I'm sitting in my recliner icing my right knee as I type. I am 65 y/o who has always enjoyed working out and as an FYI my right knee has some osteoarthritis as well as my left from years of running marathons. To answer your question it is never one specific moment where I can say to myself that the injury just happened. I have done every popular routine that has come along, P90, P90X, Insanity, Turbo Jam, Crazy Jillian, you name it. I am currently a Caroline Girvan Junkie, by far the best personal trainer On YouTube. I have done every series she has come out with. I also use 375 Dance Studio for Cha Cha, Bolero, Cumbia, Salsa, and a dozen other dance styles. I generally warm up with 20 minutes of dancing followed by a 40ish minute of resistance and dumb-bell training and then cool off with another 20 minutes of dancing. I have a tendency to hurt the wrist, knees and back and eventually can identify how I did it if I look hard enough. This last injury was lingering and I ignored it, kept stressing the knee because of my insanity with exercise. It takes weeks to recover from these but I'm hard headed and can certainly empathize. All of my injuries have been either overuse of a joint or too heavy of a dumbbell. I can only attribute them to personal obsession with staying fit and ignoring clear signs of early discomfort because of vanity!