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The flexed arm is a great demonstration of impressive strength to bodyweight ratio and is often used as a measure of upper body endurance. The United States Marine Corps uses it as a physical test with Marines required to take it (and pass it) twice a year. The minimum time they must hold in order to pass is 15 seconds and the maximum they are expected to hold in order to get a flying pass is 70 seconds. This suggests the level of difficulty of the exercise. Upper body strength improves gradually, with time and persistence. The Flex Hang Challenge will help you get stronger and will improve your overall level of fitness by increasing strength across several important, upper body muscle groups. Once you get through the challenge, like the U.S. Marines, you should aim to test yourself at least twice a year and work on workouts that help improve upper body strength; throughout the year. 

Split the total amount required into manageable sets, when necessary.