90% of the clients who come to see me for treatment need help because of a single cause - muscle overuse:
In each case problems develop because one small part of a bigger muscle is being used over and over again.
Think about the gym-rat who goes to the gym 3 times a week to build bigger biceps. He does repeated sets of bicep curls each visit. Providing his form (the way he does the exercise) is good, and he's raising enough weight so that the muscles get to fatigue, then within a year, he's going to have muscles like Popeye.
In the gym-goers case, the reason for using the muscle (the bicep) over and over again is to deliberately build up the muscle.
Most of us in our daily lives do repetitive motions: whether at work or in sport. Normally this isn't a problem, because other muscle areas nearby are recruited to help out. When we keep going though, we're trying to use muscles that haven't had the chance to get over their fatigue. so they get injured.
Injured muscles are completely normal. Let's go back to the gym-rat and his bicep curls - he probably knows that by lifting heavy weights he's actually ripping muscle fibres... that's how you build big muscles, by encouraging tiny muscle damage and then allowing it time to repair. The muscle repairs with more bulk. So injury and healing time are important parts of muscle development. That's why the gym-goer will train 3 times a week.
The really important part to get from those last 2 paragraphs is rest. Problems in muscles (and tendons, actually) occur when the body doesn't have sufficient time to heal. Resting is extremely important for your body - especially if you want to carry on whatever the activity is (work or sport) for several more years.
If the constantly repeating movement that you make is at work though, you're kind of screwed. It's not as though you can just work every other day.
So it becomes important to think about strategies that you can adopt to rest the muscle. Some people might be able to swap hands for 5 minutes every 20 minutes or so. You won't be as efficient, but you'll be able to keep going longer.
You might have to change tasks within your job more frequently: again, this isn't efficient in the short term, but in the long term it'll keep you in work.
Changing position & changing movement are key to resting fatigued muscle fibres.
Sometimes, rest can be the hardest thing to do; it is, though, the most important.
Of course, if all else fails, and you need some massage therapy to help sort out your sore muscles, you know where I am!