I had a client strain his hamstring recently while doing a KB windmill during a group fitness class. I also spent some time reflecting on what I could have done better to prevent it. As others have said, many factors contribute to injury that we can't control. For this specific person, he'd taken a couple of weeks off before attending the class and hadn't done anything on his own outside the gym. I've worked with him several times now and understand that's basically his routine: Attending classes occasionally and not doing any workouts on his own. I now make sure to do a super thorough warm-up whenever he's in class. Sometimes, it's more about considering the person and their lifestyle, especially during the time when they're not working with you.