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 classic bodyweight core exercise that primarily targets the iliopsoas and rectus abdominis, with meaningful contribution from the obliques, quadriceps, sartorius, and tensor fasciae latae. Performed with the legs fully extended on the floor, it demands greater hip flexor involvement than bent-knee variations, making it effective for building anterior core and hip flexor strength together.
How to do the Straight Leg Sit-up
- 1Lie flat on your back on a mat with your legs fully extended and your feet together or hip-width apart.
- 2Extend your arms overhead, resting them on the floor, or cross them over your chest — choose a position and keep it consistent throughout the set.
- 3Brace your core and press your lower back lightly into the mat before you begin to move.
- 4Exhale and curl your chin toward your chest, then continue lifting your torso off the floor by contracting your rectus abdominis and engaging your iliopsoas.
- 5Rise until your torso is upright and your chest is close to your thighs, keeping the movement controlled and your legs flat on the floor.
- 6Hold the top position for a brief moment, squeezing the abs.
- 7Inhale and slowly lower your torso back to the starting position under control, vertebra by vertebra, until your back is flat on the mat.
- 8Reset your brace and repeat for the desired number of repetitions.
Form tips
- Initiate each rep by tucking the chin and curling the ribcage downward before pulling with the hip flexors — this ensures the abs do meaningful work rather than the iliopsoas doing all the lifting.
- Keep your feet actively pressed into the floor throughout; if they lift, the movement becomes dominated by hip flexion and loses core tension.
- Control the descent — lowering slowly increases time under tension and reduces momentum on the next rep.
- Breathe out on the way up and in on the way down to maintain intra-abdominal pressure and protect the lower back.
- If your lower back arches aggressively off the floor at the bottom, limit your range of motion or add a slight posterior pelvic tilt before descending.
Common mistakes
- Using momentum to throw the torso upward instead of contracting the abs, which shifts load away from the rectus abdominis and increases spinal stress.
- Pulling on the neck or head with the hands when arms are placed behind the head, which strains the cervical spine and reduces core engagement.
- Letting the feet lift off the floor during the ascent, which signals excessive hip flexor dominance and a loss of controlled core involvement.
- Lowering too quickly and bouncing at the bottom, which removes tension from the muscles and relies on elastic rebound rather than muscular control.
- Holding the breath throughout the rep, which increases unnecessary spinal loading — exhale on the way up to stabilize through the movement.
Frequently asked questions
What muscles does the straight leg sit-up work?
The primary muscles are the iliopsoas and rectus abdominis. The obliques assist with trunk rotation and stability, while the quadriceps, sartorius, and tensor fasciae latae act as synergists to keep the legs anchored and support hip flexion 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 of the lower body, which places greater demand on the iliopsoas to stabilize and flex the hip. Bent-knee sit-ups reduce hip flexor involvement and are generally considered easier on the lower back, making them a common starting point for beginners.
Are straight leg sit-ups bad for your lower back?
They carry a higher lumbar load than bent-knee variations because the extended legs increase hip flexor tension, which can pull on the lumbar spine. People with existing lower back issues should proceed cautiously, focus on a neutral pelvic position, and consider progressing through bent-knee sit-ups or crunches first.
How many sets and reps should I do for straight leg sit-ups?
For core strength, 3–4 sets of 10–15 controlled reps is a solid starting point. As technique improves you can increase reps to 20–25 or add a slow tempo (3–4 seconds on the descent) to increase difficulty without needing extra resistance.
Can I do straight leg sit-ups every day?
Like any strength exercise, the abs and hip flexors benefit from adequate recovery. Training them 3–4 days per week with at least one rest day between sessions allows for adaptation. Daily high-volume work can lead to hip flexor tightness and cumulative spinal fatigue over time.
