Pouring over older threads and having a look at some 16th century and later breastplates it struck me that the (vertical) medial ridge appeared in Gothic armor a few decades before it appeared on Milanese armor. After that it becomes nearly ubiquitous in the 16th century

My question is whether the medial ridge on armor was just a fashion thing or an improvement over the earlier more globose designs.

To my knowledge, the medial ridge was initially used to strengthen the breastplate. In Gothic-style armor, it was later accompanied by fluting on the sides to even further reinforce the strength. And, it looked good. So....both, I guess. :) ....McM

The medial line down those late 14th and 15th c. breastplates that have them did not change their globose shape much, as they were more like a huge flute than an angle. Later peascod breastplates, on the other hand, do have a medial line that is an actual angle (NOT a constant one; they start out gentle and sometimes become acute at the 'bag' of the peascod. They both deal with blunt trauma extremely well, but the peascod uses a different shape to accomplish this.