But relaxation techniques may not be enough, Peeke said, since the best way to block stress hormones is through beta endorphin -- produced during exercise.

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Immediately following each session their endorphin levels were tested.

ECONOMIST: Exercise and company

Because endorphin levels can only be measured directly through an invasive lumbar puncture unfeasible, even for notoriously pain-hungry oarsmen the researchers used a readily accepted proxy: they deduced pain tolerance, and hence endorphin levels, by gradually tightening a cuff around each rower's arm.

ECONOMIST: Exercise and company

"It isn't a huge surprise when you consider many positive effects exercise can have with regards to the brain chemistry: dopamine, serotonin, endorphin, epinephrine -- these are all associated with mood altering effects, " said Dr. Cedric Bryant, the chief science officer for the American Council on Exercise.

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