The V-up, also known as the jackknife sit-up, is an advanced bodyweight exercise that challenges the entire anterior core musculature. It requires abdominal strength, body control, and hamstring flexibility to perform correctly. Many people experience lower back pain when attempting V-ups, leading to questions about their safety. This discomfort is rarely due to the exercise itself, but rather a consequence of improper execution, a lack of prerequisite strength, or insufficient body awareness.
The Role of Spinal Flexion and Hip Flexor Dominance
The V-up requires the body to simultaneously perform trunk flexion and hip flexion. The movement recruits the rectus abdominis—the main muscle responsible for spinal flexion—and the powerful hip flexors. If the abdominal muscles are not adequately engaged, the hip flexors can become dominant in driving the movement.
The psoas major, a deep muscle that originates from the lumbar vertebrae, is the most powerful hip flexor. When core stabilizers are weak, the psoas major can pull excessively on the lumbar spine while attempting to lift the legs and torso. This increases compressive forces on the lower back, especially if the pelvis tilts forward or the lower back lifts off the floor. This imbalance loads the spine without sufficient core protection.
Identifying and Correcting Common Form Errors
The most frequent error contributing to back discomfort is allowing the lower back to arch or lift away from the floor, especially at the start and end of each repetition. This often happens when the abdominal wall fails to stabilize the pelvis, leading to aggressive hip flexor recruitment. Correct initiation involves actively pulling the navel toward the spine to create a slight posterior pelvic tilt, pressing the lumbar area into the floor.
Another common mistake is relying on momentum to complete the repetition, often seen as a swinging motion of the limbs. Using the speed of the arms and legs to propel the body upward reduces the time the core muscles spend under tension, decreasing the exercise’s effectiveness and control. The movement should be slow and controlled, especially during the eccentric phase. Failing to control the descent means the stabilizing muscles are not properly engaged, which can shock the spine.
To correct these issues, athletes should adopt a “hollow body” position, which is the foundational cue for the V-up. The hollow body involves a slight tuck of the pelvis and constant tension throughout the entire core, with the shoulders and feet slightly off the floor in the starting position. Maintaining this rigid, slightly curved spinal posture throughout the entire range of motion ensures the abdominal muscles are stabilized and working to flex the trunk, rather than relying on the hip flexors to pull on the spine. If full range of motion cannot be achieved with this core engagement, the movement should be scaled back, perhaps by bending the knees or reducing the distance the torso and legs lift.
Safer Core Alternatives for Spinal Health
If back pain persists even after correcting form, or if strength for a full V-up is lacking, several alternatives can build core control with less spinal load. These exercises focus on anti-extension—the core’s ability to resist the lower back from arching. This prerequisite strength protects the lumbar spine during more complex movements.
The Dead Bug exercise trains the deep core muscles to stabilize the trunk while the limbs are moving. It forces the lower back to remain pressed into the floor as opposing arm and leg extend, effectively teaching anti-extension and coordination. Similarly, the Hollow Hold is an isometric exercise that teaches a person to maintain the specific full-body tension required for a safe V-up.
Reverse Crunches are another effective way to target the lower abdominal muscles without excessive hip flexor involvement. By keeping the torso on the floor and curling the knees toward the chest, the movement emphasizes the part of the abdominal muscles responsible for posterior pelvic tilt. These alternatives allow the necessary core strength and stability to be developed safely before attempting the V-up.