In the latest Racecar Enginering Richard Divila has an article on braking where he quotes some actual F1 data.
At Monaco after the tunnel the cars drop from 290 kph to 95 kph in 1.8 seconds covering less than 50 metres. That is 5g de -acceleration requiring a 144kg pedal load At 144kg it sounds like no power brakes were involved.
I think the 5g must be peak as not enough downforce near 95kph to get 5G. So the pedal load of 144g is of short duration. Nonetheless that's is one leg so equal to a leg press of about 300kg at the gym.
Now F1 drivers weigh about 70kg so they are , in effect, bench pressing four times their own weight. An average person should be able to do 2x their weight, over 2.5 is very good. So the 4x factor isn't impossible BUT a modern f1 car has a body layout very unsuited to hard leg presses.
In the old lay down, straight leg days the driver could toggle the knee straight using the shoulder straps as his reaction point. Today his backside is low down but his lower legs are straight out almost at chest height.
So how can he generate a pedal push equal to leg pressing four times his weight? Is the trick that his legs weigh about 20% of his weight i.e 14kg , or 7kg for one leg and at 5g that is providing 25% of the pedal load.
SO , if true he is doing effort worth about 3X body weight ( 144kg *0.75*2 = 220kg/70kg) ), in leg press terms and the leg inertia provides 25% or 35kg.
Of course every reaction has an opposite reaction so he still has to modulate the leg inertia force.
Maybe I'm over-analysing it and F1 drivers do so much fitness training now they can leg press 4 times their weight in such an awkward position?