httpv://www.youtube.com/watch?v=9f7odhDbJL8
By:Â Alicia Weber, Central Florida Personal Trainer Click here to visit JD Productions.
Due to recent requests by my fans, I made a 10-minute non-stop sit-up video (shown above) where the sit-ups get increasingly more difficult as time goes by. Sit-up endurance is definitely a measure of many muscles not just the Abs. For one, a strong back is needed to sustain endurance and proper form in sit-ups. Proper form is defined as legs bent and held down, arms bent and immovable with fingers placed above the ears. Chin is away from the chest with a forward head placement.
For a rep to count – medial elbow on both arms must cross the lateral knee on both legs (back is perpendicular to ground at apex of situp) and then the lower back must flatten and touch floor. This is how to perform proper form situps for fitness tests.
Sit-ups are contraindicated for many people including those with spinal arthritis, back spasms, back injuries, low back pain, posture problems, weak backs, etc.  Back pain is the 2nd most common neurological ailment in the U.S. and over 50 billion a year is spent on back relief accordng to the National Institute of Neurological Disorders and Strokes.  The first step in reducing back problems are finding the causes and taking steps to back pain relief and those answers are found on my page - Time to Back-up and Re-Group!
A weak, sore, and fatigued back will be the first to give out during sit-ups. The bending of the knees make the abdominal muscles work more as the upward pull of the torso from lying position to the bent knees involves the following abdominal muscles and other muscle groups:
1.)    The first 30-45 degrees (lifting the shoulder blades off the ground with full compression of the rectus abdominis m. ) involves external and internal obliques muscles acting like a corset as they tighten and flex around the rib cage. The rectus abdominis m. is compressed.
2.)   Then moving from 45 degrees to full sit (where medial elbows touch lateral knees) are where the hip flexors become involved in the pull to sit and there is a high compressive load placed on the lumbar spine (where back extensor muscle strength is important or else one could risk injury). In the initial pull phase, the hip flexors can lift the trunk from a lying posture toward the front of the leg in a sit-up. These hip flexors include the iliacus m. and psoas major m. (flexes and laterally rotates thigh), tensor fascia lata m. (an abductor), and the pectineus m. (an adductor). Then, the rectus femoris m. and sartorius m. join in as they are the only muscles in the quadriceps muscle group involved as hip flexors. They originate at the pelvis and are involved with leg flexion as the hip crosses the pelvic femoral joint.
3.)   Going from full sit to about 45 degrees down the hip flexors are once again involved as well as the abdominal stabilizing muscles. The tensor fascia lata m. assists with abduction, internal rotation, and flexion of the hip, as well as trunk stabilization. *Controlling the back and maintaining proper posture are key especially in this phase (you don’t want to just fall back!!). The back extensor muscles (mainly the erector spinae and multifidus muscles) are the main source for posterior stability for the spine and they oppose the force of gravity as they maintain erect posture and control forward flexion during a sit-up.
4.)   Finally from 45 degrees down to flattening of the back on the ground, the rectus abdominis m. goes from flexion to full length extension. The rectus abdominis m. is a long abdominal muscle that begins at the pubis with the muscle fibers extending vertically and attaching to the cartilages of the fifth, sixth, and seventh ribs (on the front part of the trunk).
Are Sit-ups really a good exercise and why do them? Sit-ups are not a good exercise for everybody. As stated in the 3rd paragraph, sit-ups are a No-No for anyone with any kind of back problem or back weakness. Sit-ups are excellent exercise for a regular exerciser that does regular abdominal exercise and back strengthening exercises. Endurance sit-ups are a good challenge for hip flexors, back, and abdominal muscle groups. Although the emphasis should be abdominals with minimal hip flexor muscles working as in proper form sit-ups shown here. Sit-ups can cover the full length of the rectus abdominis muscle in a way that can’t be achieved in a crunch or ab curl, or other (low back held down) ab exercise.   Your rectus abdominis m. and back should feel really stretched out, relaxed, and strengthened after sit-ups if you are conditioned and doing them properly.
Alicia Weber holds a 30-minute sit-up world record and she set the world record in the form explained above. She is an Official World Record Holder published in The Book of Alternative Records , Believe the Unbelievable: the Ultimate Book of World Records and the People who Pursue Them
 , and others.
Alicia also holds the proper form 30-minute situp world record with RecordSetter World Records found here.
One and a half hours later, I returned to attempt to break the bodyweight only consecutive knee tuck record, which is 310 reps. I kept going, and going, and finally my foot touched the ground and the attempt had to end at 311 reps. Unsatisfied with only being 1 rep over the record, I regrouped and returned 2 hours later with a more focused frame of mind for an exercise that requires great concentration and control…the RESULT: 402 consecutive reps – demolishing the former record!!