both have been using weight machines without incident but would like to change up their routine but every time I try to incorporate a new free weigh lower body exercise they don’t want to do it for fear it will hurt their knees. They are doing the leg press, leg curl, leg extension machines now and have no problems. Are there lower body free weight exercises that do not include, squats, lunges, steps???
You might want to think about the planes of motion as well as range of motion the entire leg works in. (sagittal, transverse, frontal).
Don’t forget that the knee is the midpoint between the ankle and the hip. Hence, you can incorporate movements that stretch and/or strengthen the muscles that surround the joints above and below the knee, that will benefit the knee.
Lastly, don’t forget to include exercises that focus on the different types of muscular contractions (isometric, concentric, eccentric) as well as tempo to add variety.
Hope this helps.