Increased Morning Heart Rate in Runners: A Valid Sign of Overtraining?

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Abstract
In brief: An elevated resting pulse rate is generally considered a marker of overtraining in endurance athletes who greatly increase their workout distance. This study supports that assumption, demonstrating increased morning heart rates in 12 men who ran twice their regular training mileage during a 500-km (312-mile) road race over 20 days. After the first week of running, morning pulse rates were slightly reduced, but thereafter they progressively increased, becoming 10 beats min−1 higher (p <.01) as the race ended. Blood pressure, oral temperature, body weight, sweat loss, and blood glucose, lactate, insulin, and Cortisol levels were not related to the increase in morning heart rate.