Flexion Leg Sit up (straight arm)
- Target muscle
- Rectus Abdominis
- Synergist muscles
- Iliopsoas, Obliques, Quadriceps
- Equipment
- Body weight
- Body part
- Waist
- Type
- Strength
The flexion leg sit up (straight arm) is a body-weight core exercise that primarily targets the rectus abdominis, with the iliopsoas, obliques, and quadriceps assisting through the movement. Performed with your arms extended overhead, it trains trunk flexion and is a solid pick for building abdominal strength without any equipment.
How to do the Flexion Leg Sit up (straight arm)
- 1Lie flat on your back with your knees bent and your feet flat on the floor, roughly hip-width apart.
- 2Extend both arms straight overhead, keeping them in line with your torso and your shoulders relaxed.
- 3Brace your abdominals and tuck your chin slightly toward your chest.
- 4Curl up by flexing your spine, sweeping your straight arms forward as your shoulders and back lift off the floor.
- 5Continue sitting up until your torso is upright and your extended arms reach toward your feet.
- 6Pause briefly at the top while keeping tension in your abdominals.
- 7Lower yourself back down under control, rolling your spine onto the floor one vertebra at a time.
- 8Return your arms overhead to the start position and repeat for your target reps.
Form tips
- Move with control on the way up and down rather than using momentum, so the rectus abdominis does the work.
- Keep your arms straight and in line with your torso throughout to maintain a consistent lever and resistance.
- Exhale as you curl up and inhale as you lower to help brace your core and keep the rep smooth.
- Press your lower back toward the floor at the start of each rep to engage the abs before your hip flexors take over.
Common mistakes
- Yanking the arms forward to throw yourself up, which shifts the work off the abs and away from real trunk flexion.
- Pulling with the iliopsoas and quadriceps instead of flexing the spine, which strains the lower back and reduces ab engagement.
- Letting the back slam down on the way down instead of lowering under control, which wastes the eccentric portion of the rep.
- Bending the arms or dropping them out of line with the torso, which makes the movement easier and inconsistent.
- Jerking the chin and neck forward to start the rep, which strains the neck rather than working the core.
Frequently asked questions
What muscles does the flexion leg sit up (straight arm) work?
It primarily works the rectus abdominis, with the iliopsoas, obliques, and quadriceps acting as synergists to flex the trunk and stabilize the movement.
Why keep my arms straight overhead?
Holding your arms extended overhead lengthens the lever, making the sit up harder and increasing the demand on your rectus abdominis compared with arms crossed on the chest.
Is the flexion leg sit up (straight arm) good for beginners?
It can be, but the straight-arm position adds difficulty. Beginners can start with arms crossed on the chest and progress to the overhead position as core strength improves.
How many sets and reps should I do?
For most lifters, 2–4 sets of 10–20 controlled reps work well. Focus on full range and steady tempo rather than chasing high numbers.
Where should I feel this exercise?
You should feel it mainly in your abdominals (rectus abdominis). If you feel it mostly in your lower back or hip flexors, slow down and focus on flexing your spine rather than pulling with your hips.
