Masturbation doesn’t result in any kind of pains or side effects; it is only a natural process of relieving ones sexual pressure. As your friends opine it isn’t the reason for your pain in muscles. Your pain in the muscles might be the result of over doing some exercises or some other issue with your body. Consult with a doctor to get rid of it.
5-6 times a week tells that you masturbate almost every day barring a single day, which btw isn’t good. Excess of anything isn’t good for the body. Limit it to 2-3 times a week or reduce it and do it only when you feel like doing it. At times abstinence is also good as it helps you maintain the energy by not losing it when you ejaculate your semen. The other facts about masturbation can be known from an article on the same topic.
http://www.askanjali.com/80558/masturbation-what-are-the-causes-and-how-do-you-control-it