I plan on doing some long distance running over the summer/off-season to keep my stamina up. I read somewhere that running over 3 miles a day stretches your calves too much causing you to lose height from your vertical. Can anyone verify this? If it's true what'd be the optimal distance to run without damaging my calves/vertical? I don't know much about muscle kinetics so any help would be greatly appreciated.

I also plan on using Air Alert to increase my vert. People on my school's varsity team swear by it, but I also know it can damage the legs if isn't done under the right conditions. I plan on using a mat or some shock absorbing surface with Air Alert. Aside from that, is there anything else I should do to maximize the effects and minimize the (potential) damage to my legs?

I'd also like to know what other people do during the off-season to improve, or just to stay at their current level, just so I can get some ideas for the future...

I do weights when I'm not in practice and then do some of our conditioning/plyos from practice on Monday Wednesday Friday. Saturday's a rest day, Sunday is practice and then Monday's back to the weightroom. Basic strength training will help, just try not to get so jacked it cuts down on your speed.

P.S. one of our coaches, who plays pro in Slovenia, told me their trainers make them stretch out their hip flexors before they work out. He swears by it, so I do it regularly.

Here is a HUGE thread dedicated to Volleyball Training on this website already:

Volleyball Training Thread

There are a ton of ideas inside for training. Its on the second page in this forum, but this is the direct link to it.

simply weight training helps so much with everything you do.

make sure you hit your core lifts upper/lower body

bench
SQUAT
hang clean

you have much more control over your whole body and what you can do if you are stronger, people think it doesnt help volleyball players much, but it really does