Animate beating heart while skeleton is stationary?

I'm trying to animate a beating heart from a single image. The heart is enclosed in the rib cage. When I try to animate the beating heart, Firefly does a good job of this but it is rotating the entire skeleton. I only want to animate the beating heart while the rib cage and the skeleton stay stationary.

Here is an example of what Firefly did with the animation:

(22) Firefly Create heart beat No movement of bones or skeleton 79772 - YouTube

The prompt I used  was "Create heart beat. No movement of bones or skeleton." I used the same image for the first frame and end frame.

Let me know how if there is a way to animate just the heart. This is for a medical presentation.

Thanks,

David