Do you only build muscle when you feel sore the next day?
So, right now I have a flat stomach, but I've been trying to build it into a two-pack, or at least some sort of muscle. I want my stomach to look fit, so I've been doing core exercises 5 days a week from an app I downloaded on my phone. Although while I'm doing the exercise I feel the pain and the soreness, the next day, my stomach is not sore. I feel like since it isn't sore the next day, it isn't building muscle or helping me at all. I know the things that I'm doing aren't "too easy" because they hurt a lot while I'm doing them. So, because I'm not feeling sore the next day, does that mean the muscle isn't building? Please helpp(: Thanks!