Frog Sit-Ups
You lie on your back with the soles of your feet pressed together and your knees splayed wide toward the floor, then curl the torso up a quarter of the way. The frog leg position is the whole trick: hips turned out and abducted, the hip flexors lose their leverage, so the abs cannot hand the work off.
Own the movement
Set up first, then execute. Every cue below is something you can check on your next set.
Before the first rep
- Lie flat, bring the soles of the feet together and let the knees fall wide until the outer thighs are near the floor.
- Slide the heels up toward your groin so the legs form a diamond — closer heels means less hip flexor involvement.
- Cross your arms over your chest, each hand on the opposite shoulder. Keep them there; do not pull on your neck.
- Flatten your lower back into the floor and hold it there before the first rep.
Every rep
Breathing & tempo — Exhale all the way out as you curl up, which is what lets the ribs drop and the spine flex; inhale on the way down. Two seconds up, one-second squeeze, three seconds down.
Sets, reps & load
Where to start, and what to change when it stops being hard.
Bodyweight, arms crossed on the chest, is the default. When 20 reps with a one-second squeeze gets easy, move the arms overhead to lengthen the lever, then hold a 2.5-5 kg plate on the chest. Add weight rather than reps once you pass 25 — beyond that you are training endurance you probably do not need.
What goes wrong
The errors that show up most often, and the cue that fixes each one.
Set a target: shoulder blades an inch off the floor, then stop. Have someone film one set from the side, or place a rolled towel under your mid-back so you can feel where the range should end.
Press the outer knees actively toward the floor throughout, or wedge a light dumbbell against each outer thigh so any drift is obvious. Stop the set when you cannot hold the splay.
Hold a tennis ball, or an imaginary one, under your chin and keep the gap constant. The neck should travel with the ribcage, not ahead of it.
Pause a second at the bottom with the shoulder blades just touching down, then start the next rep from stillness. Reps per minute is not the metric here.
Why bother
In a normal sit-up the psoas and rectus femoris pull the torso up from about 30 degrees onward, and the abs coast. Opening the hips into external rotation and abduction slackens those muscles and shortens their moment arm, so they contribute far less. What is left is a genuine spinal flexion rep, which is what you came for.
The range is deliberately short — roughly the first quarter of a sit-up, shoulder blades clearing the floor and no more. Past that point you are hinging at the hip, not flexing the spine, and the whole reason for the frog position disappears.
Where it fits
A good choice for anyone whose sit-ups turn into hip flexor work, or whose lower back complains about anchored sit-ups. Two or three times a week at the end of a session. Pair it with a leg raise or reverse crunch so you cover trunk flexion from both ends — this one pulls the ribs down, those pull the pelvis up.
If this one isn't it
No kit, too hard, or too easy — here's where to go instead.
Same job, different kit
Start here instead
When this gets easy
Asked and answered
Why put my feet together instead of flat on the floor?
Because feet flat on the floor gives the hip flexors something to pull against, and they will take over the moment the abs start to fatigue. Splaying the hips into external rotation and abduction takes that leverage away. The set gets harder at the same rep count, which is the point.
My hips are too tight to get my knees near the floor. What now?
Put a cushion or a rolled towel under each outer thigh so the knees rest somewhere rather than hovering, and slide the heels a bit further from your body. You do not need the knees on the floor; you need them wide enough that the hip flexors are slack.
Is a short-range crunch really enough to build abs?
Yes, if you load it. The rectus abdominis only has a few centimetres of shortening available between the ribcage and the pelvis — that short range is its full range. What builds it is tension and progressive load, not how far your shoulders travel.
Can I do these every day?
You can, but there is no reason to. The abs recover like any other muscle: two or three hard sessions a week with a day between them will grow them faster than daily sets of 50. Daily high-rep work mostly trains you to tolerate the burn.