You see those pink donuts under her knees? They work REALLY WELL for reducing knee discomfort, when performing kneeling exercises. As a trainer, I've tried so many different knee supports for clients that do kneeling push ups, ab wheel exercises, etc. Towels, airex foam pads, actual knee pads, etc ... and all work only to a certain degree. These knee donuts are so far, the only tool that have made it comfy for the patella, and this has been the case for all my clients, both male and female.
Why does my knee pop? Linked to an article explaining the muscle tug of war that my doctor once told me I have, and a short time of physical therapy would fix. Article has multiple exercises for the knees so I can get into races again.