jordan metzl wrote "the young athlete: a guide to sports injuries."irls are four times more likely than boys to have knee injuries. that's because the muscles around their knee are different. >> and so what we do for girls is we get them to work on strengthening the muscles on the back of their leg, the hamstring, so that equalizes the force across the knee and reduces the risk of the injury. >> reporter: strength training can stop injuries from happening and prepare you for contact sports like soccer and football. trainers say kids as young as 8 years old can lift weights safely if they're instructed by an expert. >> if you're in shape and you're fit, then i think you give yourself the best chance of not getting injured. >> reporter: pro-football quarterback troy aikman knows about getting hurt. he had to retire early after getting tackled too many times. so, even if you're just playing for fun, safety gear is key. ♪ so whether it's paintball, horseback riding or skateboarding, protect yourself. >> beginners, helmet no matter what and kneepads, elbow