I practice Tai Chi and Kung fu, I have heard developing strong abdominal muscles impedes internal progress.?

Tai-chi, and the other internal arts, specifically, liu-he-ba-fa, and baqua utilizes the internal energy. This energy is stored just slightly below the navel. As the earlier answerer had said, tension in the joints and muscles will impede the flow of this energy from the energy store, commonly known in Chinese as Tan Tien to the point of contact. And, this explains the 'relaxed and softness' of those internal martial arts, marshaling the 'soft' qi. As for 'kungfu',like hung gar, and 7-star praying mantis, the internal energy is developed through 'hard' qi-training and conditioning. Hung gar has a set, the Steel Wire Fist, "Tei Sein Chuen" which is the qi-training set and there is a lot of abdominal tensioning, like San-jin in karate, and similarly with the Grand Ancestor (Tai-ju) and 5-Ancestors (Wu-ju) Schools. In short, developing strong abdominal muscles will not impede the development of internal qi.

Strong muscles do not in any way impede the progress of internal martial art practice.

What impedes internal progress is tension, and the bulk of overdeveloped muscles. The reason overdeveloped muscles can cause problems is the restriction of motion in the joints.

So, as long as you do your stretches, and relaxation exercises you will be just fine with a strong core.

EDIT -

"Overdeveloped" muscles is the term signifying the muscular development most commonly associated with extreme bodybuilding. The muscles themselves are not 'overdeveloped' per se, but the human need for muscles so big they can no longer wipe their own butts, designates the muscles to be developed beyond their actual NEED, thus deeming them, overdeveloped. Being overly analytical can at times be detrimental.

The same is true with the over-clarification of tension.

EDIT -

Chris J... The word typed is over-clarification, not Overclassification as you put it. It does not indicate any lack of knowledge or understanding on my part. Point out to me how this "cliche" of people wanting muscles so big they can't wipe their butts doesn't have any bearing or get the point across. In that statement, the point is made that muscles aren't an issue, unless they're massive.

The point of overdeveloped muscles (or would it appease you to use "abnormally large muscle builds instead"?) is something which does pose an issue with martial arts and martial application, as I mentioned in the first post.

Instead of attacking my post which you did, you could have built on it, since you so felt the need to respond to what I wrote. I have no problem with the verbose nature of your responses; nor do I have a problem understanding the language. My problem is simply the attacking nature to my post which was unprovoked and unnecessary.

Not many people are going to be able to physically discern dynamic tension from antagonistic tension until they have spent time feeling the results. Sure, they can grasp the concept of "don't flex my biceps or brachialis, while I am extending my arm"; but without continued practice and other methods for releasing their physical tension, antagonistic tension will always be present. It is simply a physiological result of daily stress.

My over-clarification remark was that this was a bit of overkill. Tension, as the layperson (not charging any one person with that title) understands it is enough to get the point across, without over complicating the subject.

Yes, obese people will have a tough time...

Without external there is no internal. Quite frankly, to anyone half way understanding of basic anatomy the whole concept of internal vs external is pretty ridiculous anyways.

But then its so much easier coming up with catch phrases, lame concepts like physical development is detrimental to internal (which is simply synergistic structural interaction and a totally asinine demarcation), than it is to actually try and understand the intricacies of anatomical placement, physiology, kinematics, etc.

There's no such thing as an overdeveloped muscle. Muscular development is a result of a physical need placed upon the body by mechanical means (lifting weights). Tension on the other hand is detrimental IF overly antagonistic. Tension in your core is at a level necessary. And if you understand the alignment of core muscles to facilitate the stability of your platform and the integration of that platform into synergistic muscular interaction, a level of tension (ie. tone) is necessary for not just "internal" but any action. Over abundance of antagonistic tension though is the issue, (ex tension of brachialis in conjuction with triceps impedes extension of the arm)

Anyways, long story short. Yes train your core...

Edit for Fat Cobra: Overclassification? Hmm to me that sounds like an excuse for, "I don't undestand." Its ok, rather than actually taking the time to learn something you can cling to the old cliches and talk of butt wiping (or lack thereof). The irony is that I've yet to actually meet any with a developed core body that could not actually wipe there own butts. Now fatties on the other hand.....

Ridiculous old belief.

There are those that are internally strong that would not appear so from the outside. conversely there are those that appear strong from the outside yet are NOT internally agreed?

Point is... that one cannot and does NOT preclude the other.

as long as you do your stretches, and relaxation exercises you will be just fine with a strong coreas long as you do your stretches, and relaxation exercises you will be just fine with a strong core