It is widely believed the lower abs are exercised during the leg raise or other hip flexor exercises. However, it can be misleading to judge the mechanics of an exercise based on localized muscular fatigue. The primary muscle used in hip flexion is actually the Iliopsoas one of many hip flexors. The Iliopsoas, particularly the Psoas portion, happens to lie deep below the lower portion of the Rectus Abdominous. During the leg raise, the entire abdominal musculature isometrically contracts (contracts with no significant movement) to:
o Posture the spine and pelvis
+ Supports the weight of the lower body so the lumbar spine does not hyperextend excessively
+ Maintains optimal biomechanics of the Iliopsoas
# Hips are kept from prematurely flexing if the lumbar spine and pelvis does not hyperextend excessively
# Iliopsoas can contract more forcefully in a relatively slight stretched position
* Bent knee (and hip) sit-ups actually place Iliopsoas at a mechanical disadvantage
o Counteracts Iliopsoas's pull on spine
+ Many people with weak abdominal muscles are not able to perform hip flexor exercises without acute lower back pain or discomfort
The combination of the local muscular fatigue, or a burning sensation from the isometrically contracted abdominal muscles, and from the working hip flexors produces fatigue in the pelvis area which we mistakenly interpret as the lower portion of the Rectus Abdominous being exercised. In movements where the Rectus Abdominous does Isotonically contract (contracts with movement), it flexes the spine by contracting the entire muscle from origin to insertion. The spine is not significantly flexed during the leg raise. Incidentally, both the spine and hip flexes during the full range op motion Sit Up and Leg Hip Raise
