My legs aren’t “tired” they just physically stop moving
Where was the all of the above option??? 
just my personal experience, even the comments about legs stopping moving could be mental rather than physical. When I’m not looking at the steps, I don’t really feel like my legs will stop moving, but if I do, when that next step engages, suddenly I have a problem. On my last ramp test, I made it to 19:15 without looking at anything. Which was great, because I knew I would have a successful test at that point. However, come the 20:00 minute mark, it was like I hit a wall and I think it was a mental wall, not a physical one.
I agree with this, ramp test 6 weeks ago, max hr 202 (result 203 watts), 6 weeks later max hr 196 with a result of 211.
When I realize I have better things to do. 
Agreed on this. So much of this, in my experience, is mental breaking vs physically breaking.
To combat this, I now bump each ramp test value by 2%. Im a gluten for punshiment.