I came here to ask this, as well! Where does that tension come from and the relief one feels after cracking? People keep saying it's built-up gas but that doesn't really explain why we feel so much relief after cracking. (Not just when cracking our knuckles but also, or especially after, cracking our back, neck, etc.)
The article provides some hints at least but I wish there'd be more research on this topic:
> For some people, cracking can provide a sense of relief and a temporary increase in joint mobility, because it stretches the joint, taking it through a wider range of motion. One theory even suggests that cracking stimulates nerve endings around the joint, relaxing surrounding muscles while reducing pain and releasing endorphins – but this hasn't been proven.
The article provides some hints at least but I wish there'd be more research on this topic:
> For some people, cracking can provide a sense of relief and a temporary increase in joint mobility, because it stretches the joint, taking it through a wider range of motion. One theory even suggests that cracking stimulates nerve endings around the joint, relaxing surrounding muscles while reducing pain and releasing endorphins – but this hasn't been proven.