no situps and crunches

The hip flexors are part of it. That can be overcome by going into posterior pelvic tilt and doing abdominal vacuums...but the biggest reason is that they are bad for the shoulders and not to mention the back....(which again indirectly leads to the shoulders). And they are simply not as effective as some of the other choices. For flexion standing ab pulldowns can be much more effective. You can add more weight and you can posteriorly tilt the pelvis and do the vacuums which will effectively remove the hip flexors from the equation.

People go around and around about all this and are always trying to come up with modifications to the pushups and crunches to make them better....it's such a waste of time when there are so many other options. Do reverse crunches instead of crunches and it will be much better. You can add leg weights and doing hanging knee or leg raises is sort of a more advanced version of this. BTW, for hanging stuff I actually do more LOWERING that raising.

For instance for hanging leg raises I will bring the knees up all the way to my face then lower them down WHILE straightening the legs until my thighs are parallel and my legs are straight out in front and then I will slowly lower...and repeat. I do ALL of this while maintaing a posterior pelvic tilt (that means the top of your pelvis is tilted to the rear). There's just all sorts of shit you can do. And isometric stuff is good.

It's very important to find ways to recruit the external obilques more. So that's why the torsion exercises come it. And things like cable woodchops are good. ALL of this type of thing is going to strengthen the entire abdominal complex. You really don't need to worry about situps and crunches