Jack Knife Sit Up
Jack Knife Sit Up Exercise Description
The jackknife sit-up is a bodyweight trunk flexion and hip flexion exercise that trains the anterior core. From a supine position, you lift the legs and torso together to meet in a “V,” then return under control. The primary driver is the rectus abdominis, with significant help from the hip flexors (iliopsoas and rectus femoris); the external and internal obliques assist with trunk stabilization and pelvic control. Because both ends of the body move, this variation increases the lever arm and demands precise lumbopelvic control to avoid excessive lumbar extension at the bottom or collapsing at the top. When performed with a neutral cervical spine, a braced abdominal wall, and smooth, simultaneous movement of the legs and torso, it builds coordinated anterior core strength without external load.
- Muscle Groups: Abs, Core, Waist
- Target Muscles: Rectus Abdominis
- Assisting Muscles: Iliacus, Psoas Major, Rectus Femoris
- Stabilizing Muscles: External Obliques, Internal Obliques, Transversus Abdominis
- Body Area: Upper Body
- Body Parts: Core, Torso
- Body Surface: Front
- Body Symmetry: Bilateral
- Equipment: Bodyweight
- Exercise Patterns: Other
- Exercise Type: Strength
- Functional Grouping: Trunk Flexors
- Joint Involvement: Multi-joint
- Muscle Location: Stomach
- Movement Direction: Vertical
- Muscle Position: Anterior
- Gender: Male
How To Do The Jack Knife Sit Up Exercise
‘- Lie supine on a mat. Legs extended together, toes up. Arms overhead with biceps by ears.
– Set ribs down and lightly posteriorly tilt the pelvis so the low back contacts the floor.
– Brace the abdominal wall 360° and keep the neck neutral (chin slightly tucked).
– Exhale as you initiate. Lift shoulder blades from the floor while raising straight legs together, flexing at the hips.
– Reach hands toward shins/ankles, bringing torso and legs to form a controlled “V” on the sit bones.
– Keep shoulders away from ears; avoid shrugging. Maintain even, steady breathing.
– Pause briefly with a long spine—do not round the neck or collapse the chest.
– Lower torso and legs together with control until shoulder blades and heels lightly touch down.
– Throughout, prevent the low back from arching hard off the floor; stop short if ribs flare or control is lost.
– Keep knees softly extended (not locked) and movements smooth—avoid swinging the arms or using momentum.
Jack Knife Sit Up Tips
‘- Before lifting, set the ribs down and lightly posteriorly tilt the pelvis so the low back contacts the floor.
– Brace the abdominal wall 360° and keep the neck neutral with the chin slightly tucked.
– Exhale as you initiate the movement.
– Lift the shoulder blades from the floor while raising the straight legs together, moving the torso and legs simultaneously.
– Reach the hands toward the shins or ankles, forming a controlled “V” on the sit bones.
– Keep the shoulders away from the ears to avoid shrugging.
– Pause briefly at the top with a long spine, without rounding the neck or collapsing the chest.
– Lower the torso and legs together with control until the shoulder blades and heels lightly touch down.
– Throughout, prevent the low back from arching hard off the floor and stop short if the ribs flare or control is lost.
– Keep the knees softly extended (not locked) and make movements smooth, avoiding arm swinging or momentum.
Jack Knife Sit Up Beginner Introduction
The jackknife sit-up is a challenging bodyweight core move that brings the upper and lower body together at the same time. It blends bending at the waist with lifting through the hips, so the abs work hard as both the shoulders and legs travel off the floor. The main effort comes from the front of the midsection, with strong support from the hip flexors and side abs to steady the trunk and guide the pelvis. Because both ends of the body travel equally, the midsection acts like a control center for the whole movement.
During a smooth jackknife, the lifter starts on their back, then raises the torso and straight legs together until they meet in a controlled “V” shape, before returning to the ground in unison. The emphasis is on moving as one solid piece rather than snapping or swinging, which increases the demand on the lower back and hips to stay stable. Attention to a long spine, quiet shoulders, and a relaxed neck helps the exercise target the front of the core while building coordination and precise body awareness.