knee problems
Lora,
First of all, two of Cathe's strength tapes are only upper body (Pure Strength back biceps/abs, and PS Chest/shoulders/triceps). These would be fine on your knee and I encourage you to get them!
Maximium Intensity Strength (MIS) is about 60% upper body. It is an whole body strength workout in one tape.
I do not have any Firm videos. From what I hear, they are slightly faster paced and use a "tall box". Again, I am speaking without direct experience, but this does not sound exactly great on the knees. From my knowledge, Cathe stresses proper form and joint safety a little more than the Firm. She uses only an 8 inch step in MIS. On her PS leg tape (strong legs and abs) she uses a slightly higher step for only a small portion of the workout. You could use an eight in step instead in order to prevent your knee from bending at a sharp angle. Cathe's strength tapes spend a good amount of time (especially her Pure Strength series) on each body part. She encourages you to go as heavy as you can handle properly and uses up to 25 pound weights for many sections, with a 35 pound barbell for the legs (I think). You could start much lighter, of course.
The only Cathe strength tape that I haven't mentioned is BodyMax. This workout is slightly more endurance and involves higher reps. After about 30 minutes of intense muscular step aerobics you alternate extremely brief, but intense, step sections with 8 to 24 reps of barbell squats, lunges etc. The last section has five brief, but thorough upper body sections that feel like a marathon because there is absolutely no rest. The finale is 10 minutes of ab crunches. Since this workout is way over an hour, many people do only one, or two sections at a time.
I hope this helps your decision!
Irene