OK, this may not be the most surprising news… In fact, for those who do high intensity interval training, it may not be news at all, but researchers at the University of Turku in Finland have determined that high-intensity interval training (HIIT) leads to an endorphin release in the brain, which could alleviate the physical and emotional stress caused by the high-intensity exercise. A U of Turku news release reports that study participants were first injected with a radioactive compound that binds to their brain’s opioid receptors. Next, researchers used positron emission tomography to measure the radioactivity in their brains in three conditions: after a 60-min aerobic moderate-intensity exercise session, after a high-intensity interval training (HIIT) session, and after rest. The researchers found that HIIT significantly increased the release of endorphins and other opioid peptides in the brain areas controlling pain and emotions. They also found that high intensity exercise induced negative feelings in the test subjects, which was associated with higher endorphin release. One the other hand, a one-hour aerobic exercise did not induce significant release of endorphins, and it did increase pleasurable feelings and euphoria, which correlated with endorphin release.