Post Made Community Wiki by MichaelHouse
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Casting requires mental discipline but also requires large amounts of energy to flow through the body. Holding the body in the right position and directing the flow with precision requires physical strength to hold the arms and the head and the fingers just "so" while huge energies are flowing through them. If your muscles are not strong you have to take it very slowly and carefully, the more physical strength you have, the more you can rely on your body to hold the necessary position while you channel the energies.

Which is to say that strength affects casting speed and channeling rate.