Arm strength is a help, but not critical. I have never seen a master caster who was pumped up like a weightlifter. In the West, we get in shape for skiing by skiing. A mistake many people make is that they do not prepare for the physical aspect of the game. I am not requiring that you hone yourself to the razor's edge, but you have to be physically ready and overcome any weaknesses.
Many people start off on the wrong foot because of poor equipment or an unbalanced outfit.
Some examples are women or children using a rod that is too heavy or that has too large a grip. Other people use rods that do not fit their casting style and cause problems. I have seen people try to fish on windy Yellowstone Lake with a 5-weight rod, a feat that is almost impossible. Knowledgeable friends and fly shop pros will generally steer you straight on equipment. Let's try to take a snapshot of a person who has difficulty keeping their correct basic body positioning.