Chels.

How the cardiovascular system relates to performance in physical activity..
We have to get blood moving, and this is what makes it move, so I guess that's how it relates. To keep this healthy, a constant moving is most likely a necessity. Run.

Muscular System :
Without this, we'd be blobs of skin and bones. Muscle holds everything together, and it's what makes up capable of doing anything. I would have to say that any type of muscle building activity would make sense to keep this up and running, unless a fat couch potato is in your life desires.

Skeletal System :
Without a skull covering my brain, my brain would be very damaged. For example, this summer I got hit in the head with a rock. If my skull wasn't there, it probably would have sunk right into my brain. How dreadful would that have been? Horrid, I tell you! How on earth do I know how to keep this healthy? Oh gee, maybe swallow some horse sized calcium pills.