What was strange about the dumbell and why was holmes refering to it time to time

Many times in Sherlock Holmes stories, Holmes’ advice is sought in a matter because there is a total absence of clues. In this case, the police find themselves overrun with clues that all seem equally important: bloody footprints, the unusual weapon, a card inscribed 'V.V.' next to the body, the peculiar burn on the dead man’s arm, etc. The one observation that Holmes makes that seems to be of no consequence whatsoever is that there is only one dumbbell in the room. This oddity is generally looked on as unimportant by the other detectives, but, as it turns out, it is the most revealing clue. As Holmes says later, “Dear me, Watson, is it possible that you have not penetrated the fact that the case hangs upon the missing dumb-bell?”