There is a risk every time you stretch. There is a risk every time you push or pull. There is a risk with every leg curl and leg extension. There lies the reason why personal trainers have clients sign an Agreement and Release of Liability along with a medical questionnaire and, if needed, a medical clearance. Even with the best watchful eye and careful cuing, accidents happen and in a split second there is an injury. Last month I had an attorney look at my "Agreement and Release of Liability". I was apologetic about the source and even said that I had been told it would not hold up in court. However, she said that it was fine and that it would. She suggested that I bold and capitalize a few words emphasizing clients knowing there is risk in exercise and they have to assume some responsibility. There is a risk in walking across the street and there is a risk in doing nothing. Just this morning I read from The American Association of Orthopedic Surgeons (AAOS 2008). "...the worst thing clients can do is nothing, as inactivity weakens the muscles around the joint and makes them unstable". It's not cut and dry as to how much is too much or too little. If you can do something, do it and don't wait too long!
Resource: Keeping Seniors Moving by Sherry Ballou Hanson IDEA Fitness Journal January 2016