The problem with physical activity or exercise on a fast is twofold.
Firstly, this wastes vital food reserves that would otherwise be used in enabling the patient to fast for a much longer period of time if needed. This can make a profound difference between the success or the failure of a fast. Complete rest whilst fasting is merely a state of enabling and channeling ALL of the energies of the body into cleansing and healing, rather than divert these energies elsewhere into physical activity; cleansing and healing suffer as a result.
Secondly, physical activity or exercise during the fast will waste muscle tissue much more quickly and severely than while resting, and as some muscle tissue is catabolized to supply the amino acids required for the production of glycogen in the Liver, for the maintenance of the Brain and other internal organs, this supply will become depleted much sooner than it should resulting in a higher risk of Cardiac Failure: even in the obese.
This is not to say a faster should live in a vacuum of inactivity, but keep this to a minimum in the preservation of their food reserves and their health during what is after all a period of "physiological rest".
I have known several domesticated animals who when sick or injured did not move at all until they felt well and able to take nourishment and had recovered: sometimes up to 20 days without food and freely available water.
Bed rest generally was found to have by far the most beneficial results in the fasting patients of Natural Hygienic Doctors, but of course this would not apply in the well person, so if you are well, why would there be a need to fast anyway? except a bit of annual internal housecleaning from exogenous toxins.
Fasting can become obsessive and even addictive, but is merely a small part in maintaining wellness in the healthy but a major part in the recovery of a sick organism.