I'm going off the top of my head and only from personal experience here, but for me the difference is primarily about joint vs muscle pain. Joint pain is almost always bad. If you just did a ten mile hike and you don't normally do that, a couple of days of knee pain is normal. If you're mousing for your usual work day and your wrists hurt, it's a problem.

I have managed to cause instant tendinitis where it basically felt like intense muscle pain, but that was a rare shock-load situation.

Most fatigue is just a drained feeling, followed by some muscle soreness for one to three days. Soreness in tendons and joints is almost always a problem, in my experience. If it doesn't clear up in about three days of relative rest, you're heading into chronic injury territory.

> Most fatigue is just a drained feeling, followed by some muscle soreness for one to three days. Soreness in tendons and joints is almost always a problem, in my experience. If it doesn't clear up in about three days of relative rest, you're heading into chronic injury territory.

In my experience, if it doesn't clear up in three days, I'm already there (and usually also have no fucking idea what I did to cause the injury).

The more I look into this, the more I suspect that in my case it's a sensory deficit, i.e. I literally can't feel the initial pre-injury discomfort that trains most people not to move/position their bodies in ways that acutely damage joints or connective tissues.