[gravitas]

Sounds like someone is ready for the Robert Fripp guitar aphorisms! And will soon tire of them! And grow to wish you could go back and time to kill Rob's parents before he could be conceived if only to rid yourself of his smartassedness!

But he's oh so right.

Here are a few that might relate to your problem:

Tension is unneccessary tension. -RF

Moving forward feels like moving forward;
It also feels like moving backwards, and standing still. -RF

Gradual transitions take place suddenly. - RF

I'd suggest that, each time you play , stretch your whole body. Not just your fingers, but your spine, your feet, your pecs, glutes, calves, and any other bit of your body that you hadn't otherwise thought to stretch (except your genitals, which might make your significant other either very happy or very sad). Not only will you feel more relaxed by the time you pick up your guitar, you'll feel better afterwards as well.

As for building physical strength, the general rule is to play something gradually faster until you feel tension in the appendage that the action requires (fingers on the fretboard, fingers on the strings, pick on the strings, tendon in the arm, etc.). Once you reach this point, slow down slightly and build (ie, speed up). Repeat step one and step two. As far as I know, this is the best way to build chops on the guitar, drums, sport, or weightlifting. The only problem with this is that it's difficult to tell when you're making any progress. You'll only know when you step back and think of how you were playing before and how you're playing now.

I'll leave you with another Guitar Aphorism, one applicable to me and to Bob.

Helpful people are a nuisance. - RF