I would like to know when tassets started grewing longer replacing the cuisses.

I am not familiar with wearing armour myself, so I wonder what were the reasons for this transition.

Will the weight move away from the legs more to the waist giving your more moveability when on foot? Will it not impede you when turning your torso to the side?

This is a 16th century they were getting longer, by the 17th century they were connected to the knees. I'm pretty sure most people wearing these elaborate suits of armor would be able to afford a horse rather than being on foot. As you can see pikeman's tassets are shorter and a bit wider from the same time periods.

I believe armour where the tassets connect to the knees was meant to be worn over the puffy pants common at the time. Cuisses might be hard to strap/point over that. :)

Leg armour seems to have been less popular among pikemen, so the pikemen harnesses of the same time have wide tassets and a shape that would fit over those pants more easily.

These are my own ideas, so I can't cite any sources. If they don't make sense, forgive me.
The other thing besides those "nice" pants is bullet proofing. As far as I know, those elongated tassels are usually parts of harnesses with at least the cuirass proofed. Now, for me it would seem logical that armoured warriors would prefer the strongest AND lightest armour possible, especially when there are guns present. Making two layers (tassels and cuisses) shot-proof against at least pistols would make for a very heavy harness, while a thickened long tassel is lighter. Trivially, in this case lighter doesn't mean light, just lighter. :D
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I once made such a tassel for a fantasy piece, luckily for a guy with about the same size as me, so I could try it. It hinders movement much, much less as it would seem, as the lames are made with floating articulations on the front and the outer side, and with either pivoting or sliding rivets on the inside, just as on some if not most almain gorgets. As usual, if it would make fighting on foot hard or near impossible, why would anyone wear it, especially when a horse is easy to hit with bullets, and likely will meet pike formations some time....
Again, if I'm speaking nonsense, forgive me. :)

I've made a couple of pairs of these, and I can say that, made properly, they wear like a heavy pair of jeans. A person who couldn't stand to fight on foot in them couldn't stand to wear clothes. And they do fit MUCH better over the big puffy pants.

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