i agree with both posts. what would be good in terms of progression, i dont want to end up doing 100 situps since thats not really going to help my strength.
Then personally you are not looking for endurance. Which is fine, of course.
i think hugging/holding plates while i do them would be better, that way i can go up to a 25kg plate.
what do you think?
Since your primary interest is in sit-ups: you can go to a local retail establishment and purchase velcro straps. You will be able to strap the plates together, which will assist in preventing slippage. An example cut off rep limit would be about 20 to 25, since you are not interested in.......endurance.
For example: You are using 20 lbs. Your first set of reps you reach 20. Add 5 lbs to the next set (strap it together). The second set, with the added 5lbs, you reach say 12, and then the 3rd, 8. On your next session (as an example), you keep the added 5lbs on the first set, if you reach 20, then add another 5lbs.
As you progress in weight, and the weight becomes a burdon (there will be a point, where it becomes a little rediculous in putting 100lbs on your chest, for reps). You have some other options, assuming still straight flat sit-ups, which deals with momentary failure of a set. Additionally this assumes you have progressed for a few weeks, and are repping with some heavy weight. Here is an example: Lets assume you are using 20lbs to begin the first set. You fatigue at 20 reps. As soon as you fatigue at 20 reps, pull off one of the 10's (or pull off the 20, and put on a 10 real quick), and continue repping out. You will be able to perform more reps with the 10 though you fatigued at the 20. You can stop it here and call it a 2-tier set (one set, 20/10), wait the designated rest period, and repeat for another 2-tier set (second set), and so on and so forth. This is sort of like a pyramid. This is just a very brief example. Other options include performing decline sit-ups on declining bench, using progressive weight increase as you did with the flat sit-ups. I would first build your progression in weight before attempting the pyramid I exampled. As your stength increases, you can mix and match a couple of versions of the sit-up, and pyramid example.
There are better exercises out there. One example is the Hanging Leg Raise, which also can be performed with/without added weight, if you are interested.
Have you ever considered the: Front Squat? IMO, it activates the core a bit more as compared to the back squat.
Can I ask what your personal goals are?