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
Page 105: Bent-Leg Sit-up
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".