So after few months of some bodyweıght exercises I think that I ma starting to change a little my mind. So in the past I was doing the squats on my toes in their lowest phase. Plus that I was curling my back and my form wasn't good, so the core muscles weren't used so much. Add the fact that I was doing straight leg sit ups and or sit ups (I don't know their name) in which you lift your upper and lower body simultaneously, but in contrast of the v-ups you bent your legs and pull them to your torso. Never mind- my point is that I was working much more my upper thighs with these exercises and adding the fact that I had stronger leg muscles than abs made me to use more the legs even when doing ab exercises. Now I perform bent knees sit ups and try to use more my abs than legs and even after squatting there is no problem. But from other hand, a friend of my who is training Karate Kyokushin complained form the same- the sit ups are much harder after jump squats for instance. So in their gym they were making the leg exercises after the core.