What initially strikes you about this artwork is the extraordinary physique of the subject, particularly around his torso. Rubens did not normally exaggerate his muscular details so this artwork stands out in that regard.
The style of expanding the stomach and back muscles was common in the work of Michelangelo, as shown in his Last Judgement fresco. Rubens tended to be more subtle, typically taking on female portraits and a wide range of men with varying ages, physical attributes and attire.
The beauty found in this sketch suggests that it might be a completed artwork in its own right but there are some signs of its status as a practice piece in light touches found at the bottom of the drawing. Those practice lines bear no resemblance at all to the completed drawing, suggesting that he was working on something else before beginning this depiction of the toned male.
Ever since the Renaissance there has been a stream of incredible portrait artists whose raw skills in drawing would normally be the backbone to their work, whether they specialise in painting, sculpture or anything else. In recent centuries there have been supreme draughtsmen such as Gustav Klimt who was a true specialist in depicting the female form. Edgar Degas was another significant artist who practiced drawing women in ballet poses.