Jack Knife Sit-Up

Jack Knife Sit-Up Exercise Description

The Jackknife Sit-Up (often called a V-up) is a bodyweight core exercise that combines simultaneous trunk flexion and hip flexion to bring your hands and feet toward each other in a “V” position. It primarily targets the rectus abdominis while the hip flexors (iliopsoas and rectus femoris) lift the legs; the internal and external obliques assist with trunk control and pelvic stability. The movement challenges lumbopelvic control by asking the anterior core to generate and resist motion without relying on momentum. Quality execution emphasizes a neutral neck, ribs down, and smooth, controlled lowering to avoid excessive lumbar extension. Range of motion and leverage can be adjusted by bending the knees or modifying arm position to match your current control and flexibility.

Muscle Groups: Abs, Core, Waist
Target Muscles: Rectus Abdominis
Assisting Muscles: External Obliques, Iliacus, Internal Obliques, Psoas Major, Rectus Femoris
Stabilizing Muscles: Multifidus, Quadratus Lumborum, 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: Female

How To Do The Jack Knife Sit-Up Exercise

‘- Lie supine on a mat with legs straight and together; arms extended overhead, biceps near ears.
– Set ribs down and lightly posteriorly tilt the pelvis to bring the low back toward the floor; brace the abdomen.
– Keep the neck long with a slight chin tuck; avoid shrugging the shoulders.
– Exhale and simultaneously flex the trunk and hips: lift shoulder blades off the floor while raising straight legs.
– Reach hands toward shins or ankles, forming a controlled “V” without yanking the neck or swinging the legs.
– Pause briefly at the top with balanced control on the sit bones.
– Inhale and lower the torso and legs together with control, maintaining rib position and avoiding an arch in the low back.
– Keep movement smooth—no bouncing or throwing the arms.
– Options: bend the knees to shorten the lever, keep hands by your sides for balance, or lift one leg at a time.
– Work within a comfortable, pain-free range and stop if form deteriorates.

Jack Knife Sit-Up Tips

‘- Set your ribs down and lightly posteriorly tilt the pelvis so your low back moves toward the floor before you start.
– Keep your neck long with a slight chin tuck and avoid shrugging your shoulders as you move.
– Exhale as you simultaneously lift your shoulder blades and legs, flexing the trunk and hips together.
– Reach your hands toward your shins or ankles without yanking on your neck or swinging your legs.
– Pause briefly at the top, balancing on your sit bones with controlled, steady tension.
– Inhale as you lower your torso and legs together, keeping your ribs down and avoiding an arch in the low back.
– Keep the movement smooth and controlled—do not bounce or throw your arms for momentum.
– Bend your knees to shorten the lever if you need an easier range of motion.
– Use options like keeping hands by your sides for balance or lifting one leg at a time to match your current control.

Jack Knife Sit-Up Beginner Introduction

In the Jackknife Sit-Up (often called a V-up), the upper and lower body rise toward each other so the body folds into a V-like shape. This bodyweight move centers on the abs, with strong help from the hip flexors to raise the legs, while the side abs assist in guiding the torso and pelvis. The motion blends trunk bending and hip bending at the same time, creating a demanding challenge for the front of the core to move the body and also stabilize it, instead of relying on fast, swinging momentum.

The exercise highlights smooth, deliberate movement both on the way up and down, along with a relaxed neck and steady ribcage and pelvis. Control at the bottom of the repetition is just as important as the lift, especially around the lower back area. The overall difficulty can shift by changing leverage: bending the knees, changing arm placement for extra balance, or lifting a single leg instead of both are common ways to match the movement to current strength, coordination, and flexibility.