Just like anything, it may not hurt everyone, but the majority of people are going to experience tendon and ligament injuries if they run and jump in ankle weights. They are made for aerobics, not running and jumping. This kid's genetics may help him not get injured, but who knows, he may have problems in the future. If you want to add hypergravity training to your workouts, get a weighted belt. I think JumpUSA sells a good one.