i went to gym since form 4

for me no problem what so eva , as long as you do it the correct way
meaning with good motion / rhythm / posture

btw if you want to burn fat ---cardio activities is the best

if you want a 6 pack ---- make sure ur body fat is below 25%
everyone has a 6pack----- but on flubby tummy it doesnt show with all that FAT.

sit up is just for toning it.... giving it more strength N bulk
btw if youre young make sure u sit up the correct way.
if u do it wrong ur abs will look ugly/weird/not symmetrical

the most important thing is to enjoy your workouts

nothing feels better than a great body!