My experience is physical activity improves the efficiency of fasting and reduces the side-effects.
Bear in mind that in Nature, where our genes evolved, the fasting organism had MORE reason to be active than the fed one: After all, that's the one that needed to find food. Thus we were selected to be more energetic during the fast. We who fast regularly do experience this.
More than a quarter century ago I stopped advising my patients to fast passively. Since then our results have just been so much better and the side effects so much less.
Physical exercise is an integral part of the 5-step detoxification activities that we practice and which has so far given only positive results. Exercise in isolation could be problematic but if you follow it with massage and sweat immediately, it is invigorating to say the least.