Pull Ups - ArmEx(free):

Application ArmEx for everyone who wants to pull-up on the bar more. Most people will notice a huge increase in their pull-up numbers, but it is not correct because sooner or later, progress stops. This training system contains everything you need to increase the n pull-up numbers: overload, variety and regularity.
Charles Lewis Armstrong, Major Marine Corps of the United States, developed a system of pull-ups on the bar. He was preparing for this system to set a world record in the number of pull-ups on the bar. Everyone who pulls-up on the bar 12-15 times for on set, after 6-8 weeks will be able to pull-up at least 20 times.
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Based on http://armstrongpullupprogram.com/