The lower-ab sit-up is a bodyweight floor exercise performed lying supine that curls the pelvis toward the ribs, training the lower portion of the rectus abdominis. The name is misleading: there is no separate lower-ab muscle, and the movement is the floor reverse crunch, identical in pattern to thereverse sit-up. What sets it apart from a standard sit-up is which end moves. The shoulders stay down and the hips travel, so the neck stays out of the rep and the abs work in a short range. Program it late in a session as high-rep accessory work.
What Muscles Does the Lower-Ab Sit-Up Work?
- Primary: Rectus abdominis (lower portion)
- Secondary: Iliopsoas (hip flexors), internal and external obliques
- Tertiary: Transversus abdominis, pelvic floor, adductors
How Do You Set Up for the Lower-Ab Sit-Up?
Lie flat on your back on a mat with your arms along your sides and your palms pressed down. Bend your knees to about 90 degrees and lift your feet until the shins sit level with the floor. Flatten your lower back into the mat, tuck your ribs toward your pelvis, and hold the knees still over the hips. Let the abs take the weight of the legs.
How Do You Perform the Lower-Ab Sit-Up?
- Press your palms and lower back into the mat, knees bent and stacked over the hips.
- Exhale and curl your tailbone up, rolling the pelvis toward the ribcage rather than swinging the legs.
- Bring the knees over the chest until the hips leave the mat and the lower back rounds off the floor.
- Squeeze the abs for a count at the top with the shins still level, no pulling with the thighs.
- Lower the hips slowly over two counts, unrolling the spine one segment at a time.
- Set the tailbone back down with the knees over the hips and the shins level, ready for the next rep.
What Are the Key Form Cues?
- Curl, do not lift: think of rolling the pelvis under like the end of a tape measure, not of raising the knees.
- Keep the knee angle fixed: if the shin angle changes mid-rep the thighs took over from the abs.
- Press the palms down through the whole set to keep the shoulders anchored to the mat.
- Hold the ribs pulled toward the pelvis so the lower back never arches at the bottom.
- Empty your breath at the top of the curl, where the abs are shortest.
What Are the Most Common Mistakes?
- Swinging the legs to gain height: momentum carries the hips up and the abs never shorten, so slow the tempo until the curl is the only thing moving you.
- Pushing off the floor with the hands: the arms lever the hips upward instead of anchoring the torso, so keep the palms flat and passive.
- Straightening the legs at the bottom: long levers arch the lumbar spine under the weight of the legs, and that extension is where low-back pain starts, so keep the knees bent.
- Stopping before the hips clear the mat: without pelvic tilt the rep is a hip-flexor hold, so lift until the tailbone leaves the floor.
- Dropping the legs back down: the eccentric is where most of the work sits, so take two counts down.
What Are the Best Lower-Ab Sit-Up Variations?
- Tuck reverse crunch: heels held close to the glutes, a shorter lever for lifters whose back arches.
- Bench reverse crunch: performed on a flat bench so the hips start below the ribs, adding range at the bottom.
- Weighted reverse crunch: a light dumbbell squeezed between the feet for extra resistance.
- Single-leg reverse crunch: one leg extended along the floor, which halves the load and exposes a weaker side.
- Hanging knee raise: the same pelvic curl from a pull-up bar, with the shoulders loaded instead of supported.
How Do You Progress the Lower-Ab Sit-Up?
- Add resistance only after 20 clean reps: squeeze a light dumbbell or ball between the feet and jump 5 pounds at a time.
- Build each set from 10 reps to 20 before adding load.
- Work from 2 sets up to 4 across a few weeks, keeping tempo unchanged.
- Slow the lowering phase from two counts to four, or start the rep with the legs further from the chest.
How Should You Program the Lower-Ab Sit-Up?
Place the lower-ab sit-up at the end of an abs or lower-body session, after squats, hinges and any loaded core work. Two or three sessions per week suits it, for 2 to 4 sets of 10 to 20 reps with 30 to 60 seconds of rest. Pair it with a crunch or cable crunch so the trunk also flexes from the top down.