Martial arts are broadly classified into categories such as hard, soft, linear, circular, and external and "internal". From a limited review of martial arts, it can be seen that each martial art contains elements of each category, so the classifications themselves are somewhat subjective.
One vocal internal theorist teaches what he calls a ground strength vector or ground path. There are a hodgepodge of other things mixed into his theory, like breathing techniques, energy fields the human body emits, philosophy, terms from foreign languages, belief in hidden skills, and a myriad of other complexities. He considers this, his interpretation of martial arts writings mixed with conjecture, the basis of taijiquan, aikido, and all other internal martial arts. Supposedly it is a method of aligning the body to use more than just muscular strength. He also claims to have read an old martial arts book which supposedly said that in the martial arts qi is best translated as ground strength (title and author are apparently unknown).
What I find more interesting, is that the judo (which is not considered an internal martial art) book The Secrets of Judo, 1959, has a diagram (22 on p. 48) explaining vectors. Note that they mention vectors in passing, not as any basis for judo or any other martial art, and just one of many, many factors. And, they use it as a teaching tool, not to make 'This is judo, you're doing it wrong.' type of judgements.
Here is my rough drawing of that diagram
Do a search for pictures of ground path, ground vector, ground strength vector, pengjin, etc., and you will find that each has a foot pushing into the ground and a body/limb moving forward, and hence is explainable by the diagram from the judo book. In other words, there is apparently no need to refer to the internal at all, at least not to what he believes is internal.
In addition, the karate (again, which is not considered an internal martial art) book Dynamic Karate, 1966, has a similar diagram (p. 109).
Another example, one of literally thousands, is this passage from a book, one I slightly altered (by adding the terms enclosed by x's) to prevent one from figuring out immediately which martial art it comes from by looking at martial art specific terms.
Then go into your action with all the physical resources at your disposal. All your energy must flow through arm and xxxxx hand xxxxx until it strikes the target powerfully. Then, relax immediately and completely-from right fingers to left foot. 'All the physical resources' does not mean brute force. ... By power, I mean that concentrated nervous energy which all xxxxx martial artists xxxxx must know how to store, and how to release suddenly, unexpectedly, and completely whenever necessary.It could be something straight out of a taijiquan or aikido book, talking about not using brute force, the power coming from the leg, through the hands, concentrated energy flow, etc. But it is from a fencing book (On Fencing, by Nadi), from the 1940s. Last time I checked, fencing is not considered "internal", and moreover, it is "Western", and it is often claimed by some internal teachers that Westerners do not understand internal stuff properly.
To me, things like this, utterly demolish some internal teachers' claims that what they are teaching is different from external, or even specific to certain types of martial arts, movements, or training methods, and that things are missing, and so many people (except them of course) don't know about such things.
A great summary of the differences between (real) internal and external is presented here. Also be sure to read another skeptical examination of an internal guru's theories.