Although I own quite a few of them, I don't find martial arts textbooks especially useful. If one does not know how to read between the lines (and pictures), they seem to promote an inefficient way of teaching the arts, by focusing on directions on how to execute specific techniques: "1) Grab your opponent from the neck and biceps, 2) Push the neck diagonally to the back while at the same time pulling the arm in a 45 degree angle towards the opponent's front right corner", etc. Whether we like it or not, combat techniques are not like IKEA furniture, where if one blindly follows the instructions, the end result will inevitably be a couch or an armchair. The subtleties of manipulating an alive and resisting human body are infinite, so sensitivity and adaptive measures are needed every moment of the interaction, and no martial arts textbook can convey that.
Nevertheless, every now and then one can stumble upon valuable pieces of insight in books about martial arts and from my experience, these are usually located in the first, introductory chapters, where the basic principles of the art analyzed in the book are explained. For example, the other day I was browsing Total Aikido: The Master Course by master Gozo Shioda. On page 14 of the book, Shioda explains the first basic principle of aikido, namely Chusin-Ryoku, or "the power of the center line": "One of the basics of aikido is the principle of maintaining a straight center line in the body. For most people, even if they try to stand straight, their center line is not really straight. Even when we do stand straight and focus on keeping our center line fixed, we lose it again as soon as we move. If this happens, then the purpose of aikido, the development of breath power, becomes impossible. If we are able to maintain a strong centerline whatever direction we move in, we have focused power. By forging this focused power, we are also promoting strong posture, concentration and breath power ".
Now, Shioda, at a height of 1,57m and barely 50 kilos of weight, was by any measure a very small man. Still, if one watches his aikido demonstrations, it is obvious that this man, unimpressive in terms of stature, possessed extraordinary power and impressive skills, so we must assume that his writings must have some importance, right? The problem is that his descriptions seem to me pretty vague and open to interpretation (as is very common with most Asian martial arts, in my opinion). This should not be surprising: when Shioda was studying aikido under the founder of the art, Morihei Ueshiba, scientific research into the field of human movement was nonexistent, so the terms one could use to describe what happens into a moving human body had to be intuitive, which means that they lacked objectivity.
I don't even want to think about what on earth "breath power" might mean, but how about the term "center line"? Some difficulty might be also present here, in order to explain it. For example, some Wing Chun people describe the center line as an imaginary line drawn along the centre of the human body that joins the eyes, nose, throat, navel, knees, and groin as a central focus of attack. Some JKD people might add that the centerline should always be directed towards the opponent, so that all our weapons are available to use to the maximum of their capabilities. The way I understand these explanations, it seems that the center line is some sort of a topographical feature that lies on the front part of the body. In Shioda's description, I suspect that, much more than a topographical feature, the center line is a biomechanical feature of a body in motion.
Maybe then, if we turned to biomechanics and sports science, we'd get some additional insight. According to Joanne Elphinston, performance consultant to elite athletes, "...all sports require control of a central longitudinal axis (CLA) to achieve their most efficient movement. In practice this central axis is not a rigid position: it is the sense of a firm but flexible central reference point, which supports movement of the torso and limbs. Imagine a firm, thick metal cable passing vertically through the top of your head and down through the middle of your body. This cable would form an axis for your shoulders, thorax and pelvis to smoothly rotate around, but still enable you to move easily in all directions. [...] If the central axis collapses, rotational movement will be restricted due to joint compression on the concave side of the collapse, and soft tissue tension on the convex side" .
