The standard leg raise, whether performed lying down or while hanging, is a popular exercise often associated with building the abdominal muscles. While it is an effective core exercise, the primary muscles responsible for the actual lifting motion are not the obliques, which is a common misunderstanding. The standard, non-rotated movement mainly targets the muscles that flex the hip joint, leaving the obliques to function in a secondary, supporting role.
The Primary Function of the Oblique Muscles
The abdominal wall includes two main oblique muscle groups: the internal and external obliques, positioned on the sides of the torso. These muscles are named for the diagonal direction of their fibers. Their main job is to produce movement in the transverse and frontal planes, meaning they are built for twisting and side-bending actions. When working unilaterally, the obliques cause lateral flexion, or side bending, of the trunk. They also work together to facilitate trunk rotation, with the external oblique on one side partnering with the internal oblique on the opposite side to execute a twist. This inherent function explains why a straight, vertical leg raise is not their ideal stimulus.
Biomechanics of the Standard Leg Raise
The motion of lifting the legs toward the torso is fundamentally a movement of the hip joint. The largest muscles performing this work are the hip flexors, primarily the iliopsoas and the rectus femoris, which is one of the quadriceps muscles. These muscles pull the femur toward the pelvis, creating the arc of the leg raise.
The abdominal muscles, including the rectus abdominis, are involved, but their role is mostly isometric. This means they contract without significantly changing length, working to stabilize the pelvis and spine. This stabilization prevents the lower back from arching or extending as the weight of the legs is lifted.
The obliques contribute to this stabilization, helping to maintain torso rigidity, but they are not the prime movers. For hypertrophy, a muscle requires a dynamic, challenging contraction through a full range of motion. Since a standard leg raise involves minimal dynamic contraction of the obliques, it is not an efficient way to train them for size.
Modifying the Movement for Oblique Activation
To shift the focus toward the obliques, the movement must incorporate either rotation or lateral flexion. Introducing a controlled twist forces the obliques to contract dynamically rather than just isometrically. This alteration changes the exercise from a simple hip flexion movement to a compound action involving the trunk.
One effective modification is the hanging oblique raise, where the knees are lifted and simultaneously rotated toward one elbow. The “windshield wiper” is another variation, where the legs are raised and then swung from side to side in a controlled arc. These rotational movements directly engage the obliques, leveraging their primary function as torso rotators.
Superior Oblique Exercises for Hypertrophy
For maximum oblique development, exercises that center on the side-bending and twisting functions of the muscle are more effective. Exercises involving loaded rotation, like the cable woodchop, require the obliques to powerfully twist the torso against resistance. The Russian twist is another staple that forces the obliques to repeatedly contract and control rotation.
Side planks are a highly effective isometric choice that builds endurance and stability in the lateral core muscles. Introducing a side crunch or side-bending element, such as a dumbbell side bend or a side plank with a hip dip, directly targets the obliques through their full range of lateral flexion. These movements provide a more direct stimulus for the internal and external oblique muscles than a standard leg raise.