Simple. Warmup. Proper stretches and massages can help prevent tendon damage. Key thing is ALWAYS ALWAYS ALWAYS when you feel any form of actual pain stop. A vigorous stretch is one thing but actual pain is bad. You can damage your tendons big time if you don't prepare ahead of time and thus actually ruin your ability to keep playing and this applies to your keyboardist too. Stretch your shoulder areas, wrists, fingers and thumbs. It's too detailed for me to describe in this little box so just search either this site or the internet for stretching warmups. Also some massaging around the shoulders, elbows, palms and fingers help get the blood flowing making your fingers and hands more relaxed. Tension is also bad. I don't know if you have a tension problem or not but if you do it can also cause damage. Just relax as much as you can and push the envelope but only to an extent. I'd suggest that for now you take practicing easy and don't strain yourself and build up a duration before taking it to an extreme.