Campbell, S. A., Hughes, H. C., Griffin, H. E. et al. 1988. Some effects of limited exercise on purpose-bred Beagles. American Journal of Veterinary Research 49, 1298-1301.

Dogs had little inclination to exercise when released alone into the [unstructured] exercise area. Regardless of the size of the [barren] cage, dogs did not exercise unless human beings were present in the room.