I've picked up playing the bass again after an 8 year hiatus. It's been about 3 months now, and I've found on the days where I practice approximately four hours or more, my left index finger is sore right at the fattest knuckle. Sometimes to the point I can't bend it when I first wake up the next day (like this morning). I'm pretty sure my technique is solid (thumb behind fretboard, 1 finger per fret, arm is perpendicular to the fretboard, etc.). Is this common and and just part of building endurance? If not, has anyone experienced this before?