As we discussed earlier, all of our physical yoga practice does one of three things to our tissues: we
stretch the tissues, compress them, or apply a shear to them. This simple fact dictates what stops us
from going deeper into any posture. The resistance to stretching, or said another way, the limitation
on our flexibility, is due either to tension along the tissues, which resist further movement, or
compression, where two parts of the body come into contact and prevent further movement.
If tension is stopping the movement, it is felt in the direction away from the movement. For
example stand up and fold one leg backward, moving your heel toward your buttock. If the heel stops
before the calf presses into the back of the leg it may be due to tension in the quadriceps. This
tension in the quadriceps is in the opposite direction from the movement of the lower leg. If
compression is stopping the movement, it is felt in the direction of the movement. In this example,
compression may occur when the calf is squeezed into the back of the thigh or when the heel pushes
into the buttock.
In some cases whether tension or compression is limiting movement is not easy to determine, and
part of our practice is to pay attention to what is happening in the body when we move. A useful
mantra to repeat during asana practice is "What is stopping me from going further?" The answer
to that question may influence your practice considerably. 
The range of motion (ROM) we have in our joints, if it is limited by tension, can be increased
through asana practice, breathing, and even diet. When the limit to the ROM has been reached
and compression is stopping further movement, no amount of yoga will increase it; you have
reached the limit of flexibility that asana practice will provide you. However, diet, injury,
surgery, and other interventions may reduce the point of compression thus increasing the ROM.
For example, a woman nine months pregnant may not be able to touch her toes due to compression
of her belly and her legs. Yoga will not help her now! Once she has delivered her baby, the
point of compression has changed, and her range of motion in that direction will increase again.
When tissue's resistance limits the ROM, the resistance has been found to come in four main areas:
the skin, the tendon of the muscle, the muscle itself and its fascia, and the joint capsule - all
provide tensile resistance to movement. The following table shows how the resistance is
distributed relatively in these four areas: 
| Muscle (and fascia)||41% |
|Connective Tissues (joint capsule)
As shown, the biggest single limit to flexibility, when it is caused by tension, is the joints' rigidity,
followed by the muscle and its fascia. Yang yoga is excellent for opening us to the limits of flexibility
of our muscle tissue, its fascia, and our skin. Yin Yoga is required to safely open the joints to their
healthy limits.  As we continue to investigate the muscles and connective tissues, we will learn
how we can open them more deeply. The exercises to open connective tissues will be examined in the
second part of our journey down the Yin River - The Practice of Yin Yoga.
For now, let's look closer at the muscles.
1 -- A more general mantra useful at any time in life is a similar question, "What is stopping me?" The answer
to that question is also extremely illuminating, although often very difficult to find.
2 -- Johns and Wright, 1962.
3 -- It is not recommended to deliberately try to lengthen the tendons; these
tissues need to remain at their normal length to help maintain fine control of our movements. Since tendons
are directly connected to muscles, it is very difficult to target a Yin Yoga exercise strictly at a tendon.
However, since the material in a tendon is closer in makeup to a ligament, the tendon, being more yin-like,
will not respond well to yang-type exercise. Thus the muscle protects the tendon by taking up the stretch.
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