“Overmixing” is a term that’s generally applied to batters versus doughs. It means that the mixture has been agitated to the point that the gluten in the flour has been developed. Most often it’s pancake batters and muffin batters (things made via the muffin method) that are said to be overmixed, though cookie and cake batters can be overmixed, so can biscuit doughs, though I always think of that as “over-kneading“. Call me a hair-splitter, I won’t deny it.
What are the telltale signs of overmixing? For virtually all types of baked goods, know an overmixed product by its tough texture, the result of stretchy, developed gluten. Where muffins are concerned, overly large and/or long holes, or “tunnels”, are one of the telltale signs. The reason, because developed gluten networks trap and hold expanding steam. Where there is little developed gluten, much of the steam produced by a baking muffin escapes out the top and sides. In an overmixed muffin the steam has a much harder time escaping, and so forms bubbles. This, as you might expect, increases the volume of the muffin, which is why a small slightly domed, even flattish muffin is always to be preferred over one with a prominent, conical peak. For the visual thrill of such a dramatic rise is invariably paid for by a rubbery interior.