The Plywood Principle
Anatomist’s Corner
By Thomas Myers
Illustrations by Andrew Mannie
Originally published in Massage & Bodywork magazine, February/March 2003.
Copyright 2003. Associated Bodywork and Massage Professionals. All rights reserved.
The belt of the waist clearly shows the Plywood Principle -- almost-vertical muscles surround the spine and run up the front, while the layers that join the two run in successive oblique layers. The placement of these layers gives the area strength and resilience.
Plywood, that ubiquitous building material slapped onto spec houses all across America, is widely used for its amazing strength per pound. This efficiency depends upon two elements: first, the placement of the various thin plies (the grain of each is oblique to its neighbor) and, secondly, the glue that binds the plies together.
This principle is also used across the human locomotor body in a variety of situations. The "glue" parts in our tissue are the varieties of ground substance, the glycosaminoglycans and mucopolysaccharides (specialized mucous) such as hyaluronic acid, heparin and chondroitin. We will leave the discussion of these complex macromolecules for another column in order to concentrate on the simpler concept of "grain" in the connective tissue net.
Within each bone are the Haversian canals, nourishing the bone and picking up the fresh red blood cells produced in the marrow. Around each Haversian canal are layers of the protective boney lamellae -- the round "pipes" within the bone. Examined closely, each pipe consists of successive layers of collagen (surrounded and saturated with the boney apatite of calcium carbonate, calcium phosphate and other mineral salts). Each of these layers of collagen runs in a different direction -- longitudinal, circumferential, obliquely -- and this alternation lends a plywood-like strength to each canal and to the bone as a whole.
A second example lies in the annulus of the intervertebral discs, which, likewise, consists of rings of collagen fabric (this time drenched in the rubbery chondroitin sulfate) concentrically arranged around the squishy nucleus pulposus. Each layer of collagen is firmly attached to the vertebra above and below it. But to give added strength and resiliency in the many movements (including compression) to which the spine is subject, each successive layer is arranged at a different angle -- up and down, circumferential or diagonally oblique in either direction.
This arrangement gives the discs a small range of motion in every direction -- flexion/extension, lateral flexion, rotation and all combinations thereof -- but also resistance to strain in each of these directions. In this case, the Plywood Principle serves to keep the pulpy nucleus in its place unless sudden trauma, long-term misuse or a congenital weakness allows the nucleus to break through these successive layers and escape into a protrusion or, worse, an extrusion into the space outside the disc.
Muscles and Myofasciae
Our first two examples of the Plywood Principle lie in the body's skeletal structure, and while they might be of academic interest (they sure involved a lot of $5 words), the daily bread of most readers of this magazine lies in the muscles and myofasciae. It is to these tissues we turn for the remainder of the article.
If we use a microscope to examine the areolar and other dense irregular connective tissues that fill the spaces in the body (providing the plastic shopping bags around the organs), we see the fibers run in all directions, lending general strength and resilience for the diverse strains put on them. But this is not an example of the Plywood Principle, as these are not successive layers in different directions, but rather just a general mess of unlayered fibers -- more like tough felt or fiberglass than plywood. Muscles, however, clearly have directionality, and are arranged in layers.
No muscle is simply a muscle without fascia -- the fascia coexists with and determines the integrity of each and every muscle, right down to providing a supporting sac-like sarcolemma for each and every muscle cell. Therefore, we could say the direction of the muscle cells and the collagen fibers, both of which are long and thin, determines the grain of a particular muscle.
If we examine the limbs, there are very few examples of the Plywood Principle. The length of the bones and the lean nature of the muscles along the bones dictate that most muscles here run parallel or slightly spiral paths. The pronator quadratus in the arm and the popliteus in the leg are exceptions, but they simply create or prevent strong spiral movements in the limbs and are not the successive layer arrangement for which we are looking. The way the bones of the limbs develop (starting from buds and moving rapidly away from the trunk) draw the myofascial tissues into these strings parallel to the bones.
The Exemplar Abdomen
To find plywood in action, we must turn our attention to the trunk. Given how unprotected the abdomen is, how far it is from bony support and how many movements it must perform, it should be no surprise it is the largest and most obvious example of the Plywood Principle.
