Hi all!
Some comments on JeffK's exercise... It's true that this position in supine with the knees and hips flexed and rested on a chair or on the floor puts iliopsoas and all of the hip flexors almost out of the game and gives the abs the ability to work harder. But there is a typical error in performing this. Abdominal muscles are a group of muscles, not just one. One of them is transversus abdominis, a muscle with a transverse direction (from left to right), opposite to the rectus (the so-called 6-pack muscle which lies from top to bottom). One main role of the transversus is to 'stabilize' the vertebrae to which it is attached with each others, providing a 'stable' lumbar column. To give you an example, imagine a library stool with little wheels. It can be moved easily around the shelves by just pushing it. When you want to use it to reach a book at a height, you stand up on it and it becomes stable (you actually increase the friction between the stool and the floor and a larger force is then needed to be moved. It can be moved but a much larger force is needed than when you are not on it). That's in a way transversus's role, to 'compress' the lumbar vertebrae with each other so as to eliminate excessive movement (mainly sharing loads) between them. By contracting of this muscle, sharing forces acting in the lumbar spine due to the abdominals muscle group contraction as a whole don't put the spine in such a risk for tissue failure and consequently pain, compared to not properly contracting this muscle.
Therefore, you must do this exercise by pre-activating transversus abdominis. This is done by performing a so-called posterior tilt of the pelvis BEFORE performing the exercise. This is done in this way: in the same supine position, put your hand (left or right, doesn't matter) behind your back. Try to push your hand towards the floor WITH YOUR back (i.e. not the hand alone, the hand must be relaxed), that is by contracting transversus. You will notice that your pelvis tilts backwards (the lordotic back almost becomes flat). That's when you have pre-activated the transversus and 'stabilized' the lumbar spine. Without continuing the exercise, repeat this several times so as to 'understand' the movement. When you are sure of the proper pelvic posterior tilt, put your hand back to its position, perform the posterior tilt, hold it and perform the exercise. During the exercise, do NOT relax this posterior tilt. This is the only static contraction during this exercise. By doing this, the lumbar spine is protected from excessive shearing forces and of course injury.
Hope this helped somehow. I wish I had a video of this but I don’t. Take care (of your backs)!
ilias