I used to have a job where I would use my hands a lot, loosening or tightening bolts and connectors and such and I found that my hands would fatigue rather fast to the point where it almost got hard to control them. I do not do that job anymore but I do type a lot and I notice that if I type for a while that my wrist and forearm starts to fatigue quickly. A more measureable example I guess, if I try and play guitar hero on hard or anything fast my hand gets very fatigued.
Do I just need to stretch more? I work out and I am in good shape.
It's possible that it could be carpal tunnel syndrome. Men tend not to get the idopathic type of CTS, but do get the type that is caused by their work environment. Jobs which require strong, repetitive grasping or static grasping for long periods of time tend to cause the problem. Also, jobs that have a lot of vibration in them are also a problem.
However, it sounds like you are getting more of muscle fatigue within the forearms, rather than the small muscles of the hand. And in CTS, the only muscles that are involved are the adductor muscles of the thumb. CTS is mostly a sensory nerve problem. The muscles which flex the fingers and wrist are actually in the proximal forearm.
You do not describe the usual numbness and tingling of the thumb, index finger, middle finger, and half of the ring finger, that is seen in CTS. You describe more of a focal dystonia within the muscles which flex the fingers and wrist.
If it is getting to a point of it interfering with your activities, you should see a hand surgeon for an evaluation. Do not see a general practioner, because to most of them, any disorder in the hand is CTS, even when it is not. See a specialist.
There are some simple tests for CTS that you could probably do on your own, or with the help of a friend. The most sensitive one is the carpal compression test.
With the thumb, apply pressure to the base of the palm, where the crease separates the two muscle masses. It should be fairly strong pressure. This should be applied for at least 60 seconds. If tingling develops in the thumb, IF, MF, and RF; and how fast it develops, is a sign of median nerve irritation (ie CTS).
Another is the Phalen's test. Place the elbows on the table, with the hands up in the air. Then, let the muscles relax, allowing the wrists to flex with gravity. You should sort of look like a puppy begging. Again, hold for at least 60 seconds. If the fingers start to tingle, then it's a positive sign.
Another test is called the Tinel's. But, it is usually not present until very late in the process. Here, tapping over the carpal tunnel at the base of the palm, will cause lancenating (shock like) pain to the finger tips. But, this is rarely positive in the early stages.
If these are positive, then you probably have CTS. But, negative or borderline results do not rule out very early CTS.
Sorry, but those are the tests that a hand surgeon would do in the clinic. If the tests are strongly positive, then the surgeon would probably offer a patient surgery. If the symptoms are occasional and the test borderline, then symptomatic treatment may be tried first. Sometimes electrical studies are ordered to see how badly the nerve is compressed.
But, again, from what you have said, this is probably not CTS. Focal dystonia (a big word for abnormal muscle cramping) is usually treated with therapy. Stretching, muscle reeducation, then strengthening usually do the trick. In patients who get writer's cramps, using a thicker pen and writing with larger letters is often all it takes. But, with the guitar, it may be more difficult to change muscle patterns, because that is what you have learned in learning to play the instrument. musicians are some of the hardest hand patients to treat.
Hope you find a solution to your problem. Good luck.