Hand biomechanics for guitar players
The following descriptions are meant to explain the biomechanics of the hand, not to infer a preference for one style or technique over another. Any suggestions are purely in the context of reducing the tension in the hand, if you can live with a certain amount of tension, and we all can, then whatever works for you is great. If it's not working however......
The left hand
Opposition of the thumb is required to place it behind the neck of the guitar. It is accomplished by abductor pollicis longus (see De Quervains tenosynovitis), palmaris brevis and the three muscles on the thenar eminence. In combination they flex, adduct and medially rotate the thumb, bringing it to the hand, or in our case to the back of the guitar.
Placing the fingers on and off the strings is accomplished by the flexors and extensors and moderated by the intrinsic hand muscles. In the example here the flexors (red line) are contracting to pull the finger down on to the fret. The extensors (blue line) have to stretch to allow this to happen. When you extend your finger to take it off the fretboard the opposite happens.
As we've seen with our discussion of the wrist the extrinsic hand muscles will have to work overtime when the wrist is not in a neutral position. This in turn adds to the load of the smaller intrinsic muscles causing fatigue and cramp.
Flexion of the metacarpo-phalangeal joint (the large knuckle at the base of the finger) brings the fingers in toward the hand, or in our case to the fretboard. It also increases the force needed by the interossei to abduct (seperate) the fingers. Again this extra energy requirement will lead to fatigue in the hand and eventually cramping.
The right hand
Fingerpickers obviously face different challenges to flatpickers. Everyone though, particularly for right hand technique, needs to have soft relaxed hands, we know this already. Just focusing on relaxing your hands is not enough though. They need to be in a comfortable position and for this to happen your whole body needs to be relaxed, your shoulders should be soft and in the right position to get the arm at a good angle to approach the guitar properly.
Then there's the problem of knowing how to relax in the first place, if it were that easy we'd all be doing it. This is why the relaxation techniques in the program are graded, it's the best way to learn what is for some, a challenging task.
Flatpicking
Holding the flatpick between the thumb and first finger requires opposition of the thumb and flexion of the first finger, providing you don't hold it too tightly this should be a comfortable position and not cause any problems. Picking the strings requires, to various degrees, lateral deviation, supination and pronation of the wrist and flexion and extension of both the first finger and the thumb. Strumming chords will also involve flexion and extension of the elbow if you are leaning over a thin guitar close to your body, but more often rotation of a flexed shoulder.
Resting the wrist on the bridge creates tension in the extensors of the dorsal forearm as well as restricting access to the strings. Your hand will relax considerably more once you get it off the guitar completely.
Fingerpicking
Fingerpickers need to deviate their wrist to the ulna side in order to orient the knuckles parallel with the strings. The movement of the fingers is not dissimilar to the left hand although there shouldn't be any abduction and less force is required because we're not pushing into the fretboard.
The same principles apply as here as they did for the left hand, in order for the intrinsic hand muscles to work best the larger ones need to be relaxed. A gentle curve rather than a claw in the right hand will reduce the tension in the extrinsic flexors and therefore the intrinsic muscles as well. I'm positioned for pattern picking here so my index finger is up on the G string which makes it claw more than it would if I was playing melodic lines more like a classical player.
To avoid wrist flexion the thumb picks the bottom strings by adducting and abducting up and down roughly parallel to the strings, depending on the guitar.