From an antropometric Handbook :
Pressing Pedal .... age 31 - 50; A pedal 400mm length and 30mm wide, placed inside a 55mm wide space to restrict the amount of the foot that can be placed onto the pedal.
Sex Mean (Nm) S.D (Nm) Range (Nm)
Male 329,183 129,99 173,00 - 579.70
Female 237,99 106,80 121,50 - 435,60
So Rambo can push about 1450 N or about 300 lb
I have no idea, for how long this force can be maintained.