Sit-Ups

What the sit-up actually trains

A sit-up is a trunk flexion movement in which you lie on your back with the knees bent and rise all the way to a seated position, then return under control.

The rectus abdominis produces the first part of the movement, curling the ribcage towards the pelvis. The external and internal oblique assist with that flexion and hold the trunk stiff. Once your shoulder blades are clear of the floor and the spine has flexed as far as it usefully can, the hip flexors take over and rotate the pelvis and torso up over the hips. That group includes psoas major, iliacus and rectus femoris.

The hip flexor involvement is the source of decades of argument, and the argument is mostly nonsense. Yes, the psoas is heavily involved in the second half of a sit-up. No, that is not a defect. The psoas is a muscle. It is allowed to work. The idea that hip flexor participation makes an exercise dangerous or invalid was never supported by good evidence, and it has cost a lot of people a perfectly useful movement.

What is true is that the abdominal contribution is concentrated in the first thirty degrees or so. Beyond that, the abs are working isometrically to keep the trunk rigid while the hips do the rotating. If you only care about the rectus abdominis, a crunch delivers most of what a sit-up delivers in less time.

The counter argument is that the isometric portion is real training, the range is longer, the movement loads easily with a plate held at the chest, and it scales from a struggling beginner to a strong lifter without changing anything about the setup.

Setting up and executing

Lie on your back with the knees bent to roughly ninety degrees and the feet anchored under a pad, a bar or a partner's hands.

Cross the arms on the chest, or rest the fingers lightly beside your ears. Never lace them behind your head and pull.

Take a breath in and brace. Curl the head and shoulders off the floor first, then keep peeling the spine off the floor segment by segment.

Continue until the torso is close to vertical, and pause briefly at the top without slumping into a rounded heap.

Lower slowly over two to three seconds, reversing the same order, so the lower back touches down before the shoulders. Touch the shoulders down lightly and begin again.

The mistakes that cost you the most

Yanking on the neck is the fault that gives sit-ups their bad reputation. Hands laced behind the head, elbows dragging the skull forward, chin buried in the chest. It loads the neck, it does not load the abs any harder, and it makes people believe the exercise itself is the problem. Fingers light beside the ears, or arms crossed on the chest.

Throwing the arms is the same idea with a different limb. A hard swing forward generates enough momentum to carry you upright, and momentum does not build muscle.

Dropping back down discards the lowering phase entirely. That is half the reps you are paying for.

Starting from the hips, hinging up with a flat rigid back, skips the spinal flexion that the rectus abdominis actually produces. Curl first, then rise.

And chasing rep counts is the most common strategic error. Sets of eighty tell you something about your patience and very little about your abdominal muscles. Escamilla and colleagues found meaningful differences in muscle activity between abdominal exercises, but no exercise overcomes the problem of a load that is far too light.

Sets, reps and rest by goal

Load it. That is the whole recommendation.

For muscle gain, three sets of ten to fifteen with a minute rest, holding a plate at the chest once bodyweight reps become easy. For strength, three sets of eight to twelve with ninety seconds and a heavier plate. In a fat loss phase, three sets of fifteen to twenty five with forty five seconds.

If you can perform more than twenty five controlled reps, you do not need more reps, you need weight. A five kilo plate held against the chest is a substantial increase in difficulty, and a decline bench raises it again without any equipment at all.

The abdominal muscles are not special. They contain the same fibres as everything else and they respond to progressive overload the same way. The habit of training them exclusively with high rep bodyweight sets is why so many people conclude their abs simply do not grow.

Schoenfeld and colleagues found a dose response between weekly hard sets and hypertrophy. Six to ten hard sets of direct abdominal work per week is a reasonable target, spread across two or three sessions.

Refalo and colleagues found stopping a rep or two short of failure produces comparable growth to going all the way, which is convenient here, because the last rep of a hard sit-up set is usually the ugliest.

