Straight Leg Sit-up
- Target muscle
- Iliopsoas, Rectus Abdominis
- Synergist muscles
- Obliques, Quadriceps, Sartorius, Tensor Fasciae Latae
- Equipment
- Body weight
- Body part
- Waist
- Type
- Strength
The straight leg sit-up is a bodyweight core exercise that targets the iliopsoas and rectus abdominis, with meaningful contribution from the obliques, quadriceps, sartorius, and tensor fasciae latae. Performed lying flat with legs fully extended, it demands greater hip flexor involvement than bent-knee variations and builds functional core strength through a full range of motion.
How to do the Straight Leg Sit-up
- 1Lie flat on your back on a mat with your legs fully extended and feet together, toes pointing toward the ceiling.
- 2Extend your arms overhead or cross them over your chest, depending on your preferred starting position.
- 3Brace your core and press your lower back toward the mat before initiating the movement.
- 4Exhale and curl your torso upward, leading with your chest rather than your chin, until your torso is roughly perpendicular to the floor.
- 5Keep your legs flat on the mat throughout — do not let your heels rise.
- 6Pause briefly at the top, feeling the contraction in your abs and hip flexors.
- 7Inhale and slowly lower your torso back to the mat under control, resisting the urge to drop quickly.
- 8Reset your brace and repeat for the desired number of reps.
Form tips
- Initiate the movement by curling your spine — think chest to knees, not head to knees — to keep the rectus abdominis engaged rather than relying entirely on the hip flexors.
- Keep your feet in contact with the floor throughout; if they lift, reduce range of motion or anchor them under a stable object.
- Control the lowering phase — a slow eccentric builds more core strength than dropping back down quickly.
- Fix your gaze slightly upward and forward on the way up to maintain a neutral neck position and avoid straining it.
Common mistakes
- Jerking the head and neck forward to initiate the sit-up, which strains the cervical spine and takes tension off the abs.
- Letting the heels rise off the floor, which reduces stability and shifts the workload away from the intended muscles.
- Using momentum to swing up rather than controlled muscular effort, shortening the effective range where the core is loaded.
- Holding your breath throughout the rep, which raises intra-abdominal pressure unnecessarily — exhale on the way up, inhale on the way down.
- Hyperextending the lower back at the bottom of each rep instead of maintaining a neutral spine, which can place excessive stress on the lumbar discs.
Frequently asked questions
What muscles does the straight leg sit-up work?
The primary muscles are the iliopsoas and rectus abdominis. The obliques, quadriceps, sartorius, and tensor fasciae latae assist throughout the movement.
What is the difference between a straight leg sit-up and a bent-knee sit-up?
Keeping the legs straight increases the lever arm and demands more work from the hip flexors (iliopsoas) compared to the bent-knee version. This makes it more challenging but also means the hip flexors may fatigue before the abs do.
Is the straight leg sit-up safe for my lower back?
It can be performed safely with proper form — brace your core, avoid jerking movements, and control the descent. If you have existing lower-back issues, consult a health professional first, as the increased hip flexor demand can pull on the lumbar spine.
How many reps should I do?
Beginners can start with 2–3 sets of 10–15 reps with full control. As your core strength improves, increase reps or add sets. Prioritize quality over quantity — a slow, controlled rep is worth more than several rushed ones.
Where should I put my hands during a straight leg sit-up?
Arms crossed over your chest is the standard starting position. Placing your hands behind your head is common but can tempt you to pull on your neck — if you do this, keep your elbows wide and let your abs do the work.
