Disclaimer: I'm surrounded by doctors and medical books, but have no official schooling in the subject myself. The medical knowledge discussed below is the result of personal research with the attempt to improve fiction writing, not to cure and/or diagnosis people.
I get it, your main character Tiffany just broke things off with her fiance and instead of drowning her pain in booze or chocolate she decided to do something useful – knit a blanket while crying and watching chick flicks. Sadly, she knits herself into a splint and it doesn’t bring the man back.
I’ve already talked about the secret dangers of the violin, but the truth is any character’s job or hobby can result in repetitive motion injuries. That is, strain from performing the same activity over and over for a long time. It’s a usefully little injury that can be used as either a device to get your character to stop working for the day, or if severe enough switch jobs/take a vacation. Or provide a reason for a bit of pampering from a significant other ^_~.
Results vary from eyestrain (such as looking at a screen for too long – be it a movie marathon, a computer work binge, or a video game obsession) to backaches (due to posture) to tendinitis. Tendinitis is where joint tendons develop tiny tears and so they swell and cause pain. Think tennis elbow. And don’t forget carpal tunnel syndrome, a condition in the wrist were tissues enlarge and pinch the nerves around it causing great pain.
Everything form typing to hammering to pushing around desks can result in a repetitive motion injury, meaning pretty much everyone in your story can develop something. Not that you want them all to, but the option is there.
Real life examples: what have you guys strained yourself doing?
Me - stretching, and hunching to look at my laptop.