More or less... nervousness is an emotion that is caused by a mixture of insecurity, doubt, excitement, and incresed adrenaline flow.... Normally the best way for a musician to overcome being nervous is to expose yourself to situations that you're placed in front of a crowd on numerous occasions. The more you do it the more confident you'll be... the more confident... the less nervous. It doesn't really have anything to do with your physical fitness... Nervousness is an emotional state not a physical ailment. You can take a musician that has never played in front of a large crowd before.... Have him run three miles a day... do yoga everyday... and put him in front of a crowd of 300 people with a guitar and a microphone and they'll still be peeing down their leg... gauranteed. The one thing I do agree on is the sports thing... Joining sports teams does put you in the spotlight... It helped me tremendously because I was fairly use to being in the spotlight.
Like I said though... my advice... Practice your songs until you can play them blindfolded... literally. Practice until you can play them without much thought. Then practice them some more. The next thing is work out some small gigs... like open mics or something where you're playing in front of small groups of people. I know musicians that make good money playing paid gigs that still go to one open mic a week... Also do some warming up exercises before you go out on stage... play some scales to limber your fingers up... go through any rough spots of any songs that you have... just so you have it fresh in your mind when you have to play it. It's always a good idea to do something to take your mind off of the show right before you go out there. Have friends around to talk to or if you're old enough have a beer or two to calm your nerves a bit.... that's one thing about beer... it's pretty much liquid courage.
That's my advice... it doesn't involve any work out routines or having to join sports teams or dojos... or doesn't require you to contort your body in a bunch of weird positions for the sake of enlightenment.... It's just about confidence... and believing and trusting in yourself that you knwo how to rock a crowd... Not difficult.
