In a fat loss phase, three sets of fifteen to twenty five with thirty seconds keeps the pace up.

If you can already exceed those numbers, the answer is not more repetitions. Hold a light dumbbell between the feet, slow the tempo to four seconds each way, or pause in the extended position for two seconds a rep.

Schoenfeld and colleagues found a dose response between weekly hard sets and hypertrophy, and the operative word is hard. Sets that stop being challenging stop counting, which is the structural problem with this movement.

Refalo and colleagues found stopping a rep or two short of failure produces comparable growth to going to failure, which is convenient here because the final reps are the ones where the torso starts rocking.

Two or three sessions a week is fine. There is very little fatigue to manage.

The honest recommendation is to use this as filler volume and to put your real abdominal progression into something you can load, such as a cable crunch, a machine crunch or a weighted hanging leg raise.

Where it fits in your split

At the end of a session, as a cheap two minute addition.

On push, pull and legs it fits the end of leg day. On an upper duo it works as a finisher for either session. On a bro split it belongs after the loaded work on abs day. On an upper and lower split it fits the end of a full body or lower session. On a full body programme it is convenient, since any bench will do.

The main argument for including it is that it costs almost nothing. It needs no equipment, no setup and no recovery, so if you have two minutes at the end of a session, it is a better use of them than sitting on your phone.

The main argument against is opportunity cost. If those two minutes could instead be spent on a loaded exercise, spend them there. Abdominal muscles grow in response to load and progression, and this movement offers neither in any meaningful quantity.

Somewhere between the two lies the sensible answer: keep it in the programme, do not build the programme around it.

Variations worth knowing

Holding a light dumbbell between the feet is the single most useful change you can make, because it finally introduces load and therefore progression.

The flat bench knee tuck, lying on your back rather than sitting, reduces the hip flexor emphasis slightly and is easier on the lower back if the seated version feels uncomfortable.

The V-up brings the arms and legs together over the middle, which lengthens the range and adds a real spinal flexion component.

The hanging knee raise performs the same tuck from a bar, where bodyweight loads the abdominal wall far more effectively and where a dumbbell can be added between the feet.

The lying leg raise straightens the legs on the floor, lengthening the lever considerably, and is a step up in difficulty for most people.

And a paused version, holding the extended position for two seconds every rep, adds time under tension without needing any equipment at all.

What the evidence says

McGill has argued for choosing trunk exercises based on what job they train and what they cost the spine, which is a useful lens for an exercise like this one that mixes hip flexion with a modest abdominal demand.

Escamilla and colleagues compared commercial and common abdominal exercises with electromyography and found that many popular bodyweight movements produce lower abdominal activity than their reputation suggests.

A second study by Escamilla and colleagues on traditional and nontraditional abdominal exercises found considerable hip flexor involvement in leg movement patterns, which describes this exercise accurately.

A third study by Escamilla and colleagues on core muscle activation across abdominal exercises showed how much the choice of movement, rather than the effort put into it, determines which muscles work.

Vigotsky and colleagues cautioned that electromyography amplitude does not straightforwardly predict hypertrophy, so activation comparisons should inform exercise selection rather than settle it.

Schoenfeld and colleagues found a dose response between weekly hard sets and hypertrophy, which is precisely why an exercise that cannot be made hard eventually stops contributing.

Refalo and colleagues found stopping short of failure produces comparable hypertrophy to training to failure, which suits a movement whose last reps tend to lose their form.

Common questions

Why do I feel this mostly in my hip flexors?

Because they are doing most of the work. Drawing the knees towards the chest is hip flexion, and the psoas and iliacus are the muscles for it. Adding a small pelvic tuck at the end of each tuck brings the abdominals in, but it will never become a pure abdominal exercise.

How is this different from a lying knee tuck?

Barely at all. The seated version leans the torso back and adds a little more anti-extension demand; the lying version is easier on the lower back. Pick whichever is comfortable and do not programme both.

Should my back be rounded or straight?

Slightly rounded is fine and slightly rounded is what happens naturally when you tuck the pelvis under. What matters more is that the torso angle stays fixed rather than rocking back and forth with the legs.

Can I do this every day?

You could, but you would be spending time on an exercise you have already outgrown. If your abdominal training is easy enough to repeat daily without fatigue, it is not loaded enough to be productive.

What should I move on to?

Anything you can load. A cable crunch, a machine crunch or a hanging leg raise with a dumbbell between the feet will all do more for abdominal size, because all three can be made harder next month.