Generally speaking, you can always watch the trainer on the far left of the screen. He or she will always be doing the most basic version of the workout. You can always feel free to modify to your comfort level—you’ll still get a great workout!
Here are some more specific modifications:
Squat modification: hold onto a chair and bend your knees very slightly.
Push-ups: Knee pushups; pushups against a wall; pushups against a table / at an incline. If none of these are possible, do chest press or fly. If knee pushups are still too much, you can also bring your knees to a 90 degree angle, like a “tabletop” position, and do pushups from there. You’ll still feel the burn!
Batmans: You can do things like the batman standing, or you can leave the legs and chest in contact with the ground and lift the arms and upper back only.
Windshield wipers: keep your knees bent, feet on the floor. Do the same movement except the feet stay planted on the ground and the knees move back and forth. If the back and forth of leg movements bother you then just use the arm move without the legs moving at all.
Dive bombers: you can go back to your wall pushups. Here’s a helpful video that helps breaks down dive bombers: https://www.youtube.com/watch?v=XmbIIGnzAZA
Pulsing superman: leave your chest and legs in contact with the ground, lift just the arms and neck and pulse.
Note on Knee Injury / Weakness: Squats are by far the healthiest thing to do for weak knees. The only knee issue where squats would be contraindicated would be those who have posterior cruciate ligament issues.
Knee pain with exercise is actually normal and can be beneficial especially if the pain is bilateral (both sides) and comes away from the exercise session (i.e. walking up and down steps, etc.). This is usually a transient occurrence and potentially a positive sign the knee joint is becoming stronger.
However, if the knee pain is on one side only, is felt during exercise or is sharp and more painful in nature (versus soreness or a dull ache), then there may be issues. The trick is to alter the range of motion so that you do partial squats. The fact is there is no way to avoid squat as if you sit and stand you will always need to squat. So it is best to continue to train this movement.
Note: Remember that as you get stronger you might be able to do some of the original movements. As you can see from the above modifications, they are very similar so if there’s anything that we didn’t cover here, you can just use this same approach – simplify the movement the best you can…if the movement doesn’t work at all then do substitute a similar movement.