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.
There are several internal gurus who teach, mostly online, what they call internal strength, internal skills, pengjin, ground strength vector, or ground path. Apparently no videos of them using these techniques to spar with resisting opponents exist. In my experience, these people are far too interested in debating definitions and if you defend yourself using external force that isn't enough because it isn't "high level".
What I find much more interesting, however, are the physics. For example, 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.
Here is my rough drawing of that diagram
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 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.
All punches start from the floor; the power flows from the push of the legs, through a twitch of the hips and into the shoulders, which remain loose and relaxed while the arm shoots out like the end of a whipTaekwondo: The State of the Art, p. 108
To me, things like this perfectly address internal guru 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. Add to that list their general lack of video evidence, and avoidance of live, realistic testing, including banning their critics from seminars (because they know they will be tested, and not with a play-along student), and their martial arts advice is hard to take seriously.
This is not to say internal martial arts or concepts like peng and qi don't exist or that taijiquan is ineffective as a martial art, far from it.The book Tai Chi Dynamics, by Chuckrow, is a great resource and has a valid description and real information on peng, for example. Also Peter Lim's page and Stephen Goodson's page are great resources on real internal strength.
Find a good teacher, and guard your wallet from the internet gurus.
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