Another widely publicized effect of endorphin production is the so-called "runner's high", which is said to occur when strenuous exercise takes a person over a threshold that activates endorphin production. Endorphins are released during long, continuous workouts, when the level of intensity is between moderate and high, and breathing is difficult. This also corresponds with the time that muscles use up their stored glycogen. Workouts that are most likely to produce endorphins include running, swimming, cross-country skiing, long distance rowing, bicycling, weight lifting, aerobics, or playing a sport such as Ultimate Frisbee, basketball, rugby, or American football.
However, some scientists question the mechanisms at work, their research possibly demonstrating the high comes from completing a challenge rather than as a result of exertion. Studies in the early 1980s cast doubt on the relationship between endorphins and the runner's high for several reasons:
The first was that when an antagonist (pharmacological agent that blocks the action for the substance under study) was infused (e.g. naloxone) or ingested (naltrexone) the same changes in mood state occurred when the person exercised with no blocker.
A second piece of evidence is much more simple. It turns out that scientists cannot make a runner's high occur in the lab with any certainty. This makes it very difficult to study, much less prove that endorphins cause the runner's high.
A study in 2004 by Georgia Tech found that runner's high might be caused by the release of another naturally produced chemical, the endocannabinoid anandamide. Anandamide is similar to the active chemical, THC, found in marijuana. The authors suggest that the body produces this chemical to deal with prolonged stress and pain from strenuous exercise, similar to the original theory involving endorphins. However, the release of anandamide was not reported with the cognitive effects of the runner’s high; this suggests that anandamide release may not be significantly related to runner's high.
In 2008, researchers in Germany reported that the myth of the runner's high was in fact true. Using PET scans combined with recently available chemicals that reveal endorphins in the brain, they were able to compare runners’ brains before and after a run. The runners the researchers recruited were told that the opioid receptors in their brains were being studied, and did not realize that their endorphin levels were being studied in regard to the runner's high.
The participants were scanned and received psychological tests before and after a two-hour run. Data received from the study showed endorphins were produced during the exercise and were attaching themselves to areas of the brain associated with emotions (limbic and prefrontal areas). 
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