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It is a deep relaxation technique whereby a person tightens or tenses a muscle group, holding for a certain amount of time (around 5-10 seconds) and then relaxes those muscles completely for a longer time (10-20 seconds). Then they move on the the next muscle group. By tensing the muscles first it allows for a deeper relaxation after. Often this will be done in a progression (starting from the muscles in the face moving down the body). It's helpful for anxiety, tension, and minimizing stress reactivity in the body. An American physician by the name of Edmund Jacobson is credited with describing this process in the 1930's. It is not from Yoga although during the relaxation or savasana part of Yoga, some may use it to improve relaxation.