Why does hard labour help with my tmj Issues?

I have tried a mouthgard, deprogrammer, massage therapy, massaging my jaw myself, and nothing seems to work to help with TMJ exept for heavy lifting for some reason.
Whenever I have done heavy lifting I feal more relaxed in the muscles of my mouth and my jaw doesn't click and opens wider. Does this mean that my tmj is caused only by muscle issues in my mouth?