There is a big difference between internal "strength" and normal strength. Notice "strength" is in quotations because it is really not strength in the normal sense of the word.
(And, BTW, I'm gonna take the word of my teacher on this who can demonstrate the difference to me in person vs. the words of people who I have never met and who's actual skill level and level of experience is unknown to me.)
Internal strength uses the internal frame, the other uses the external frame.
Internal and external frame both use muscle, ligaments, tendons and bones. The WAY they are used is fundamentally different. The difference starts in the mind (relaxed vs. excited) and translates to the body.
External frame uses muscle based strength. Internal frame does not uses muscle based strength. In fact, it refrains from using strength at all. Force is something different than muscular
strength.
You can be forceful without using muscular strength. You can have muscular strength and not able to generate significant force.
Of course, some muscle is used in Neijia or else we would not be able to move. But the muscle is not used to generate the force. In Wai Jia this is fine, this is normal strength. But for Neiji something different is used most commonly referred to as Jin.
Good coordination of the external frame is not the same thing as coordination of the internal frame. Doing "whole body" external frame exercises (like powerlifting) will not help your internal frame. In fact it may hurt the development of the internal frame if you can't not delineate the two.
Can you learn both?I believe so. That's exactly what I'm trying to do. But through my inquiry into both I realize that they are very different, even though on the surface they may look similar. If you are going to learn both you have to realize the difference and learn to switch gears.
There are a lot of Nei Jia practitioners out there who practice their art using the external frame. They tell you, there is no real difference between the two. This is incorrect...it's PC language.
Their art is weak because they can't understand the difference. They give other real Neiji players a bad name by their ineffective use of the external frame for techniques that require internal frame movement. They then have to rely on pseudo-scientific hocus pocus because the art as they practice it does not work.
There is a real tangible difference. No one on this forum can tell you the difference. You have to find a good teacher who can show you the difference.
Peace.