If your doing 1000 sit ups a day to get a sixpack you will have some back pain . A sixpack isnt easy to get
First of all you need a base power and endurance . Core traning . Second the nutrition is also important .
At least 3 years of traning to build it up. Im training now over 25 years and i learned how to get ripped in the last five years . Calestenics exercices are great vor a six pack not 1000 reps . Some exercise are just holding exercises. The best core exercises are almost full body tension like front lever or V sit. These exercise are not beginner exercises you better try step bye step bild up core try to eat healthy ( im always cheating 😂i like eatting ) each day stronger and fitter 💪😁