Although many exercisers still use them, leg lifts are a contraindicated exercise. The idea that lifting ones legs off the ground for abdominal development is flawed at the biomechanical level. The rectus abdominis connects the sternum to the pelvis. The abdominals do not attach to the femur; therefore the contraction of the abdominal musculature will pull on the pelvis in the supine position – not the legs. The proper contraction of the abdominals in the supine position will cause a posterior pelvic tilt as the pelvis rotates toward the sternum. This is commonly seen during the abdominal curl-up, or crunch as it is better known. The limited range of motion employed, approximately 30° of spinal flexion, is driven via isolated contraction of the rectus abdominis. To the contrary, when leg lifts are employed the abdominals take on only an assistive role as the hip flexors take over as the prime mover. The iliopsoas, or hip flexor, is actually a muscle group comprised of the iliacus and psoas major (the psoas minor attaches to the pelvis). Additionally, the rectus femoris also crosses the hip and serves the dual role of hip flexion and knee extension.
The long resistance arm created by the extended legs during leg lifts creates resistive torques that pull on the attachment sites of the hip flexors. Under this resistance the attachment site of the psoas major pulls the lumbar spine into an arch while the concurrent pull of the iliacus on the iliac spine further increases the lordotic curve due to an increasing anterior pelvic tilt. When in this position, the abdominals cannot serve as a mover until the pelvis moves posteriorly. Therefore, supine leg lifts are a (contraindicated) hip flexor exercise that places unnecessary strain on the structures of the spine. One caveat to the supine exercise is once the legs reach a position that allows for posterior pelvic rotation the abdominals contribute to the movement. Therefore, in a supine position a common adjustment is to flex the knees and hips to 90 degrees as a starting point and the action more efficiently addresses the abdominals, albeit through a limited range of motion.
A better way to properly address the abdominals using the distal load of the legs is to perform the exercise from a hanging position. Again, the movement requires a posterior pelvic tilt, but due to the removal of the forced fulcrum created by the ground, the abdominals gain a level of mechanical advantage. Simply hanging and flexing the hips to 90 degrees is a hip flexor exercise, but once the pelvis rotates backward, the abdominals are responsible for the brunt of the forces. The exercise from the hanging position creates a closed chain environment which increases stability requirements and often explains the “swinging” action in those who cannot properly maintain position. Modification to the hanging requirements are commonly seen when exercisers use elbow supports by a machine or the bar straps which provide mechanical advantage and help reduce the stress placed on the shoulders particularly for those with tight lats. The exercise begins from an extended body position and is completed when the knees reach the height of the chest. Whether using extended or flexed knees the emphasis is placed on pelvic rotation rather than hip flexion. Common errors occur when incomplete movements are used or when momentum is generated from hip flexion or swinging actions dominate the force production through rotational inertia.