Jack Split Crunches
- Target muscle
- Iliopsoas, Rectus Abdominis
- Synergist muscles
- Adductor Brevis, Adductor Longus, Gluteus Maximus, Obliques, Pectineous, Quadriceps, Sartorius
- Equipment
- Body weight
- Body part
- Waist
- Type
- Strength
Jack Split Crunches are a bodyweight core exercise that combines a crunch with a simultaneous scissor-split leg motion, making them a dynamic challenge for the hip flexors (iliopsoas) and abs (rectus abdominis). The split leg action also recruits the inner-thigh adductors, quads, and obliques as synergists. They are an effective tool for building core endurance and hip-flexor coordination without any equipment.
How to do the Jack Split Crunches
- 1Lie flat on your back on a mat with your legs extended together and your arms resting at your sides or lightly supporting your head with your fingertips.
- 2Engage your core by drawing your navel toward your spine and pressing your lower back gently into the mat.
- 3Simultaneously lift your shoulders off the mat into a crunch and raise both legs a few inches off the floor.
- 4In one controlled movement, split your legs apart as wide as comfortable — like the bottom of a jumping jack — while holding the crunch position.
- 5Bring your legs back together with control, keeping them hovering just above the floor.
- 6Lower your shoulders back toward the mat in a controlled descent, without fully releasing tension at the bottom.
- 7Repeat for the target number of reps, maintaining a steady breathing rhythm — exhale on the crunch and split, inhale on the return.
Form tips
- Keep your lower back pressed firmly into the mat throughout the movement to protect your lumbar spine and ensure the abs — not the hip flexors alone — do the work.
- Move your legs and torso at the same time so the crunch and split happen in sync, maximizing core engagement.
- Avoid pulling on your neck with your hands; use your fingertips only for light support and drive the crunch from your upper abs.
- Control the return phase — resisting gravity on the way down builds more total-rep tension than simply dropping back to the start.
- If you feel your lower back arching off the mat, reduce the range of leg split or raise your legs slightly higher until core strength improves.
Common mistakes
- Letting the lower back arch off the mat during the leg split, which transfers load away from the abs onto the lumbar spine and increases injury risk.
- Jerking the head and neck forward instead of curling through the thoracic spine, which strains the cervical spine and reduces abdominal activation.
- Rushing through the reps with momentum rather than controlled muscle action, reducing tension on both the abs and hip flexors.
- Allowing the legs to drop all the way to the floor between reps, which releases core tension and makes each repetition less effective.
- Splitting the legs so wide that the lower back loses contact with the mat, sacrificing spinal stability for range of motion.
Frequently asked questions
What muscles do Jack Split Crunches work?
They primarily target the hip flexors (iliopsoas) and abs (rectus abdominis). The inner-thigh adductors (adductor brevis and longus), obliques, quads, sartorius, and pectineus all assist during the split leg motion and crunch.
Are Jack Split Crunches suitable for beginners?
Yes, with a modification: beginners can reduce the range of the leg split and keep the legs higher off the floor to ease the load on the lower back. Build up gradually as core strength improves.
How do Jack Split Crunches differ from regular crunches?
Regular crunches isolate the abs with no lower-body involvement. Jack Split Crunches add a simultaneous scissor leg motion that also challenges the hip flexors, adductors, and quads, making them a more demanding full-core exercise.
How many reps should I do per set?
For core endurance, aim for 3–4 sets of 15–25 reps with a short rest. For a conditioning circuit, perform them for time (20–40 seconds per set) at a controlled, rhythmic pace.
Why do I feel this more in my hip flexors than my abs?
This usually means your lower back is arching off the mat. Focus on pressing your lumbar spine into the floor before and during each rep. If the arching persists, raise your legs higher or reduce the split range so your abs can maintain tension throughout.
