Question for Penny: I don't think I want to ever do exercises like jumping (plyometrics), squats and running up the stairs. I hear it will totally destroy the knees. So I don't know why trainers would even recommend this?
Answer: Your body was designed to do incredible things like jump, squat, skip, run and a whole lot more. If you don't use your body to do these movements, your ability to move this way effectively will inevitably weaken.
When you say "I 'hear' it will totally destroy the knees," you need to take a closer look into who and where you're hearing this from. How reliable is that "one" source, or was it "MANY" highly credible sources? What "exactly" did you hear, or "thought" you heard? Don't conclude so quickly without getting all your facts straight. You can only hurt your knees if you don't know "how to workout properly."
For example: Wearing the wrong shoes that don't provide proper support; not executing a proper warm up before you start going "hard core" on your workout; not stretching properly after a workout; not starting slow taking one step at a time up the stairs until your body adapts and builds strength before you go running two steps at a time; not taking breaks or having proper rest days in between. If any of the latter points lag, then YES, you can expect to get hurt.
Plus, to avoid injuries in anything you do, proper form is KEY. (Yes, there are actually proper techniques that accompany each exercise move to maximize performance and avoid injuries).
Also, do you have an injury from the past that you did not properly tend to? If so, exercises that require your body to move a certain way may aggravate that past injury. You may not be aware of those side effects and could easily mistake your current movement or workout as "the source" of the problem.
So if you think you're going to hurt your knees doing exercises like jumping or running the stairs, then you DO NOT know how to workout properly. Ego may be your driving force instead of mind-body awareness and knowledge. It's not the workout that is the problem; it's the approach a person takes with the workout that can cause the problem.
Hope this gives you a better sense of how you can be more empowered in your body — and prevent injuries!