I exercise a lot and do a lot of dance, badminton, and karate, almost every time after, I get really sore or pull a muscle. Now I’ve been doing this for like 3 years! I really don’t understand why I get so sore easily and pull my muscles a lot. My friend did a 2k rowing test, and after the test, she wasn’t sore. But after I did 2k, my legs were really sore and we both basically have the same athletic abilities. help!!! 
ps. ruba535 doesn’t not work that well
Well, looks like there’s a chance there that you’ve been over-using your muscles, doing too much exercise in one session or subjecting same muscle to repeated stress & strain over a period of time…….
Overused muscles easily gets you into “trouble” – soreness, cramp, tightness, injuries, pulls, etc.
Here’s a detailed explanation on overuse injury: http://www.perfect-body-toning.com/exercise-injuries.html
Your friend’s muscles not sore – well each one’s muscle and body condition are different; so you can’t make a comparison like that.