YouTube / @Masunho

The B in BTS stands for "buff" ... 'cause three members of the massive K-pop boy group proved they've got muscles to go along with their pipes -- putting their fit figures on display during a recent workout with a bodybuilder!!

The swole sesh was documented on "Physical: 100" star Ma Sun-ho 's YouTube page this week ... when V , Jung Kook and RM met up with him at John Reed Fitness in L.A.

Sun-ho put the musicians through the gauntlet during their workouts ... with shoulders, arms and core being the focus.

The three impressively made it through each challenge ... and even got the praise of the fitness guru, who said while they all have things to improve, they're dedicated to getting better. Instagram / @natural_masunho

The boys are working hard in and out

See Full Page