Where it fits in your split

At the end of a session, when nothing else depends on your trunk being fresh.

On push, pull and legs it fits at the end of legs day, or on push day if legs are already long. On a bro split it is the main flexion movement on ab day. On an upper and lower split either lower session works. On a full body programme it is a fast finisher two or three times a week. On a bodyweight programme it goes anywhere, because it needs nothing.

Do not do them before squats or deadlifts. Cholewicki and colleagues showed how much load the lumbar spine carries during heavy lifting, and the trunk musculature is what manages that load. Fatiguing it first is a poor trade for a few extra ab reps.

The other thing worth saying plainly: no amount of sit-ups will reveal your abdominal muscles. Spot reduction does not work. Training the abs makes them bigger; whether you can see them is decided by body fat. Separate projects, and the sit-up addresses one of them.

Variations worth knowing

The decline sit-up is performed on an angled bench, which lengthens the range and makes the bottom of the movement considerably harder, and is the natural next step once flat sit-ups are easy.

The weighted sit-up, holding a plate or dumbbell at the chest, is the most direct way to progress and the one most people skip.

The crunch stops once the shoulder blades leave the floor, keeping the work in the rectus abdominis and removing most of the hip flexor contribution.

The Swiss ball sit-up lets the spine extend below horizontal at the start, adding range at the stretched end, which is a genuine advantage.

The V-up raises the legs and torso together, adding a substantial hip flexor and lower abdominal demand.

And the cable crunch, performed kneeling, is the easiest flexion movement to load in small increments, which makes it the best choice if progressive overload is your priority.

What the evidence says

Escamilla and colleagues examined muscle activity across a range of commercial and common abdominal exercises and found substantial differences between them, which supports selecting a few movements deliberately rather than doing everything.

A second study from Escamilla and colleagues compared traditional and non-traditional abdominal exercises, including sit-up variations, and found the traditional movements held up well against the marketed alternatives.

A third study from Escamilla and colleagues compared Swiss ball and traditional abdominal exercises and found unstable surface work does not automatically produce more abdominal involvement.

McGill's review of core training is the origin of much of the caution about repeated spinal flexion, and it is worth reading in full rather than through the summaries, since the argument is about total flexion volume rather than a blanket prohibition.

Cholewicki and colleagues measured lumbar loads during very heavy lifting and showed the demands the trunk musculature manages, which argues against fatiguing it before heavy compound work.

Schoenfeld and colleagues found a dose response between weekly hard sets and hypertrophy, which applies to the abdominal wall as much as to anything else.

Refalo and colleagues found stopping short of failure produces comparable growth to training to failure, so there is no need to grind out the final ragged repetitions.

Common questions

Are sit-ups bad for your back?

Not inherently. The concern comes from work on repeated spinal flexion, and McGill's writing on core training is where most of it originates. The sensible reading is that very high volumes of flexion are worth avoiding, not that flexion itself is dangerous. Three sets of twelve loaded sit-ups is not high volume.

Do sit-ups just work the hip flexors?

They work both. The abs produce the first part of the movement and then hold the trunk rigid while the hip flexors rotate you up. Psoas major, iliacus and rectus femoris being involved is normal, not a fault to engineer away.

Sit-ups or crunches?

If you want the rectus abdominis and nothing else, crunches deliver most of it in less time. Sit-ups add range, an isometric portion and easier loading with a plate. They overlap heavily, so pick one as your main flexion movement rather than doing both.

Should my feet be anchored?

Anchored is the standard version, allows more reps and lets you add weight. Unanchored reduces the hip flexor contribution and is much harder. Neither is more correct, so choose based on whether you want to load the movement or make it harder without equipment.

How many should I do?

Fewer than you think, with weight. If you can do more than twenty five clean reps, add a plate rather than adding repetitions. Three sets in the range of ten to fifteen with a load that makes that range genuinely difficult is worth more than several hundred unloaded reps.