The ability to exert maximal force into what? A lift? a sprint? A punch? The method of exerting maximal force into each of those separate activities is different for each one. Your example is even more idiotic than mine. Fighter A is weak and can only exert 800N of force with his legs... doing what the fuck exactly? Squats? box jumps? cumming? Being able to exert 800n of force doing a squat has jack shit to do with how much force you can exert into a punch. The fighter isn't learning how to take the technique he uses to generate the force of a squat to generate that same force into the punch. No, just no. He uses the technique of punching to generate the force of the punch. You don't exert 800n of force doing a squat, then store that power in your calves and quads to later use to exert that force in an entirely different sequence of movements. This is bro science at its finest. The technique of the punch is what generates power. Conditioning, building and strengthening your muscles through repetitive drilling of the techniques is what gives your muscles the strength and speed at which to generate maximum force through said technique.