The jackknife workout, often referred to as a V-Up, is an advanced exercise designed to maximize core strength and stability. It requires the simultaneous lifting of the upper torso and the legs from a supine position. The goal is to bring the upper and lower halves of the body together to meet near the center, forming a distinct V-shape. This folding action effectively targets the entire abdominal wall, making it a highly efficient move for core conditioning.
Anatomy of the Movement
The primary muscle group driving the jackknife movement is the rectus abdominis, the superficial muscle responsible for the visible “six-pack” appearance. This muscle facilitates spinal flexion, or the bending of the torso toward the hips, as the shoulders lift off the floor. The folding action also heavily recruits the hip flexors, specifically the iliopsoas group, which are responsible for raising the legs toward the midsection.
The obliques, the side abdominal muscles, engage to stabilize the torso and prevent unwanted rotation during the lift. The deeper transversus abdominis muscle contracts throughout the movement to increase intra-abdominal pressure, which supports the spine. The shoulders and latissimus dorsi muscles are also engaged to help anchor the upper body and control the movement, especially when performing the exercise with arms extended overhead.
Proper Step-by-Step Execution
To perform the standard floor jackknife, begin by lying flat on your back on an exercise mat, extending your arms straight overhead and keeping your legs straight. Before initiating the lift, engage your core by drawing your navel toward your spine to create a neutral spine position. This helps protect the lower back and ensures the abdominal muscles are activated.
As you exhale, simultaneously lift your upper body and legs off the floor in a controlled manner. Keep your arms and legs as straight as possible, reaching your hands toward your feet or shins. The peak contraction occurs when your torso and legs are raised at about a 45-degree angle to the floor; briefly hold this position.
Inhale as you slowly lower both your torso and legs back down to the starting position. Maintain control during the descent, resisting the urge to let gravity drop your limbs quickly. For maximum muscle engagement, keep your heels and shoulder blades hovering slightly above the floor before beginning the next repetition.
Modifications and Safety Considerations
Modifications can make the jackknife more accessible for those still building core strength. A bent-knee jackknife, where the knees are drawn toward the chest, significantly reduces strain on the hip flexors and lower back. Progression steps also include performing only the upper-body crunch portion or a single-leg jackknife, lifting one leg at a time toward the opposite hand.
Advanced variations increase the challenge, such as holding a light weight for a weighted jackknife or using a stability ball. The stability ball jackknife is performed in a plank position with the shins resting on the ball, then rolling the ball inward by flexing the hips and knees, which introduces instability.
To avoid cervical strain, do not pull on the neck or tuck the chin into the chest. Focus on leading the movement with your chest and maintaining a neutral gaze. If you feel excessive back arching or pain in the lumbar region, your core strength may be insufficient for the straight-leg version; revert to a bent-knee modification. Always prioritize a slow, controlled tempo, as momentum reduces muscle engagement and increases the risk of injury.