Core · Spinal flexion
Sit-Ups
The sit-up is an abdominal exercise where you lift your entire torso from lying flat to an upright seated position, then lower back down. It trains the rectus abdominis alongside the hip flexors, which take over the second half of the movement. Usually trained for 3 sets of 12–25 reps.
Also known as: Sit up, Situp, Full sit-up
- Muscle group
- Core
- Movement pattern
- Spinal flexion
- Type
- Compound
- Equipment
- Bodyweight
- Difficulty
- Beginner
What muscles does the sit-ups work?
Primary movers
- Rectus abdominis
- Hip flexors
Assisting muscles
- Obliques
- Transverse abdominis
A sit-up has two distinct phases. The first — curling the head and shoulder blades off the floor — is driven by the rectus abdominis flexing the spine. The second — bringing the torso up to vertical — is driven mainly by the hip flexors pulling the pelvis and trunk together.
That second phase is why sit-ups are more of a compound movement than crunches. The iliopsoas and rectus femoris do substantial work, which is not a flaw but does mean the abs are not the only limiting factor.
It also explains why sit-ups can bother some people's lower backs. Strong hip flexors pulling on the lumbar spine, especially with the feet anchored, can increase lumbar compression. Keeping the feet free reduces how much the hip flexors can contribute.
The obliques assist throughout and take on more work if you add rotation at the top. As with crunches, the abs respond to load — once bodyweight sit-ups become easy, holding a plate is a better progression than doing more of them.
How to do the sit-ups
Setup
- 1
Lie on your back with your knees bent at roughly 90 degrees and feet flat on the floor.
- 2
Leave your feet unanchored unless you specifically want the hip flexors involved.
- 3
Cross your arms over your chest, or rest your fingertips lightly at the sides of your head.
- 4
Tuck your chin slightly, keeping space between your chin and chest.
- 5
Press your lower back into the floor and brace your abs.
Execution
- 1
Exhale and begin by curling your head and shoulder blades off the floor.
- 2
Continue rolling up one vertebra at a time until your torso is upright.
- 3
Keep your chin tucked and your neck neutral throughout.
- 4
Pause briefly at the top without resting on your hip flexors.
- 5
Lower back down under control over two to three seconds, reversing the same rolling motion.
- 6
Let your shoulder blades touch down fully before starting the next rep.
Common sit-ups mistakes
Pulling on the neck
Hands laced behind the head yanking it forward, which strains the cervical spine and takes work away from the abs.
The fix: Cross your arms over your chest instead. If you keep your hands by your head, rest the fingertips only and never interlock them.
Using momentum to swing up
Throwing the arms forward and bouncing off the floor. It raises the rep count while dramatically reducing what the abs actually do.
The fix: Roll up deliberately over two seconds. If you cannot get up without swinging, do crunches until you are strong enough.
Not lowering fully
Stopping halfway down and bouncing into the next rep, which cuts out the eccentric — a substantial share of the stimulus.
The fix: Lower until your shoulder blades touch the floor on every rep, under control.
Anchoring the feet unnecessarily
Hooking the feet under something lets the hip flexors dominate and can increase lumbar stress, especially at higher rep counts.
The fix: Keep the feet free. If they lift off the floor, that is useful feedback that your abs are not yet doing enough of the work — regress to crunches.
How many sets and reps?
| Goal | Sets | Reps | Load | Rest |
|---|---|---|---|---|
| Hypertrophy | 3 | 12–20 | Weighted, 1–2 RIR | 60 s |
| Bodyweight | 3 | 15–25 | Bodyweight, controlled | 60 s |
| Beginner | 2–3 | 8–12 | Bodyweight or assisted | 60 s |
Two or three ab sessions a week is enough. Once 25 clean bodyweight reps is comfortable, hold a plate against your chest rather than chasing higher counts — very high-rep sit-up work builds endurance rather than muscle and accumulates a lot of lumbar flexion. If sit-ups aggravate your lower back, planks and dead bugs deliver core strength without the repeated spinal flexion.
Sit-Ups variations and alternatives
Crunch
EasierA shorter range that stops at the shoulder blades, removing the hip flexor phase entirely. More isolated to the abs and gentler on the lower back.
Weighted Sit-Up
HarderA plate held against the chest adds direct load. The natural progression once bodyweight sets get long.
Decline Sit-Up
HarderPerformed on a decline bench, which increases the resistance through the whole range and lengthens the movement.
V-Up
HarderLegs and torso rise simultaneously to meet in the middle, which increases the demand on both the abs and hip flexors considerably.
Hanging Leg Raise
AlternativeTrains the abs from the bottom up by moving the pelvis toward the ribs instead. Emphasises the lower portion of the rectus abdominis.
Frequently asked questions
Are sit-ups or crunches better?
Crunches isolate the abs more cleanly because they stop before the hip flexors take over, and they involve less lumbar flexion. Sit-ups involve more total musculature and a longer range. For building the abs specifically, crunches are the more targeted choice; for general trunk strength, sit-ups are fine. Neither is necessary if you already train planks and leg raises.
Should I anchor my feet during sit-ups?
Generally no. Anchoring lets the hip flexors do much more of the work and can increase stress on the lumbar spine. Keeping your feet free means the abs have to control the movement. If your feet lift off the floor without anchoring, that is a sign to regress to crunches for a while.
How many sit-ups should I do a day?
Train abs like any other muscle — two or three sessions a week, three sets of 12–25 controlled reps, rather than a daily total. Hundreds of daily sit-ups build endurance in a small muscle group and accumulate a lot of repeated spinal flexion for very little extra benefit.
Do sit-ups burn belly fat?
No. Spot reduction does not work — you cannot target where your body loses fat by training the muscle underneath it. Sit-ups strengthen the abs. Whether they become visible depends on your overall body fat level, which is a function of total energy balance rather than any specific exercise.
Why do sit-ups hurt my lower back?
Usually the combination of anchored feet and dominant hip flexors pulling on the lumbar spine, sometimes made worse by using momentum. Unanchor your feet, slow the movement down, and reduce the volume. If it persists, switch to crunches, planks and dead bugs, which build core strength without the repeated loaded flexion.
Hercules tracks every sit-ups you log, shows you last session's numbers before you lift, and flags when your progress stalls.
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