If we look at the abdomen from the front, we can see there are four layers, arranged from surface to deep, in roughly the same pattern as the British flag -- the Union Jack. The innermost layer is the transversus abdominis, which traverses the entire abdomen from one set of lumbar transverse process to the other. Though it consists of two muscles, they often act as one, squeezing the belly like you might squeeze a toothpaste tube. We use the transversus to initiate emptying motions in the bladder, rectum and stomach -- and even the uterus. This squeeze, called the Valsalva Maneuver, can be felt most easily when coughing. (Tone in the transversus has been recently shown to be very important in stabilizing the sacroiliac joint in conjunction with the lumbo-sacral multifidus muscles.)
If the transversus creates a horizontal ply, the internal oblique creates a ply that is, well, oblique. The internal oblique runs diagonally up and forward from the iliac crest, tying it to the ribs on the same side, up (via the external oblique) to the ribs on the opposite side and even to the opposite hip, moving straight across the lower belly just like the transversus.
The other two bands are created by the external oblique, a broad sheet of muscle which extends downward and forward to connect the lower ribs to the same-side hip, the pubic bone and the opposite hip (via the internal oblique). Together, these form the cross of St. Andrew (x). The most superficial muscles of the group, the rectus abdominis, connect the pubic bone to the 5th rib on each side. However, these muscles, whether they are washboard abs or washtub abs, are so well known we need not belabor them further. Along with the transversus, this forms the cross of St. George (+).
Now, we have a vertically running ply, a diagonally running ply under that, another diagonal ply in the opposite direction underneath that and the deepest ply running horizontally.
The connective tissue sheets (abdominal aponeuroses) around these muscles don't exactly run the way we have described the muscles. Fascially, the external oblique runs in front of the rectus abdominis, the internal oblique fascia splits around it, and the transversus fascia runs behind it above the hara and in front of it below the hara. The net result of all these fascial shifts is that the rectus starts at the 5th rib as the most superficial muscle and ends at the pubic bone as the deepest, attaching to the back of the pubic tubercle and connecting with the anterior pelvic
floor and suspensory ligament for the bladder.
Although we are staying with the same configuration of muscles, it is worth noting the abdominal muscles (sans rectus) form a similar plywood arrangement viewed from the side of the body. The outer part of the external oblique goes down and forward from the ribs to the iliac crest, and can be felt by grabbing the waist muscle and turning the chest away from the hand. The internal oblique, running up and forward from the crest to the ribs, can be felt by grabbing a little deeper and turning your chest toward your hand. Still running horizontally, the transversus can be felt by pushing into the side waist and coughing gently.
Again, staying with this configuration, the plywood layer runs around to your back. Although it cannot be felt here, as it is now thin fascial layers under the latissimus, the plywood arrangement continues under the edge of the erector spinae to attach to the transverse processes of the lumbar vertebrae.
Another area where the Plywood Principle can be usefully observed is in the back of the neck. If we work our way down, layer-by-layer, we see the most superficial muscle is the trapezius. At this level, the trap fibers run down and out toward the acromion like the branches of a pine tree. They act to lift and stabilize the scapula in arm movements, and to provide frustrating trigger points for neuromuscular therapists.
If we lift the trapezius off, we see the next muscle is the splenius, running down and in like oak tree branches from the skull to the central spinous processes below. These muscles splint the underlying erectors and act like the reins of a horse to rotate the head. You can feel the splenius by resting your client's supine head in your hands and pressing gently past the trapezius in the upper outer neck with your fingertips. Place your thumbs on the skull in front of the ear to prevent the skull from moving, and then ask your client to rotate in a "no" motion. The splenius will pop against the fingertips on the side the head is turning toward as it meets the resistance of your thumbs.
Deep to the splenii is the straight-up-and-down palm tree trunk of the semispinalis. This cord is clearly palpable in the laminar groove deep to both the trapezius and the splenius. Strum your fingers horizontally across the upper neck on either side of the spinous processes to feel this muscle. This plywood arrangement serves to protect the back of the neck as well as offer multiple angles of movement and stabilization. By knowing this, you can find your way to any of these layers and never be lost in the laminae of the neck.
The Complex QL
Our last example is from one anatomist's speculative, but interesting, take on the Plywood Principle within the quadratus lumborum (QL). Most books show the QL as a simple quadrate muscle, with straight-up-and-down fibers passing from the iliac crest and iliolumbar ligament to the bottom of the 12th rib, pausing to attach to the transverse processes on its way (in front of the abdominal fasciae we were discussing before). Viewed this way, the QL could provide some lateral flexion, but would mostly be used to stabilize the ribs on the hip.
