u probly already know this but, being tense is bad. it slows u down and causes all medical issues. only advice is really onoly play as fast as u can without tensing up.

try and get close to this limit. so play just before u get tense. and try and slowly get faster but not so much as u get tense. this will alow u to be less tense at faster paces. which will give u more speed. and u wont get stiff.