The Jackknife Sit-Up, often called a V-up, is a challenging bodyweight exercise that engages the entire midsection by requiring simultaneous movement of the upper and lower body. It is an advanced core movement that pushes beyond the range of a standard sit-up, demanding high levels of coordination and muscular endurance. This exercise is named for the way the body folds in the middle, resembling a closing pocketknife.
Defining the Movement and Targeted Muscles
The Jackknife Sit-Up is fundamentally a full-body flexion that articulates at the hips, folding the torso and legs toward the body’s midline. This movement intensely targets the Rectus Abdominis, the long, flat muscle that runs vertically down the front of the abdomen and is responsible for flexing the trunk. The simultaneous raising of the legs places a substantial demand on the lower section of this muscle.
Several other muscle groups play an important role. The deep-lying Hip Flexors, including the iliopsoas and the rectus femoris, are recruited to lift the legs against gravity. Stabilization is maintained by the Obliques (internal and external) and the deepest core muscle, the Transverse Abdominis, which prevent excessive rotation and maintain a neutral spine. Controlling two separate limbs simultaneously makes the Jackknife Sit-Up an intermediate-to-advanced movement.
Proper Execution Step-by-Step
To begin the Jackknife Sit-Up, lie flat on your back on a cushioned surface. Extend your legs straight and reach your arms overhead, keeping them parallel to the floor. Ensure your lower back is in a neutral position, avoiding excessive arching or pressing it forcefully into the mat.
Initiate the movement by engaging your abdominal muscles and simultaneously lifting your legs and torso off the floor. As you rise, keep your legs straight and bring your arms forward, reaching toward your shins or toes. This contraction should fold your body into a distinct “V” shape, with your hips acting as the hinge point. Exhale forcefully during this upward movement to maximize the abdominal contraction.
Briefly pause at the peak of the contraction, maintaining tension in the core. The descent should be slow and controlled, maximizing muscle engagement and protecting the spine. Slowly lower your torso and legs back down toward the starting position while inhaling. Control the entire movement, avoiding the tendency to drop quickly, and stop just short of letting your arms and legs completely rest on the floor before initiating the next repetition.
Common Mistakes and Modifications
A frequent error is relying on momentum, swinging the arms or legs to generate upward force rather than using core strength. Rushing repetitions shortens the range of motion and prevents full muscle engagement, especially during the controlled, eccentric phase of the descent. Another fault is pulling on the neck with the hands, which can strain the cervical spine; the hands should only guide the arms toward the feet.
Focus on a slow, deliberate tempo, which forces the core to stabilize the movement throughout. If the full extension is too difficult, beginners can modify the exercise by performing Jackknife Knee Raises, which involves bending the knees and bringing them toward the chest instead of keeping the legs straight. This reduces the leverage and strain on the hip flexors.
To increase the challenge, the movement can be progressed by holding a light weight, such as a small dumbbell or plate, or by wearing ankle weights. Another advanced variation is performing the exercise with the upper and lower body hovering a few inches off the floor in the starting position, maintaining constant tension. Maintaining control over the entire range of motion is paramount for building true core strength.