For all you ripped crafters

When I do that--not in years, I confess--I suppose it's my forearms, my hands, and my neck and upper back, because I bend over my work in a way that's terrible posture. (I notice it, sit straight, and as soon as I forget, I bend over again.)

If the hand sewing involves piercing something with your needle which resists piercing, like canvas or leather, the biceps will get involved, too.

Maryn, who used to sew and craft more than she does now

I crochet on a regular basis. I would possibly include the wrists to Maryn's list. What's the lighting like for the tailor? Candlelight/firelight in a pre-industrial society? Gaslight? Or modern day florescent lighting? Then you have to factor in eyestrain.

Oh, duh, of course the wrists. And light is a huge factor, and probably a part of the reason I peer so closely at the work. I can't imagine doing fine work by firelight or candlelight.

Maryn, recalling the forger who went blind in The Great Escape

Maryn pretty well covered it. It's not muscles, but my fingertips always get destroyed too. I never learned to use a thimble.

Achy hands in general. I'm an artist and I've noticed if I'm using color pencils they start to ache from the pressure. I think anything you need to apply pressure too-any craft-would do the same. I guess I'll find out better when I get around to making my first teddy bear. Got all the stuff but I'll probably be sewing by hand.

Index finger and thumb get coarsened, almost calloused, if not using a thimble. Scratches where a needle slips across the other fingers.
But if a thimble IS used for any length of time, the finger becomes nesh and almost sweaty (like underneath an elastoplast). I used to make sandals and the thumbs became hardened and the skin was harsh, nails shortened.
After years, say, in old age, the joints would be swollen, perhaps arthritic.

I sew about 6-12 hours a day by hand and find it's my neck and back shoulders that get the most strain. (I have to peer fairly closely due to bad eyesight.) Never had a problem with wrist or forearm. Thumbs, sometimes.

Callouses: fairly heavy on the index and middle fingers of my left hand (I'm right-handed), right index and about midway down the ball of the right thumb, where the end of the needle is.

Occasionally the thread end of the needle will pierce the thumb around the callous if it's a heavy seam, which hurts. And I'm sure I can't be the only one who's ever sewn their finger to the fabric by the needle slipping through the callous unnoticed.

Oh, and don't forget the inattentive seamstress who occasionally stitches the callus to the work, passing the needle right through it. I've done that.

Maryn, who's also machine-stitched through a fingernail

I've done way too much craft work over the years - so much so that now, simply using scissors to cut the length of a piece of paper will send my hand into a nasty cramp - visible and grotesque. My left hand cramps too from some actions.

One of my biggest problems during winter months has been dried out hands that crack around the fingertips so much that thread actually gets caught in the cracks. And, of course, if I stop long enough to get the fingers healed, I don't get things done when I want. Vicious cycle. Puma

Well it depends on context of your question too. Sewing isn't going to get you buff in any way, shape, or form. Honestly from all of the people in my family who have been sewing for years, there isn't really a visible difference. They have really strong fingers (with callouses, as have already been covered) but their hands don't look muscular or anything. Also note that "strain" doesn't necessarily mean that you are working out a muscle group. Bad posture, which is pretty common when sewing or dealing with anything small, really, is going to put strain on your back and shoulders but it isn't "working out" those muscles. In fact it may lead to chronic upper or lower back pain and in very rare or prolonged cases a permanent change in posture, such as a visible hunch.