The leg raise, typically performed while lying on the back (supine leg raise), is a fundamental exercise involving lifting the legs against gravity. It is often perceived as the definitive way to target the “lower abs,” but this oversimplifies the complex biomechanics involved. The exercise is challenging because it requires a precise balance between movement and stabilization across multiple muscle groups. This analysis clarifies the anatomical targets of the leg raise, distinguishing between the muscles that move the legs and those that stabilize the core.
The Primary Core Muscle Targets
The primary intended target of the leg raise is the Rectus Abdominis, the long, superficial muscle extending vertically along the front of the abdomen. While its dynamic role is flexing the spine, its main function during a traditional leg raise is isometric stabilization. The Rectus Abdominis works intensely to prevent the lumbar spine from arching or hyperextending under the heavy load created by the legs acting as a long lever.
The common notion of isolating the “lower abs” is inaccurate because the Rectus Abdominis is a single muscle that contracts as a whole unit. However, electromyography (EMG) studies suggest that the lower fibers show higher activation when the pelvis is stabilized against the force of the legs being lowered. The degree to which the lower fibers are emphasized depends on successfully maintaining a posterior pelvic tilt, which is the slight curling of the pelvis toward the ribs.
The Obliques (external and internal) also play a significant, secondary role as synergistic stabilizers during the leg raise. They engage isometrically to resist rotational forces that could cause the torso to twist, ensuring the spine remains straight and centered. The Transverse Abdominis, the deepest core muscle, also contributes by acting like an internal corset to compress the abdominal contents and further stabilize the spine.
The Essential Role of the Hip Flexors
While the core muscles stabilize the torso, the actual work of lifting the legs is primarily performed by the hip flexor group. This group includes the powerful Iliopsoas, a collective term for the Psoas Major and the Iliacus muscles. The Iliopsoas originates on the lumbar spine and pelvis and inserts onto the femur, making it the main muscle responsible for hip flexion.
When the legs are lifted, the hip flexors are the prime movers, bringing the femur closer to the torso. The difficulty stems from the long resistance arm created by the extended legs, which places a high demand on these muscles. The Rectus Femoris, one of the quadriceps muscles, also crosses the hip joint and assists the Iliopsoas in this lifting action.
The frequent sensation of burning or fatigue deep in the lower abdomen is often incorrectly attributed to the abdominal muscles. This feeling is more accurately a sign of the Iliopsoas reaching muscular fatigue, especially the Psoas Major. If the core is not braced strongly enough, the hip flexors will pull the pelvis forward into an anterior tilt, causing the lower back to arch off the floor. This action shifts nearly all the work away from the Rectus Abdominis.
Optimizing Form and Targeting Specific Muscles
Effective execution requires maintaining constant, controlled tension in the abdominal wall, especially during the eccentric (lowering) phase. The most important form cue is ensuring the lower back remains pressed into the floor, which demands consciously engaging the core to maintain a slight posterior pelvic tilt. If the lower back begins to arch, the core stabilizers have failed, and the Iliopsoas is dominating the movement at the expense of spinal safety.
To maximize Rectus Abdominis activation, the range of motion should be limited to the point just before the lower back lifts from the floor. Beginners can significantly reduce leverage and strain on the hip flexors by performing bent-knee raises (knees bent to 90 degrees). This modification shortens the resistance arm, allowing the user to focus almost exclusively on stabilizing the pelvis with the core muscles.
For those seeking to increase core demand, the lowering phase must be performed slowly and with strict control, as this eccentric contraction is highly effective for muscle strengthening. Advanced variations, such as the hanging leg raise, introduce a dynamic element. The core must not only stabilize against hyperextension but also dynamically flex the spine to raise the legs higher than 90 degrees.
This final curl-up of the pelvis towards the chest is the true dynamic work for the Rectus Abdominis, transforming the movement from a pure hip flexion exercise into a powerful core flexor. The height to which the legs are lifted is less important than the degree to which the descent is controlled, especially in the final 45 degrees of the range of motion. Prioritizing spinal stability over the depth of the leg drop is the primary factor in shifting the exercise’s emphasis from the hip flexors to the abdominal muscles.