Many people who are too young for arthritis have nevertheless experienced the phenomenon of arthritis-like joint pain and stiffness when they get up after being still. Why would that happen?
It’s probably a non-specific reaction to practically anything that can go wrong with a joint. I think of this as the “arthritization” of joint pain. No matter what is actually wrong, it often feels arthritic.
The sensation of stiffness probably rarely involves any actual physical impediment to movement. Instead, it’s likely caused by neurological inhibition with roots in local inflammation (regardless of source) as well as top-down modulation of movement. The functional purpose of that system is to limit risk exposure for a troubled joint. In other words, it’s how the body says, “Careful now… no sudden moves… just test that out a bit first….” This inhibition might even be so basic that it’s regulated largely by spinal reflexes — no brain-imposed inhibition at all, just an automatic system for clamping down on movement of vulnerable joints.
Our nervous system trying to protect us is a huge theme in the science of pain. It’s the main reason that “pain is weird.”