From:
www.exrx.net
Incline Situp
During the first portion of the sit up, the abdominal muscles flex the spine. The hip flexor muscles flex the hip to complete the movement. If the abdominal muscles are not strong enough to counter the Psoas' pull on the spine, the lumbar vertebrae can be forced into hyperextension. This can occur during other hip flexor movements as well.
In the book "Strength Training for Young Athletes" by Kraemer and Fleck, the cover states "Includes over 100 safe exercises for 18 muscle groups and 16 sports. They include two sit up exercises with the feet anchored
o Page 105: Bent-Leg Sit-up
o Page 106: Bent-Leg Sit-up with a Twist (on incline)
Steven J. Fleck, PhD and William J. Kraemer, PhD are probably the most well respected scientists studying resistive training. Fleck and Kreamer have dedicated their careers in investigating researching, and writing both scientific and main stream publications on weight training.
Kreighbaum (1996) states: "The physical condition of the performer dictates how safe and effective these exercises will be in the strengthening the abdominal"
For those with no history of lower back pain during hip flexion, situps or leg-hip raises can be considered so the abdominal and hip flexors can be exercised in a single exercise. A determination should be made if the client has adequate abdominal strength to counter the psoas' pull. Like the lower back integrity previously discussed, this biomechanical deficiency can be easily corrected (see links immediately below). Crunches, or half sit ups can be prescribed for the first months before the introduction of hip flexor movements.
Incidentally, the bend of the hip severally diminishes Psoas' mechanical efficiency and consequently its pull on the lumbar spine. See tension potential. Kreighbaum (1996) adds: "For a performer with weak abdominal, the hip-flexed position is the best".
Also see
o Abdominal Weakness
o Hip Flexor Inflexibility
o Lordosis
o Lower abdominal myth
Flexible hip flexors and strong abdominal muscles are particular important before performing the Decline Situp, Roman Chair Situp, Leg Raises and Hip Raises since the hip becomes fully extended.
As for having your feet anchored effecting movement from the origin rather than the insertion, this essentially does not effect its activation. The muscle contracts through out itself despite the end that moves (origin or insertion). Are the forces through the Latissimus Dorsi some how different in pullup as compared to the pulldown? Your old physics or biomechanics book will clarify the forces are indistinguishable. Incidentally, the crest of the pubis is often referred to as the origin of the Rectus Abdominis, anyway. Obviously, man made classification systems do not perfectly fit all constructs; these semantics merely propitiate rhetorical arguments.
In a horizontal position, the anchoring of the feet will allow you to stabilize the lower body without extending the legs to counter the leverage and momentive forces of the upper body during its extended leverage in the lower position. Remember when the hips are extended the Psoas is activated more. In an incline motion, the anchoring of the feet prevents you from sliding back, or falling on your head.
If a client has such weak abdominal that their psoas is pulling on the back, corrective exercise should be performed months before they start on intense hip flexor exercises. To say sit ups are bad, or any other exercise is bad is a gross over-generalization.