In The Physiology of the Joints, Dr. Ibrahim Adalbert Kapandji suggests the QL is more complex, implying it has three layers, only one of which corresponds to the vertical layer shown in most other books. The second passes down and in from the 12th rib to the transverse processes in an oak tree pattern. This portion would tie the ribs to the lower back. Paired with the scalenes at the neck end of the ribs (they attach to ribs 1 and 2), envision this portion as providing a suspensory system for the entire rib cage.
The third layer, according to Kapandji, runs up and in from the iliac crest to the transverse processes in the pine tree pattern. This configuration would provide stability for the lumbar spine on the pelvis, and be a part, along with the upper psoas, of the very top of the leg -- where walking movements are initiated.
In dissection, I have not been able to locate these layers as clearly as Kapandji, but the concept works for me in such structural patterns as lateral shifts of the rib cage (where I will work with the hip-to-lumbar portion of the QL on the side the lumbars are leaning toward) and the spine-to-ribs portion on the side (where the lumbars are leaning away). Maybe I am imagining I feel the layers, but working with the image produces gratifying clinical results.
I'm sure you can think of other examples of the Plywood Principle in action (like the middle traps, the rhomboids and serratus posterior superior, and the erectors). Understanding this principle allows you to see how our bodies can be really strong when using only very thin sheets of muscle. Applied to soft-tissue work, the Plywood Principle allows us to be quite specific about releasing these thin layers by adjusting the direction in which we work.
Thomas Myers, Certified Advanced Rolfer, LMT, NCTMB, studied directly with Drs. Ida Rolf and Moshe Feldenkrais, and has practiced integrative bodywork for more than 25 years in a variety of cultural and clinical settings. Myers directs Kinesis, Inc., which develops and runs training courses internationally for manual and movement therapies. He served as a founding member of the NCBTMB and as a chair of the Rolf Institute's anatomy faculty. His articles have appeared in a number of magazines, and his book on myofascial continuities, Anatomy Trains: Myofascial Meridians for Manual and Movement Therapists, was published in 2001 by Harcourt Brace. Myers retains a strong interest in perinatal and developmental issues around movement. His practice in Boston combines structural integration, physiological rhythmic sensitivity and movement. He lives, writes and sails on the coast of Maine.
Originally published in Massage & Bodywork magazine, February/March 2003.
Copyright 2003. Associated Bodywork and Massage Professionals. All rights reserved.
The belt of the waist clearly shows the Plywood Principle -- almost-vertical muscles surround the spine and run up the front, while the layers that join the two run in successive oblique layers. The placement of these layers gives the area strength and resilience.
 The belt of the waist clearly shows the Plywood Principle--almost-vertical muscles surround the spine and run up the front, while the layers that join the two run in successive oblique layers. The placement of these layers gives the area strength and resilience. |
This principle is also used across the human locomotor body in a variety of situations. The "glue" parts in our tissue are the varieties of ground substance, the glycosaminoglycans and mucopolysaccharides (specialized mucous) such as hyaluronic acid, heparin and chondroitin. We will leave the discussion of these complex macromolecules for another column in order to concentrate on the simpler concept of "grain" in the connective tissue net.
 In this diagram of the inside structure of bone, you can see the deep spongy trabeculae on the left, the more solid compact surface bone in the middle and the periosteum and blood vessels on the right. The blood vessels run through the bone in Haversian canals, and the successive rings of bone around the canals, called lamellae, each run in a different direction - a deep example of the Plywood Principle. Note there is an external circumferential system around the outer edge of the bone that has the same plywood property. |
A second example lies in the annulus of the intervertebral discs, which, likewise, consists of rings of collagen fabric (this time drenched in the rubbery chondroitin sulfate) concentrically arranged around the squishy nucleus pulposus. Each layer of collagen is firmly attached to the vertebra above and below it. But to give added strength and resiliency in the many movements (including compression) to which the spine is subject, each successive layer is arranged at a different angle -- up and down, circumferential or diagonally oblique in either direction.
 Here we see an intervertebral disc sitting on a lumbar vertebra. The nucleus pulposus is soft jelly. The annulus is tough, cartilage-impregnated fiber, arranged in successive oblique layers - a manifesting of the Plywood Principle. |
Muscles and Myofasciae
Our first two examples of the Plywood Principle lie in the body's skeletal structure, and while they might be of academic interest (they sure involved a lot of $5 words), the daily bread of most readers of this magazine lies in the muscles and myofasciae. It is to these tissues we turn for the remainder of the article.
