Stretching cramps

I knew the skin around that area would be pliable, but I had no clue the effort my left hand was about to extend. To get taut skin, I had to maintain a strong stretch. Half way through and the meaty chicken leg part of the thumb (muscles of the thenar eminence) was cramping up faster and harder than normal.

We continued for about 30 minutes more and we took a break.
What to do? Well, I treated it like any cramp and covered all the bases:

Causes - temporary fixes
Muscle fatigue - * stretch, or Counterstrain - see below (thanks Sharon)
Heavy exercise - took a break.
Dehydration - drank water.
Pregnancy - nope, ha.
Hypothyroidism - nope, my thryroid is fine, thanks.
Reduced levels of magnesium or calcium - possibly.
Alcoholism - sure hope not
Medications - nope.

Why so detailed? Why not. Sure, I knew my muscles were simply worn out, but taking extra time and caution for my body is important.

* Related links: Hand stretches one, two, three. (PDF version of three) - Counter strain one, two, three.