My 13 year old son loves doing both gymnastics and sprinting 100m/200m. He is currently training both. Will the muscle build in the different training pose a problem? He has a very muscular upper body(from Gymnastics training), will it cause problem in his speed? He is said to be explosive which will not be an advantage to him for sprinting. Is it true? He usually could not maintain his speed in the last 10metres. Is it due to his gymnastics training?Thanks.