As previously stated, get full clearance, including aquatic exercise clearance. Avoid full extension and flexion. Does the client wear any type of brace? I would assume so, but if not, consult their phyisican/surgeon to find out if they could or should be wearing a brace. I have had cliients that were told to wear a brace, but didn’t like to do so. This type of client will have very little stability in the affected knee(s), if they have not had adequate training for knee stability. Or if they have not kept up with their exercises for the knee. Checking to see that they have had any rehab is a good precaution. Don’t assume they have. If they haven’t, I would have them ask their doctor to recommend rehab before working with them. Getting full clearance would usually cover this, but I thought it was worth mentioning. As Nancy said, aquatic exercise is great for this type of client. But be sure to watch the range of motion in the water as well. And wearing a brace in the water is still advisable. Training all of the leg muscles through the glutes and into the low back is also a great idea. Work all motions of the ankles, knees (as permitted), hips, and work the stabilizing muscles of the low back/back/abdominals/hips. Strong stable muscles surrounding the knees and at the joints above and below will take a lot of pressure off the knee joint. When working the other leg joints. Be sure to avoid putting stress on the knee joints unintentionally. Such as, place the contact point of straps or pads for leg/hip ab/adduction, flexion/extension above the knee joint as opposed to putting on an ankle strap. Or doing side planks with a roller/pad above the knee instead of planking from the feet. Take a second to consider what is happening at the knee joint during all exercises. Modifiy any exercise that could be putting unnecessary stress on the knee joint. Contact me through my profile if you would like to ask me more questions or want some feedback on ideas you have.