I've got a stag (deerhound) cross, she's just over 7 months now. I only let her run in the yard which is good enough size for her to get up a little speed but a couple of times she's yelped while playing with the others. Nothing hurt her, it was I believe as many are saying in this thread... that their bones and joints are still soft. I won't let her out on any long runs. I had her on the fairgrounds a couple of times and let her run one time, luckily nothing happened but she's getting keen on small animals now so she'll be on a long leash.
Good thread this, glad to read it.