Indeed he is! In a recent episode of Infinity Challenge, Yoo Jae Seok had to hold on to a bar and then jump off as part of a game. When MC Yoo was holding on to the bar, people screen captured his growing arm muscles. He used to be rather skinny but eagle-eyed netizens noticed he might be working out nowadays. One netizen commented, "Since he started working with Kim Jong Kook, I guess he started working out." I don't know what got him to work out but it sure seems to be working. Other netizens' comments include, "Wow, that almost looks photoshopped.", "Yoo Jae Seok, he's always the best.", and "He always works so hard." I'm pretty sure no one viewed Yoo Jae Seok as the type of person to be carrying around some guns but nothing lasts forever and neither did his weak image. His hard work to become successful paid off because now he is Korea's number 1 MC but now it seems his hard workouts are paying off as well. Congrats to MC Yoo on his new figure. Hope he keeps himself healthy for a long time!