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A "brisk" walk means raising your heart rate to your target heart rate or to the point when you are breathing rapidly but can still hold a conversation. Calculating your target heart rate is easy: Subtract your age from 220, and multiply it by 0.7.
"Just 30 minutes of this type of exertion creates energy and sharpens your mind," Dr. Merrell says. Also, researchers at the Neurotrophic Lab at UCLA found that a molecule responsible for brain plasticity, which aids in quick thinking, is elevated for a full day after a session of moderate exercise.