The guitar. Wonderful instrument, but also a potential source of problems when you push yourself too hard. I just came out of a bout of tendonitis, as I overloaded the tendon in my fretting hand, and most notably the tendon that connects my thumb to the rest of my body. This is called ‘De Cuervain’s Tendinosis’, and it can happen when you grip incorrectly. I usually warm up before I start practice. I stretch and massage my fingers so they are nice and limber and flexible. I paid attention to my stretching and warming up routine, and discovered I often overlook my thumb, even though you use that thing very intensively every time you grab a chord. With thrash and death metal I haven’t needed to grab overly complicated chordings, now that I am messing with black metal, more traditional chording is way more prevalent. Add a guitar with heavier gauge strings, and you end up with an overworked thumb, in the fretting hand.
Lesson learned? Stretch all your fingers, also your thumbs. It’s so important.