If we use a microscope to examine the areolar and other dense irregular connective tissues that fill the spaces in the body (providing the plastic shopping bags around the organs), we see the fibers run in all directions, lending general strength and resilience for the diverse strains put on them. But this is not an example of the Plywood Principle, as these are not successive layers in different directions, but rather just a general mess of unlayered fibers -- more like tough felt or fiberglass than plywood. Muscles, however, clearly have directionality, and are arranged in layers.
 Dense irregular tissue (that which forms the bags around the organs) is more like fiberglass, house wrap or tough felt than plywood. This tissue is very tough, but not arranged in the distinct layers we are examining in this article. |
No muscle is simply a muscle without fascia -- the fascia coexists with and determines the integrity of each and every muscle, right down to providing a supporting sac-like sarcolemma for each and every muscle cell. Therefore, we could say the direction of the muscle cells and the collagen fibers, both of which are long and thin, determines the grain of a particular muscle.
 The reddish muscle fibers and the white cotton-candy-like connective tissue create the directionality, or grain, of the myofasciae. |
If we examine the limbs, there are very few examples of the Plywood Principle. The length of the bones and the lean nature of the muscles along the bones dictate that most muscles here run parallel or slightly spiral paths. The pronator quadratus in the arm and the popliteus in the leg are exceptions, but they simply create or prevent strong spiral movements in the limbs and are not the successive layer arrangement for which we are looking. The way the bones of the limbs develop (starting from buds and moving rapidly away from the trunk) draw the myofascial tissues into these strings parallel to the bones.
The Exemplar Abdomen
To find plywood in action, we must turn our attention to the trunk. Given how unprotected the abdomen is, how far it is from bony support and how many movements it must perform, it should be no surprise it is the largest and most obvious example of the Plywood Principle.
 The limbs do not demonstrate the Plywood Principle since the muscles run nearly unidirectionally along the bones. |
If we look at the abdomen from the front, we can see there are four layers, arranged from surface to deep, in roughly the same pattern as the British flag -- the Union Jack. The innermost layer is the transversus abdominis, which traverses the entire abdomen from one set of lumbar transverse process to the other. Though it consists of two muscles, they often act as one, squeezing the belly like you might squeeze a toothpaste tube. We use the transversus to initiate emptying motions in the bladder, rectum and stomach -- and even the uterus. This squeeze, called the Valsalva Maneuver, can be felt most easily when coughing. (Tone in the transversus has been recently shown to be very important in stabilizing the sacroiliac joint in conjunction with the lumbo-sacral multifidus muscles.)
 A simple way to visualize the plywood layers of the abdomen is as a British flag, centered around the navel. B: Of course, the reality is more complex, as this detail from the edge of the ribs shows. The vertical rectus abdominis plunges through layers as it passes from ribs to pelvis, passing first under the external oblique (here folded back to the right), then under half of the internal oblique fascia and finally deep to a pocket in the transversus fascia. |
If the transversus creates a horizontal ply, the internal oblique creates a ply that is, well, oblique. The internal oblique runs diagonally up and forward from the iliac crest, tying it to the ribs on the same side, up (via the external oblique) to the ribs on the opposite side and even to the opposite hip, moving straight across the lower belly just like the transversus.
 A waist is a terrible thing to mind! The transversus (thin, deep and out of sight) is a belt that goes from one side of the spine to the other, holding the organs in and up, and supporting the vulnerable lumbar spine. |
The other two bands are created by the external oblique, a broad sheet of muscle which extends downward and forward to connect the lower ribs to the same-side hip, the pubic bone and the opposite hip (via the internal oblique). Together, these form the cross of St. Andrew (x). The most superficial muscles of the group, the rectus abdominis, connect the pubic bone to the 5th rib on each side. However, these muscles, whether they are washboard abs or washtub abs, are so well known we need not belabor them further. Along with the transversus, this forms the cross of St. George (+).
Now, we have a vertically running ply, a diagonally running ply under that, another diagonal ply in the opposite direction underneath that and the deepest ply running horizontally.
The connective tissue sheets (abdominal aponeuroses) around these muscles don't exactly run the way we have described the muscles. Fascially, the external oblique runs in front of the rectus abdominis, the internal oblique fascia splits around it, and the transversus fascia runs behind it above the hara and in front of it below the hara. The net result of all these fascial shifts is that the rectus starts at the 5th rib as the most superficial muscle and ends at the pubic bone as the deepest, attaching to the back of the pubic tubercle and connecting with the anterior pelvic
floor and suspensory ligament for the bladder.
