“The effects of physical exercise stress on the endocannabinoid system in humans are almost unexplored. In this prospective study, we investigated in a crossover design and under field conditions at different altitudes the effects of physical exercise on the endocannabinoid system (ECS) in 12 trained healthy volunteers…

We conclude that the ECS is activated upon strenuous exercise whereas the combination with hypoxic stress further increases its activity.

In summary, physical exercise activates the endocannabinoid system, whereas the combination with high altitude enhances this activation.”

http://www.ncbi.nlm.nih.gov/pubmed/22101870