TETSUAssisted Sit-Up vs Flexion Leg Sit Up (Straight Arm)
The Assisted Sit-Up and the Flexion Leg Sit Up (Straight Arm) both train the abs as their primary muscle. The Assisted Sit-Up is an assisted movement while the Flexion Leg Sit Up (Straight Arm) is bodyweight-only. Here is how they compare and when to pick each.
Side-by-side comparison
| Assisted Sit-Up | Flexion Leg Sit Up (Straight Arm) | |
|---|---|---|
| Primary muscle | abs | abs |
| Body part | waist | waist |
| Supporting muscles | hip flexors | hip flexors and quadriceps |
| Equipment | assisted | bodyweight |
| Steps in our guide | 6 | 5 |
How each one starts
Assisted Sit-Up — first steps
- Sit on the edge of a bench or have someone hold your feet down.
- Lie flat on your back with your knees bent and feet flat on the ground.
Flexion Leg Sit Up (Straight Arm) — first steps
- Lie flat on your back with your legs extended and arms straight above your head.
- Engaging your abs, lift your upper body off the ground while simultaneously lifting your legs towards your chest.
Full step-by-step guides are linked below.
Which should you choose?
The Assisted Sit-Up reduces the load so you can own the movement before going unassisted. The Flexion Leg Sit Up (Straight Arm) needs no equipment and scales through leverage, tempo and reps. If your gym has both options, neither is a wrong choice: pick the one that feels strongest through a full range, and swap after 6 to 8 weeks to keep the abs adapting.
Full form guides
Common questions
Is the Assisted Sit-Up better than the Flexion Leg Sit Up (Straight Arm)?
Neither is universally better — both train the abs as the primary mover. The Assisted Sit-Up reduces the load so you can own the movement before going unassisted, while the Flexion Leg Sit Up (Straight Arm) needs no equipment and scales through leverage, tempo and reps. Choose based on your equipment and which movement you can progress consistently.
Can I do both the Assisted Sit-Up and the Flexion Leg Sit Up (Straight Arm) in the same program?
Yes, but usually not in the same session — they overlap heavily on the abs. A common setup is one as your main movement and the other on a second training day, or alternating them block by block.
Do the Assisted Sit-Up and the Flexion Leg Sit Up (Straight Arm) work the same muscles?
The primary muscle is the same: the abs. Supporting muscles differ slightly — hip flexors for the Assisted Sit-Up versus hip flexors and quadriceps for the Flexion Leg Sit Up (Straight Arm).
Track it in Tetsu
Tetsu tracks both movements separately, keeps your PRs per exercise, and tells you which one is actually progressing. Free to start.