Although we are staying with the same configuration of muscles, it is worth noting the abdominal muscles (sans rectus) form a similar plywood arrangement viewed from the side of the body. The outer part of the external oblique goes down and forward from the ribs to the iliac crest, and can be felt by grabbing the waist muscle and turning the chest away from the hand. The internal oblique, running up and forward from the crest to the ribs, can be felt by grabbing a little deeper and turning your chest toward your hand. Still running horizontally, the transversus can be felt by pushing into the side waist and coughing gently.
 Seen from the side, the three abdominal layers still form a plywood layer. |
 A lot of people, including practitioners, do not realize the abdominal layers of fascia go all the way around to the spine, passing between the latissimus and quadratus lumborum. |
 In the neck, the trapezius fibers run down and out. In the next layer, the splenius runs down and in. The semispinalis, like most of the erector spinae, runs straight up and down. |
Deep to the splenii is the straight-up-and-down palm tree trunk of the semispinalis. This cord is clearly palpable in the laminar groove deep to both the trapezius and the splenius. Strum your fingers horizontally across the upper neck on either side of the spinous processes to feel this muscle. This plywood arrangement serves to protect the back of the neck as well as offer multiple angles of movement and stabilization. By knowing this, you can find your way to any of these layers and never be lost in the laminae of the neck.
The Complex QL
Our last example is from one anatomist's speculative, but interesting, take on the Plywood Principle within the quadratus lumborum (QL). Most books show the QL as a simple quadrate muscle, with straight-up-and-down fibers passing from the iliac crest and iliolumbar ligament to the bottom of the 12th rib, pausing to attach to the transverse processes on its way (in front of the abdominal fasciae we were discussing before). Viewed this way, the QL could provide some lateral flexion, but would mostly be used to stabilize the ribs on the hip.
 Dr. Ibrahim Adalbert Kapandji, in The Physiology of the Joints, postulates that the quadratus lumborum, seen here from the front, has not one, but three layers, with three plywood angles of pull. |
In The Physiology of the Joints, Dr. Ibrahim Adalbert Kapandji suggests the QL is more complex, implying it has three layers, only one of which corresponds to the vertical layer shown in most other books. The second passes down and in from the 12th rib to the transverse processes in an oak tree pattern. This portion would tie the ribs to the lower back. Paired with the scalenes at the neck end of the ribs (they attach to ribs 1 and 2), envision this portion as providing a suspensory system for the entire rib cage.
The third layer, according to Kapandji, runs up and in from the iliac crest to the transverse processes in the pine tree pattern. This configuration would provide stability for the lumbar spine on the pelvis, and be a part, along with the upper psoas, of the very top of the leg -- where walking movements are initiated.
In dissection, I have not been able to locate these layers as clearly as Kapandji, but the concept works for me in such structural patterns as lateral shifts of the rib cage (where I will work with the hip-to-lumbar portion of the QL on the side the lumbars are leaning toward) and the spine-to-ribs portion on the side (where the lumbars are leaning away). Maybe I am imagining I feel the layers, but working with the image produces gratifying clinical results.
I'm sure you can think of other examples of the Plywood Principle in action (like the middle traps, the rhomboids and serratus posterior superior, and the erectors). Understanding this principle allows you to see how our bodies can be really strong when using only very thin sheets of muscle. Applied to soft-tissue work, the Plywood Principle allows us to be quite specific about releasing these thin layers by adjusting the direction in which we work.
Thomas Myers, Certified Advanced Rolfer, LMT, NCTMB, studied directly with Drs. Ida Rolf and Moshe Feldenkrais, and has practiced integrative bodywork for more than 25 years in a variety of cultural and clinical settings. Myers directs Kinesis, Inc., which develops and runs training courses internationally for manual and movement therapies. He served as a founding member of the NCBTMB and as a chair of the Rolf Institute's anatomy faculty. His articles have appeared in a number of magazines, and his book on myofascial continuities, Anatomy Trains: Myofascial Meridians for Manual and Movement Therapists, was published in 2001 by Harcourt Brace. Myers retains a strong interest in perinatal and developmental issues around movement. His practice in Boston combines structural integration, physiological rhythmic sensitivity and movement. He lives, writes and sails on the coast of Maine.
