Why was Holmes curious about single dumbell?

Holmes is curious because no one works out with a single dumb-bell. It makes no sense to onlt have one.

“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!”