How much cardio is too much cardio?

[Marsoc]

It's a simplified picture Marsoc.
(I'm also pretty certain sprinter do squats and lift weights, not just sprints)

Point is that cardio training at a certain level inhibits muscle growth.
Draining your muscle of ATP, glucose&nutrients during endurance training sends a direct signal to the mTOR complex to halt protein synthesis.
(Or that's the studies I have seen)

Your point about keeping protein intake high is very important though.
You can probably attain a lot of muscle mass and still do lots of cardio if supplementing enough, keeping BCAA, glutamine and glycogen levels high.

But at one point you will cross the threshold were muscle mass can't be attained or even held unto with lots of cardio.
At least, I believe that to be the case.
I also believe that amount of muscle mass to be fairly high,
higher than what most people will get naturally.

Yeah I known sprinters do weight training of sorts..especially front squatsand stuff for the core..just saying that sprints intervals alone build muscle mass opposed to just running long endurance cardio alone. But your right about it stalling gains or whatever..I seen this bodybuilder at my gym an he said the only cardio you will find him doing is running bAck in line at the buffet