In Sketching Men, veteran art instructor Koichi Hagawa, PhD explains how to quickly capture the dynamic male form through two distinct styles of sketching:

Very rapid (1-3 minute) line drawings that capture the essence of the subject's posture and movement—perfect for recording athletic action poses in the moment
More finished tonal drawings, which take a bit longer to render (7-10 minutes), but fill in lots of interesting texture and wonderfully realistic details and nuances, including the play of light and shadow, three-dimensional form and a sense of mass and balance