This is an excerpt from Identifying Postural Imbalances Through Yoga- Revised Edition by Vayu Jung Doohwa.
Line and Section Imbalances
The actual movements of the body are rather different from those illustrated in anatomy books. A single muscle does not operate in isolation, but as a part of the synchronous action of a combination of muscle structures, interconnected with the fasciae.
When a person bends forward, the hamstrings are not the only muscles to be lengthened. Every part of the back side of the body is affected—starting from the forehead and continuing through the cervical, thoracic, and lumbar spines, the gluteus, and the hamstrings, to the soles of the feet. The whole extended back side of the body will form, in this case, the back line. In contrast, when the body is bending backward, the front muscles, which are being stretched, will form the contiguous front line.
Obviously, there are many more links between the joints and the muscles of the body, including the lateral and spiral lines of the body (for a detailed analysis, see Anatomy Trains by Tom Myers*). But, if we understand the connection between the two main lines (the back and front lines), we can identify the cause of the imbalances of the front and back sides of the body. (I am also in the process of writing a book on the symmetric imbalance of the lateral and spiral lines.)
Figure 1.6 depicts four important sections of joints along the back and front lines of the body.
Why did we divide the front and back line into the simple four sections rather than into joints and muscles? Simply because these four sections are already divided by the major joints of the body, representing a practical functional unit.
The human body is made up of 206 bones, over 400 skeletal muscles, and more than 230 movable and semi-movable joints. When we observe yoga poses during their practice, we realize that the conventional tools, such as X-ray or muscle testing, for examining postural imbalances are not practical. Accordingly, we look for simple tools to use on our yoga mats.
For doctors and therapists, those conventional methods are essential. The patient can totally passively relax while the physical therapist palpates each joint and muscle separately. During yoga practice, however, no single muscle or joint acts in isolation; in fact, several muscles and joints work together like a single functional unit. That is why dividing the sections, which are made up of the major joints like the hip and spine, is practical. We can move cervical joints without moving thoracic and lumbar joints; however, when we move for example the thoracic or lumbar joints, we must move some of the other joints as well. This is the reason why we can divide the whole spine into two sections, where the cervical region is a separate functional unit, and the thoracic and lumbar regions together form another functional unit.
Section 1 has at least 8 movable joints, but when we move, all cervical joints work as one functional unit to perform flexion or extension.
Section 2 has at least 17 joints, but when we move, all thoracic and lumbar joints again work as one functional unit to flex or extend the spine.
Section 3 comprises the hip joint and sacroiliac joint; the latter is semi-movable and should therefore not move, in order to provide stability to the pelvis.
Section 4 has at least 16 movable joints, but the ankle joint is the most important one to ensure dorsiflexion and plantar flexion of the foot.
The knee joint is located between Sections 3 and 4.
If the forward bending poses are easier to perform than the backward bending poses, the back line is relatively effectively lengthened, as demonstrated in Figure 1.7, while the front line is less so, representing an imbalance, as illustrated in Figure 1.8.
In general, if the forward bend postures are easier to perform than the backward bend postures, we can safely assume that the back line is better lengthened than the front line. This may indicate a line imbalance.
In other words, imbalances can manifest through the whole structure of the front or back side of the body. In many such cases, a safe generic solution to correct a line imbalance would be to practice more of either forward or backward bending poses, whichever are more challenging.
Sections are parts of the front and back lines as defined above. A section imbalance is even more common than a line imbalance. In a section imbalance, there is a significant difference in the ability to lengthen the muscles of a specific section compared with the muscles in the other sections of the same line. When we learn to observe sectional imbalances, we can identify certain sets of joints and muscle imbalances, which represent one of the main causes of postural imbalances.
*Myers, T.W. 2014. Anatomy Trains, Churchill Livingstone: Edinburgh.