Over-stirring muffin mixtures
The key to making great muffins is not overmixing the batter. Once the liquid ingredients are added to the dry, mix the batter by hand just until the flour is moistened, for no more than about ten seconds. Too much mixing can cause the muffins to be dry, tough or misshapen.
The batter should be visibly lumpy, and you may see pockets of dry flour. That's OK! It's extremely important not to overmix the batter, or the resulting muffins will be too hard.
Is there some scientific reasoning behind this claim?