Some of you might note that from the vague terminology used in some Asian martial arts we have moved into geek language territory, so we'd better try and make it simple: if you have seen the movie Karate Kid 2, you probably remember that little spinning drum Mr Miyagi used as an analogy to help Daniel understand the "secret technique" that was to save him during the final fight (if you haven't seen the movie, you haven't missed much, but you can check the final fight scene and the drum I'm talking about here and before you ask, yes I too believe that Ralph Macchio's central longitudinal axis is kinda crooked). OK, now try to think what would happen if the stick which runs through the middle of the drum, was made from soft rubber instead of wood - most probably, it would collapse under the weight of the drum, so rotation would be hard to achieve, if at all possible. Well, that's what happens if one lacks control of the CLA or, as Gozo Shioda might say, is unable to maintain a "strong centerline". And, obviously this does not only happen in martial arts practice - whether you play golf, tennis, or ice hockey, or you want to pack some serious power in your punches and make your throwing techniques more efficient, you must rotate your torso, and rotation will be strongest when it is performed around a strong, clearly defined axis.
So, how does one establish this central longitudinal axis? Well, it is mostly about maintaining a neutral position of the spine (keeping those gentle curves at the cervical and lumbar regions intact) with the least effort possible. In other words, you need to have an ideal posture, described once again by Joanne Elphinston as "simply and buoyantly supporting yourself against gravity, and allowing your body structures to move and interact in their least stressful, most effective relationships. An ideal dynamic posture should make movement easier, helping you to establish a central axis for balanced motion and allowing you to breathe freely" . Unfortunately, nowadays this is easier said than done in our urbanised society, with most people spending many hours a day hunched in front of a computer, squeezed in a car seat, or collapsed on the couch in front of a television set.
It always strikes me as a paradox when I see Systema RMA practitioners in seminars trying to perform what is taught by instructors with perfect posture, using a less than ideal posture themselves. The problem is two-fold, in my opinion. On one side we have the students that focus on the end goal of each movement they try to perform, ignoring the process through which the goal can be achieved, what F.M Alexander, founder of the Alexander Technique, called the "means whereby": if one only cares about delivering a mighty Systema punch, he tends to "think with his fists", which makes focusing on the ideal posture impossible. On the other side, although all the Systema RMA instructors I have trained with (including Mikhail Ryabko and Vladimir Vasiliev) have an ideal posture, they never actually refer to it as an essential ingredient of their skills. What they do is repeatedly urge the practitioners to relax - the problem is that bad posture, by its mechanical structure generates tension in order to be maintained, which means that relaxation is by definition unattainable...
A protracted and backwardly rotated head, lack of mobility in the thoracic spine, abducted or winged shoulder blades, externally rotated hips, are all factors that contribute to a bad posture and will make your progress in martial arts (or any other type of) training arduous and, most probably, laden with injuries. If you want to check whether your posture is good, the mirror won't help you much - having a partner video your training sessions will. After you see yourself training, go check out videos of top practitioners of martial arts and top athletes and make a comparison (you can check once again the demonstration by Gozo Shioda above, perhaps a DVD with work by Mikhail Ryabko, but also videos of Mohammed Ali, Michael Jordan and even some accomplished dancers or gymnasts - ideal posture is not exclusively found in one martial art, sport or discipline). If you discover that you don't fare that well, you've got work to do, but please stay away from the "military approach" to posture: the advice to "stand up straight, head up, shoulders back, stomach in", will lead you away from your goal, by causing increased effort and muscular tension in your body. The basic Systema exercises (push-ups, sit-ups, flat-foot squats) can help you a lot, as long as you focus on executing them with a "long spine", imagining (not actively trying) that your head is moving away from your pelvis. Some stability training for your core, pelvis and shoulder girdle will also work miracles. Still, the best methods to improve your posture utilize subtle cues in order to activate neurological reflexes that stimulate your posture quickly and easily (the Alexander Technique is one of the most sophisticated I know off). You might want to try some lessons in one of those. And most importantly, when you're training, try not to think with your fists and feet. Before you try to perform any movement, even you warm-up exercises, just perform a mental check of your posture, until the neutral position of the spine becomes a habit for you (the slow training methods utilized in Russian Martial Art make this awareness possible). Because, just being able to stand does not in any way mean you're in a position to fight.
 Shioda, Gozo. Total Aikido: The Master Course. Kodansha, 1996. p. 14
 Elphinston, J. Stability, Sport and Performance Movement. Lotus Publishing, 2008. p.17
 Ibid. p. 61