Maybe not on point to this discussion, but I can't help adding my two cents when it comes to bio-mechanics and the service motion:
PROTECT YOUR ROTATOR CUFF - that linkage of muscles that allows the shoulder to rotate the arm can become the source of many aches, pains and even debilitating injuries ...
Do some research on the best ways to keep the muscles loose and the blood flowing to feed them.
I believe that serving is not so natural to the joint's designed use and serving should be afforded due respect in terms of potential overuse and injury ...
A couple of cliches apply, such as: "the ounce of prevention vs. pound of cure" rule ... as well as "the chain is only as strong as the weakest link ..."