Back flexion, alternately known as trunk or spinal flexion, is the act of curling the spine forward, as one does during an abdominal crunch. This motion is facilitated by a complex system of core musculature, as supported by bones and ligaments. Unlike flexion of the knee or elbow joint, back flexion occurs not at a single joint but at a number of intervertebral joints, all acting in tandem to curve the spine forward.
To produce back flexion requires a contraction of the abdominal muscles. The primary flexor of the spine is the rectus abdominus or “six-pack” muscle, which runs longitudinally from the base of the ribcage to the front of the pelvis. Assisting in spinal flexion are the internal obliques, which lie beneath the rectus abdominus and run diagonally from the middle of the rib cage to the sides of the pelvis. To a lesser extent this movement is supported by the external obliques, which run perpendicular to the internal obliques but are located between the rectus abdominus and the internal obliques in the abdominal wall. Both the external and internal obliques are also involved in trunk rotation, or twisting of the spine, and lateral flexion, or side-bending of the spine.
The most effective exercises for producing back flexion — and therefore strengthening the abdominal muscles — are crunches and sit-up exercises, or any movement that requires a forward curling of the spine against resistance. These exercises may be prescribed for individuals experiencing back pain, for in the absence of acute back injury, muscles weakened by sitting such as the abdominal muscles and glutes may be strengthened to relieve pain and stress on the lower back. Exercise experts often eschew crunch machines, which may encourage poor technique, in favor of movements that utilize internal resistance.
Internal resistance, or bodyweight, exercises may include variations on crunches such as the stability ball crunch. An oft-recommended exercise for its activation of the deeper abdominal muscles as well as the rectus abdominus, the ball crunch involves lying on one’s back on a stability ball with feet planted on the floor. Placing the hands lightly behind the head to support the neck but without pulling forward on the head, one should draw in the abdominal muscles and curl the shoulder blades off the ball, lifting the chest toward the ceiling. Pausing at the top, one should slowly lower back down until the back is extended and abs stretched slightly before repeating.
Another back flexion exercise that works the abdominal muscles is the crunch with legs elevated. For this movement, one should lie on the floor with both legs up in the air and knees bent up to 90 degrees, keeping the low back pulled down toward the ground. With hands placed lightly behind the head, one should draw in the abdominal muscles and curl the shoulder blades off the floor, pausing at the top and slowly lowering to the start position. Experts recommend exhaling on the lifting portion and inhaling on the lowering portion of any crunch exercise.