To protect the knees, the first absolutely essential point to look at is getting the right equipment. No matter what practice you are doing, the equipment you use will have a huge impact. Running is also the best example.
The abdominal trainer is suitable for training needs all the way from the beginners' to the upper level. 6-in-1 fitness equipment allows you to perform crunches, rotate oblique muscles, biceps curls, chest press, leg stretches, side lifting, push-ups, stretch exercises.