Yep, having had knee surgery before, the Standing Terminal Knee Extension exercise is definitely one that I’m familiar with! 🙂 I totally agree that defining what type of knee problem your client has and whether the physician has given him any ‘restrictions’ would be a great place to start. Also, I cannot imagine that the high-intensity/plyometric activity of an Insanity workout is great for his knees, and wondered (out loud) as I read your question as to whether your client’s physician KNOWS that he is doing such a high intensity/ballistic/plyometric exercise? I DOUBT that the physician knows about this, and doubt even more that if the physician did know, that he/she would condone it. You may want to have your client discuss this with his physician moving forward, or better yet, have that discussion with the physician yourself with your client’s permission.
Once you have a handle on the nature of your client’s knee issue, and a ‘conversation’ with his physician, you will be in a much better place for planning his workouts moving forward.