Understanding basic physics can greatly increase your anatomical understanding of how to approach the practice of asana. When I was taught yoga in the 80s and 90s many teachers at the time emphasized muscular practices which actually acted in opposition to natural forces and served to produce marma (obstruction) in the bodily tissues. I have a feeling many still teach this way and want to write this article to help shed light on this actually simple but apparently complex subject. Note: in this article the word energy is not so much defined as in physics but is more the subjective feeling of force in the body.
A key concept from physics that we will need to understand is called the normal force. The normal force is defined in wikipedia as "the component, perpendicular to the surface of contact, of the contact force exerted on an object by, for example, the surface of a floor or wall, preventing the object from penetrating the surface." In mathematical terms if we assume the person standing on a floor the equation is given as
N = mg
where N is the normal force, m is the mass of the person, and g is the gravitational field strength. (wish I had a picture to show at this point, maybe in the book...) The full equation is actually this :
Where the angle theta is the angle of the inclined surface measured from the horizontal.
In simpler terms what we are dealing with here is Newton's third law of motion which states (as per wikipedia again) "When a first body exerts a force F1 on a second body, the second body simultaneously exerts a force F2 = −F1 on the first body. This means that F1 and F2 are equal in magnitude and opposite in direction." When we push against something, that something pushes back with equal and opposite force. Herein lies the key to understanding "energy" in the body when applied through yoga or any other physical activity or discipline. When you attempt to jump, do you first go up or do you go down? If you aren't sure, try it just to be sure. To jump we have to push down. When you see astronauts in space, to go in one direction they have to push off from a wall in the opposite direction.
Where is the normal force "felt"? It is felt in the bones as these are the solid structure component of the body which transfers force. Try pushing your hand against the floor or wall and attempt to feel the bones directly. You will notice an inner line of force transmitting itself through the body. Where does it start? Where does it terminate? These are important investigations.
The next thing to consider is the body in the gravitational field just standing. Try standing and just notice the bones. What is the basic force which comes back from the floor when you are just standing on it without effort? Now push your shins and feet into the floor and notice what happens. When you do this, calling the shin force S, we then get
N = mg + S
In other words we have increased the normal force and we feel something coming back through our bones! This is energy, the feeling of force coming back through the bones. What is it that moves the shin down? It is the muscles known as plantar flexors which means the muscles that act to push the top of the foot away from the front shin. Tibialis posterior is the key muscle here, located in the deep posterior compartment of the calf. The same muscles push our heels off the ground in standing vajrasana (vajrasthana). The tibialis posterior is very special as well. Thomas Myers in his excellent book Anatomy Trains has demonstrated that the tib post muscle is connected fascially all the way through the deep line of the body which connects the pelvic floor, the respiratory diaphragm, and even the tongue (can any one say bandha?).
Understanding these basic principles of Newton's third law of motion and the concept of the normal force we are ready to attempt to put this into the body in yoga. The first thing we may want to consider is this question "what is it that inhibits the normal force from acting?" The answer to this is any attempt to contract the muscle in opposition to the downward force. In other words, the instruction to "lift your thighs" or "tighten your quads" in a standing pose is actually defeating Newton's third law! This is because when you contract your quadriceps in a standing pose, the muscular force is acting upwards which acts to subtract from the equation. If we call quad force Q then our equation would look like this:
N = mg - Q
Now our normal force is actually less than the normal force and we feel less energy coming through our bones! Not to mention that we are constricting the flow of any upward energy by contracting the muscle around the area of transfer. So stop tightening those thighs!
So now that we've explored the normal force and what potentially inhibits it lets take another look at asana. Try standing with feet apart as if you are about to go into trikonasana, with one foot turned out and the other turned in. Now push the shins into the floor. You may notice that the thighs want to contract. If they do, then a standing wave is set up in the leg and you aren't going to get the energy up farther than the lower hip. What is required is for the shin and foot to exert pressure down and out while simultaneously attempting to relax the thighs and groin/hip region. Try it. What do you feel? If you are successful you will notice that the energy flows all the way up into the spine. The two 45 degree vectors of return force up the legs add up to produce a vertical vector of power that pushes right up the central column. And if the legs behave properly (not attempting to cancel the normal force) then there will be a clear pathway for the force line to follow.
Experiment with this technique in different standing poses to begin with. The seats are harder to get this action with but it definitely can be done. Try sitting. Make sure that the sit bones contact the floor firmly. If not you will need to sit on a hard surface. Notice that if there is any tension in the hip you may be unconsciously attempting to lift out of the seat which will reduce the normal force. What is required first is to relax into the ground. Mentally put the seat down and watch/listen/feel the subtle force that comes back up through the hip and into the spine. In other words sit down to lengthen the spine up! I find that with the seats a lot of the work involves getting out of the way of the normal force, in other words learning to relax in the right places.
The same principle is applied in the inversions. In Sarvangasana, the shoulder stand (more properly called all-limbs pose) we need to get the neck down against the ground as much as possible. This goes against some common "wisdom" which says to lift the neck up and put the shoulders down. The problem with this is that the spine is what is required to lift and the best direct line to the spine is the spine itself of which the neck is a part. If we can truly release and surrender the neck to the floor then the spine will be allowed to receive the normal force fully and the transfer of energy/force will be allowed to move its way up the spine. The same applies in the headstand and is also why the variation of pushing slightly into the forehead (learn this with a qualified teacher please) gives more energy than just resting on the mid crown.
Explore these principles in your practice. I have much more to say on the topic but this should give a good start. Hope it helps.