we allow that individual s ability to come through. reporter: aaron von frolich runs cross fit in new york city. one of 900 cross fit programs in the u.s. as long as there s proper coaching, proper guidelines and we re teaching them basic proper movement, then there s no chance of this becoming a problem. reporter: but pediatric sports medicine specialist paul stricker says cross fit training can be too intense for some kids. strength training has its benefits but it has risks. if you have the wrong child at the wrong time with the wrong amount of experience and the wrong amount of supervision it can be quite risky. reporter: our intention is to make children safer in their daily lives. such as lifting a backpack or around the field of play when they have to run, cut, jump, etc. the parents we talked to weren t thrilled. what type of a difference have you noticed in your boys in the last month and a half. i think the focus. i think the energy and just they just love it.