How do each of these body systems relate to performance in physical activities?
The Cardiovascular system:
It keeps your blood pumping, without it you would DIE! So its a good thing we have it or else we would die and not have the ability to participate and enjoy physical activites.
The Muscular system:
It allows you to move around and hold yourself up; without it you would probably be lying on the ground without the opportunity of physical activity.
Skeletal System:
This system protects all those important organs it your body that allow you to move and think and everything else! Without this system you wouldn't be able to enjoy physical activities because you would have to be very careful to not fall or bump into anyone or anything because you would also most likely die! also it basically holds everything in your bod in place so without it you would be a blob.
What is necessary to keep each of these systems healthy?
Exercise! that would do it! Not just once in your life and then live as a couch potatoe, but on a daily basis.