Unfortunately, injuries are prevalent in the stringed instrument world. Some musicians get tendinitis, carpal tunnel syndrome, and even focal dystonia. When it comes to RSIs (repetitive stress injuries), all of them are serious and need to be attended to immediately.
Here are some, but not all, of the causes for RSIs and a few resulting injurues:
1) Poor Body and Hand Posture = tendinitis, back pain, neck pain, hip pain, carpan tunnel
2) Pressing too hard in the left hand = tendinitis, carpal tunnel
3) Playing too hard in the right hand = tendinitis, carpal tunnel
4) Playing with tension all the time (or playing too hard all time) = see #1 & 2
5) Playing too much! = see #1 & 2
The good news is that we can prevent these injuries from occurring. The great news is that these methods also help you improve as a player! Here are a few examples (but not all) of how to prevent each off the above injuries. Using these techniques (and this mind-set) also helps you improve!
1) RSIs from Poor Body and Hand Posture
Fix your posture and constantly be aware of it. Get e teacher or coach to help and practice in front of a mirror all the time.
Exercise 5-6 days/week! This is good for you in general, but it also trains your body to rebuild muscles faster. It's also great for your back and overall health! Try yoga and running, cycling, or swimming.
2) RSI Pressing Too Hard with Left & Right Hands
This happens all too often. We (guitarists) do not need to press very hard with the left hand to get a clean sound. Remember that the string only need to touch the fret (metal) and NOT the fret board (wood). Practice pressing lightly on the left hand.
For the right hand, you need to play lighter. Don't worry so much about volume. Play at a medium volume so that when you play a piece of music you have room to increase volume when needed. Practice playing your music very dynamically; take the low volumes really low and the high volumes really high (but don't overdrive the instrument and don't play for too long at a loud volume). Relaxation is the key. Stay relaxed!
3) Too much tension
This is easier said than done. but.....relax! The guitar might feel difficult, but it generally only feels that way when you are tense. Try playing everything in a relaxed state. Make the music and movement feel easy. DOo't try so hard.
4) Playing too much.
This is a all too common. Most PROFESSIONAL CONCERT PERFORMERS practice (with guitar in hand) for no more than 3-4 hours/day, 6 days a week. (And as a concert nears, they tend to practice less, or taper off their practice as the concerts nears).
No one practices for 3-4 hours straight with no breaks. That's a recipie for RSIs. Take breaks every 5-15 minutes. Practice a musical phrase for 5-15 minutes, stop, stand up, stretch, and work on a different phrase. Listen to your body. If it hurts, STOP PLAYING.
Breakup your practice sessions. I do 2/day: morning for 1.5 hours and after lunch for 1.5 hours. Try 3 or 4 shorter sessions split throughout the day.
Most empirical studies show that practicing more than 405 hours a day has DIMINISHING RESULTS (i.e., you get worse!). Remember that it's not quantity, it's quality. You can practice for 15 minutes/day and master a piece in a short amount of time if you practice efficiently and TAKE BREAKS!