Here is one definition, since there is no clear and precise one:
“Exercise is physical activity that is planned, structured, and repetitive for the purpose of conditioning any part of the body. Exercise is used to improve health, maintain fitness and is important as a means of physical rehabilitation.”
As for the second part of your question, it all depends on what exactly are you looking for. For example, what type of musicians are you referring too (orchestra, rock band, marching band, etc.)? What type of instrument (guitar, drums, saxophone, piano, etc.)? Professional level or weekend recreational musicians? Will the musicians be sitting or jumping and moving around the stage?