The Spin: a helical view of anatomical terms
In this post, we will explore the helical nature of morphology in human anatomy and how we can frame it in existing anatomical terms. If you haven’t already, check out my post on chirality before you read this as the concept of right- and left-handedness in anatomy is an important prerequisite for understanding how our limbs are set up as pairs in the first place.
First, let’s get clear on our anatomical terms of position as we look at the big picture: humans on a planet.
Or, getting from your arse to your elbow without demonstration.
Anatomical terms have arisen in biology over the centuries as humans grapple with their discoveries in biology, and work out how to talk about them with minimal ambiguity. In so doing, we have created a lot of ambiguity! Language is funny like that– as soon as we name our concepts, they change and/or become obsolete. This is no less true in anatomy and histology.
Because many of our anatomical terms are interdisciplinary and come from zoology, we get terms like caudal and rostral; ventral and dorsal. In order to put these terms (and ourselves) into perspective, we have to consider that much of the wider animal kingdom has a decidedly different relationship with the earth.
Consider an animal such as a fish. The fish face is at the front of its straight central nervous system (CNS). The term rostral refers to this arrangement, where the animal’s CNS runs roughly parallel to the surface of the earth and everything at the brain end is said to be rostral. A rostrum is also a raised platform where you go to make a speech or receive an award. Your rostrum is your face. For more about how these terms have changed since we stood up on two feet and our faces slid to the fore, see my post on Which end is up?
For now, as we move toward understanding the helical bend of our limbs, let’s think of our many four-legged friends. When they are active and roaming around doing the business of survival, they live in a prone world. Belly downwards, vulnerable areas hidden away in the region least exposed to ever-present dangers that would suddenly threaten to impose themselves. Consider how unique we are, midriff exposed, soft abdomen confronting a hostile world. There isn’t even a name, that I can think of, for the bipedal situation relative to the earth (other than perhaps “upright in anatomical position.”). Humanity stands on two feet, bellies facing one another… why? To make communication easier? To raise the stakes of hand-to-hand combat?
Even as we ponder the evolutionary role of the chest-bump, our ventral-surface-at-the-fore position is connected to the exponential growth of the overlarge human brain. Communication and vulnerability have conspired to pump up the volume in the frontal cortex and allow us to contemplate our situation as no other species can and discuss it ad nauseum. The human condition in Euclidian terms is described in three dimensions as we deviate in planes from anatomical position:
- flex, extend
- abduct, adduct
- supinate, pronate
Working with gross anatomy this way, in terms of planes and axes in 3D, is called the stereotactic approach in neuroanatomy and I think that’s a good way to think about it. Head on over to this post for more details on the so-called Anatomical Planes. There are further dimensions that yogis call “koshas” and these are explored in the study of subtle body anatomy. We can credit that massive frontal lobe for the possibility of contemplating dimensions in the first place. Moving about in three dimensions is complex enough for most of us, but there are humans whose brains are comfortable moving around theoretically in 4, 5, even 11 dimensions, which in itself is a kind of yoga!
Once we have a foundation in biotensegrity, it is easier to see the helical nature of tissues at every scale from the smallest particles up to the level of our limbs. Because we are helical beings, it makes sense that the natural way to move would be along the helical lines, and not according to the mechanical axes described in classical Newtonian biomechanics.
Rotation, along with pronation and supination, is the key anatomical term for deepening our experience of asana safely and effectively.
Pronation and supination describe directional rotation relative to the surface of the earth, and we can feel these opposite movements in yoga as we explore a range of body geometry in action. It is the concern of this post to show that all our words for anatomical movement can be viewed as snippets of spirals at various scales. What’s more, this view will make it easier for us to maximise our power in yoga practice, moving deeper into our bodies with the greatest grace while minimising the risk of injury.
Understanding the helical movement of our limbs is crucial to deepening our experience in asana as teachers and students of yoga.
Flexion and extension together form the quintessential movement pair that thrusts our bodies into existence along the miraculous spine. Flexion is known as the “primary curve” due to its domination during embryology. Extension becomes possible as the “secondary curve” as our bodies differentiate into an ever-adapting sea of tensioned compliancy. Sadly, our behavioural tendencies often obliterate the power of this is secondary curve as we age. For more on how to combat this insidious aspect of the “ageing process”, have a look at the importance of back-bending as we explore the amazing benefits of regularly extending your spine in yoga.
Locally, the Flexion/extension movement pair takes place across joints all over the body. Flexion is typically defined as a movement that brings two bones together over a joint axis. Extension takes place as two bones move further apart over the joint axis. Take the elbow joint as an example. The elbow is said to flex as the flexor muscles of the upper arm, biceps brachii, contract across the hinge joint and pull the forearm closer to the upper arm. The elbow extends when its extensors, the triceps, pull the forearm away from the upper arm. This is the view of classical biomechanics, based on Newtonian principles of mechanical leverage, and it is important for yoga practitioners to have a basic familiarity with this model.
As we advance in asana practice, it is easy to feel from experience that our bodies defy mechanistic limitations due to their continuously adaptive nature. Through correct breathing, we can attenuate forces through our tissues that would cause a mechanical model to explode. Yoga practitioners, we have to see the extension/flexion model for what it is: a simplified abbreviation for a much larger and way cooler pattern of helical movement. Sure, the forearm moves closer to the upper arm when we flex the elbow. But the “elbow” is actually a transmission node of helical power across a continuous network of highly sensitive crystalline cytoskeletons. For an expert assessment of “elbow,” you’ve got to check out Graham Scarr’s paper. What we see as “flexion and extension” is just a split-second frame within a film about spirals.
Moving the limbs away from and closer to one’s midline is known as abduction and adduction, respectively. This is achieved through flexion and extension relative to a particular joint axis and, as such, the Ab/adduction pair fits into our helical view of anatomical terms at the semantic level of flexion/extension.
Rotation & pronation/supination
If Flexion/extension is a split-second frame of movement, then we could look at a longer frame and see them as part of supination/pronation. Looking again at the action of the elbow as it flexes, we have to consider that what is classically referred to as a “hinge joint” is actually a system of three joints:
- the humeroulnar joint
- the humeroradial joint
- the superior radioulnar joint
Word origin: Latin prōnus (“turned forward, bent or inclined”).
The position in which the individual is lying while facing down, exposing the dorsal side of the body. In anatomy, the dorsal side of vertebrates is the side nearest the vertebrae.
Elise and myself demonstrating a double-decker Chaturanga Dandasana, a prone position commonly found in dynamic yoga practice.
Word origin: Latin supīnum (lying on its back)
The position in which the individual is lying while facing up, exposing the ventral side of the body. In anatomy, the ventral side is:
(1) On or toward the direction of the front or lower surface of an animal.
(2) On or toward the mesentery or belly.