What you're feeling is probably deflection in the joists. When you do your jumping jacks
you're loading one, maybe two joists more than the rest of the joists in the floor system.
You can't eliminate it, but you can sure minimize it by adding 2x10 blocking between the joists, which is what is called load sharing. Every several feet in each joist space, add a block of 2x10 and nail it in. You can offset the blocks an inch or two to facilitate nailing.
The blocks, when tight and nailed, will take the load of your footsteps and "load share" it to other joists.
It is really easy and it does work pretty well, without spending a lot of money unless your floor joists are just drastically over-spanned.