it means u worked those muscles more then you are used to if u had over done it they would not only be tight they would also hurt the more u work a muscle the tighter it gets which is why u should stretch before and after workouts and intermitantly dureing long workouts to keep them from tightening up to much later u may try a massage or stetching to ease the stiffness
I am a weightlifting fitness nut. The first thing I tell people when they work out with me weather doing arms, legs, back, running, swimming, does not matter, that their stomach and back muscles will be sore, maby really sore for a few days afterwards. This area, your tummy, sides, and lower back is called your CORE. It SHOULD be the strongest part of your body, but, if you look around at todays kids as well as adults, they have this ;tire; around their whole midsection so far, sometimes they have blower back fat. This is because they have little ot no CORE strength. To get around this either do sit ups/crunches and weighted along with squaTS EVERYDAY, OR JUST STAY IN MOTION. i TAKE IT BY YOUR PIC YOUR YOUNG!! Get off the computer and GO PLAY. Im 38. I BASE Jump, sky Dive and Moto Cross, IM IN THE best shape of my life. I still get sore...so will you...so get out...have fun and stay in motion....sorry, I get nuts talking about fitness to the young ones! HAVE FUN!