Lady TL, I'm sure you're
strengthening your muscle as well as getting a good cardio workout on the Stairmaster.

But in order to
build muscle, you have to stress your muscles to the point of getting small, microscopic tears in them. Which is a good thing, not a bad thing because when the small tears heal, you build additional muscle.
In order to stress the muscle sufficiently, you need to be working with a load that limits you to 12 or maybe 15 reps before the muscle is completely fatigued and needs to rest (like in a set of weight lifting exercises). The time frame is around 60 - 90 seconds until exhaustion. Cardio, unfortunately, doesn't limit us to 15 reps at a time. Most of us can do the exercise for a half hour or so, and so it's good for muscle strengthening but not muscle building.