By Bernie Clark, May 1st, 2016
{A Book Excerpt}
The following excerpt is from Your Body, Your Yoga written by Bernie Clark, author of The Complete Guide to Yin Yoga, with a foreword by Paul Grilley.
FIGURE 1 The ‘What stops me? spectrum
There exists an important question in life and in yoga, one that we will benefit from asking any time life presents us with a challenge or high drama: “What stops me?” While we will look at this predominately from a physical perspective, please be aware that what is stopping you may have no physical component at all–it may be psychological or emotional. (For a discussion on these other edges see the article Playing your edge .)
Physically, we reach our edge when one of two things occurs: our tissues can stretch no further, or our body is hitting itself. We will use the term tension for the former: tension arises when the body’s tissues can elongate no more, and further movement is therefore restricted. This tension can be found in our muscles but also in our fascia, ligaments and joint capsules. A common example of tension occurs for many people in their hamstrings. If your hamstrings are short and tight, when you try to come into a seated or standing forward bend with straight legs, you feel the tension in the back of your legs. The tension in your hamstrings restricts your range of motion (see figure 2). While muscles are the predominant focus of many yoga practices, there are many other causes of tension, as shown in figure 1: fascia, ligaments and joint capsule also contribute to tensile resistance.
The second reason we may be unable to progress further is compression. This occurs when one part of the body comes into contact with another part and further movement in that direction is therefore not possible. There are several kinds and causes of compression. The first we will call soft compression–what occurs when flesh comes into contact with other flesh. A good example is presented in figure, 2b where the flesh of our chest hits the flesh our legs. The second we will call medium–what arises when our bones compress our flesh. An example is when the bone of your pelvis (technically called the ASIS, or anterior superior iliac spine) compresses into the flesh of your thigh in a lunge posture. The third kind we will call hard; this is the unyielding compression of a bone hitting another bone.
FIGURE 2 Tension (a) and compression (b) limit our range of movement. 
Tension is felt in the direction away from the movement, indicated in (a) by the shading along the hamstrings. Compression is felt in the direction of movement, indicated by the shading along the chest and thighs in (b).
FIGURE 3 The points of compression experienced in a seated 
forward fold may be gotten around temporarily, but eventually, compression is the ultimate edge beyond which we cannot go.
From a technical point of view, there are several kinds of stress that we can apply to our tissues, and each form of stress has a limit. However, for our purposes, it is sufficient to look only at tension and compression. The stress of tension and compression can be arranged in a spectrum that starts with the weakest form of tensile resistance to movement and moves all the way to the finality of bone-on-bone compression. I call this the What stops me? spectrum (or WSM? for short–see figure 1). We can view our yoga practice as moving us from the far left of the WSM? spectrum to the far right, at which point the game stops. But for many people, the progress is not so linear. Our biography dictates how fast we move along this spectrum, but our biology may reorder the major stopping points. For example, due to your unique anatomical biology, you may not be able to stretch out your tensile resistance in one particular posture, because your hard edge is so close: you get stopped before you even get started.
In general, beginners to yoga will find that tension restricts their range of motion. In time, the tension will diminish. At some point, however, the student will find her restrictions are no longer due to tension–she will reach points of compression. An exception may arise when the tension occurs in joint capsules: capsular ligaments provide restraining tension that may greatly limit the range of motion in that joint. This tension may not disappear over time and may be the ultimate limit to how far she can go. A good example is the capsular ligaments of the hip joint that prevent extension beyond 30°. Our intention in yoga practice is not to become hypermobile in the joints; thus, we accept the tensile limitations presented by our joint capsules and do not try to stretch them further.
Unlike tensions, the points of restriction caused by compression will not change with further practice. When compression arises, you will have reached a fundamental limit to your range of motion–for that posture in that direction. It is important to acknowledge this point. Once you have reached compression, you cannot go further, but sometimes you can go around. In the example of the yogini who is doing a deep-seated forward fold in figure 2b, she cannot go any further because her upper body is contacting her lower body. She has reached a point of compression. However, if she abducts (spreads) her legs apart, as shown in figure 3a, she can go around this compression and continue her journey, continue to work through tension, until the next point of compression is reached–in this case, the floor! She can’t go further because her chest is hitting the floor. We could allow her to go further either by digging a pit beneath her or by building her a platform: we could put blocks under her sitting bones and calves and lift her higher (see figure 3b). Now she could continue her journey until, finally, at some point, some part of her body would hit another part of her body, and this would be a point of final compression. Ultimately, after you have stretched out your tissues as much as you can, you will reach the end of your progress for that posture. To try to go further now may be dangerous, and injuries often happen to students who try to push through compression.
These two concepts of tension and compression will resurface many times in our investigation of the body and will dictate what movements are available to us. While the concepts are simple, their manifestations are varied.
This material is from the second chapter of Your Body, Your Yoga written by Bernie Clark, with guidance by Paul Grilley and many other yoga teachers, therapists and biomechanical researchers. The book is now available for ordering via www.YourBodyYourYoga.com and other book vendors.