Muscle Joseon is a Manga/Manhwa/Manhua in english language, Comedy series, english chapters have been translated and you can read them here. You are reading chapters on www.mgeko.nl fastest updating comic site. The Summary is

"What's this?" "Kyaaaa!!!" "Wh-Why did I suddenly turn into a beanpole?!” Choi Young-jik was a Food and Nutrition academic nerd. A certified fitness enthusiast with a first-grade qualification in Lifestyle Sports Instruction. But then… One morning, waking up with all his bountiful muscles gone, he finds himself in the body of Grand Prince Suyang, the second child of Sejong the Great. Unable to hide his excitement at the thought of seeing the Great King Sejong in person and wondering whether it is a dream or reality, Young-jik goes to see his father, the Great King Sejong. "Uh...? Is this the Great King… Sejong?" Before him sits a pot-bellied man who looks like he's having trouble even breathing. What on earth is this? The Great King Sejong is severely obese and diabetic, and all the officials are suffering from chronic illnesses... Having been a fitness trainer for 15 years, the gym rat pride of Choi Young-jik begins to ignite passionately. “Well, shall we spread the fitness fever in Joseon for once?!"