Recently I had problems with my right arm, to the point I couldn’t pick anything up like a full jar. My doctor put it down to a strain injury that he recently had experienced himself. He said he didn’t believe it until it happened to him and that it is caused from having your wrist bent upwards all the time. This is an injury that takes weeks to fix. What he told me was that your arm and wrist should be facing downwards, which is not that easy if you develop habits like lying in bed with your laptop.
The fix is using a mouse pad like the one shown above and being aware of your arm position. It keeps your wrist up and this is an injury to avoid because of its lengthy healing time. If you want to save yourself weeks of grief, they are available in a variety of shapes and colors.