Q:

How late where plumed helmets and muscle cuirasses used by Roman/Byzantine soldiers?

According to the osprey book Byzantine Imperial Guardsmen by Raffaele D’Amato, Byzantine imperial guardsmen where known to have worn muscle cuirasses and also plumed helmets.
Its gotten me curious and i would like to know how late soldiers in the Greco-Roman road made use of classically inspired armour.
How late can we trace the use of things such as Corinthian/Attic/Roman plumed helmets and muscle cuirasses? and where they merely for decorative purposes or actually used in combat?

A:

Muscle cuirasses were rigid and rather uncomfortable; they also required custom fitting. So did lorica segmentata, but it was far more practical in battle. It's hard to say for sure whether the muscle cuirass was seriously used in battle by the Romans, but certainly it was exclusive to particular officers, such as a legate. They are very common in depictions,as it was a classic piece of armour and so you can imagine it was mainly a glorified status symbol. I know this doesn't really answer your question of how late, but this gives you a background. You might say that they were outdated long before, but were used to make showy officers. 

