What was the significance of the missing dumbbell as explained by Holmes?

The fact that there is only one dumb-bell is significant because dumb-bells come in pairs. No serious athlete would only have one. So the question is.... where is the other?

“Dear me, Watson, is it possible that you have not penetrated the fact that the case hangs upon the missing dumb-bell? Well, well, you need not be downcast; for between ourselves I don’t think that either Inspector Mac or the excellent local practitioner has grasped the overwhelming importance of this incident. One dumb-bell, Watson! Consider an athlete with one dumb-bell! Picture to yourself the unilateral development, the imminent danger of a spinal curvature. Shocking, Watson, shocking!”