Fascia, Shear and Anisotropy

Fascia, shear, and why tissues don’t behave like sheets

Fig 1. Thin, filmy collagen fibres lying perpendicular to a deeper collagenous formation

One of the most persistent errors in how fascia is talked about in manual and movement therapy circles is the idea that it behaves like a single, directional sheet. That it is an independent layer, with a clear line of pull, a predictable vector and a tidy mechanical response.

It’s an understandable mistake to make. Many of the images we use encourage it, and much of the language we rely on then reinforces and perpetuates the misunderstanding: release, line, chain, superficial, deep, and so on. But when you look closely at connective tissue under load or strain, the single-sheet model falls apart in the cold light of reality.

What fascia actually demonstrates is not tidy tension along a single axis, but what is more accurately described as shear: multidirectional sliding, redistribution, and reorganisation that depend on context, hydration, history, and the speed of loading. (Chaudhry et al., 2008, Schleip et al., 2012) This matters, because if we misunderstand how tissue behaves, we misunderstand what our interventions are doing and just as importantly, what they’re not doing.

Pulling tissue does not mean fibres align with the pull

When connective tissue is placed under tension in dissection or under the microscope, a curious thing happens. You pull in one direction, but the fibres you see don’t obligingly line up and follow the pull, but instead they diverge and appear to fan out. Some appear to run almost perpendicular to the applied force whereas others disappear and reappear as the tissue shifts.

This isn’t because the tissue is “disorganised”, but because fascia is not a pre-tensioned sheet with fixed fibre orientation. It is a hydrated, three-dimensional network whose changeable behaviour emerges under load.

Under tension, what you are observing is shear deformation: layers and microstructures sliding relative to one another, rather than stretching uniformly in the direction of pull. In a hydrated system, this sliding is enabled by fluid and ground substances, not just collagen fibres behaving like ropes. Remember that what we see in most dissection images is dried fascia and not a representation of a pressurised, fluid based system. (Yahia, 1993, Benjamin, 2009).

Crucially, when you release the tissue, rehydrate it, and then apply load again, the visible fibre arrangement often changes. The directions we see during one event are not permanent tracks, but are outcomes of that specific action, possibly never to be repeated. This is what I mean when I say these patterns aren’t patterns as such but instead are contextual outcomes.

This single observation and statement undermines a great deal of popular fascia narrative. Sorry about that!

Fig 2. If you can see fascia, it's broken or being broken. These images show an unnatural perspective of a hidden tissue.

Allow me to introduce - Anisotropy.

Direction matters, but not in the way you might think and the behaviour of fascia falls under the definition of a material that is anisotropic. This means that its mechanical properties vary depending on direction. Wood for example, is stronger along the grain than across it. (Benjamin, 2009)

The opposite is isotropic, where the material behaves the same in every direction.Steel rods, rubber balls and most fluids are commonly treated this way in simple models at least. Biological tissues however are very rarely isotropic. Fascia, tendon, ligament, muscle, bone, skin, are all anisotropic to varying degrees. (Yahia, 1993; Benjamin, 2009).

So far none of this is controversial or contentious.The mistake comes from assuming that anisotropy means there is a preferred or fixed direction that can be targeted or “released”.

In reality, anisotropy in fascia is load dependent and history dependent. The fibres you see aligning in one direction are not necessarily the fibres that will dominate under a different load, speed, or posture. They reflect what the tissue has been asked to manage at that moment, not a universal blueprint. (Chaudhry et al., 2008).

Secondly, there is never a single layer of fascia wherever fascia is thought of as a layer. In every instance where fascial fibres are laid down, there may be arrangements that that appear superficially ‘sheet like, but when examined more closely, are made up of a multitude of overlapping and interwoven tissues. (Benjamin, 2009).

Most popular fascia models impute fixed fibre directions (lines, sheets, chains etc). Anisotropy immediately destabilises those models because it implies directional behaviour is contextual, not absolute. In other words, instead of one preferred direction, fascia responds according to how, where and how fast load is applied at any given moment. (Findley & Schleip, 2007).

Returning to wood as an example of an anisotropic material, we know that it will split more easily along its grain, rather than across. We don’t however say that wood splitting more easily along the makes it somehow dysfunctional.Fascia will certainly lay down strong directional patterns, but these will always be offset by counter directional fibres. If fascia behaved like wood, it would split more easily in one direction.I can assure you that it doesn’t.

For manual therapists, this matters because it challenges the idea that you are “following the fibres” in any literal sense. The fibres you feel under your hands are not necessarily the fibres that matter most functionally, nor are they fixed targets waiting to be corrected.

Fig 3. Fluid within fascial layers moves under pressure in a constant flow

Shear, not stretch is fascia's dominant behaviour

Once we understand fascia as anisotropic in this way, another assumption begins to unravel. If a material’s behaviour varies with direction, context and loading history, then pulling on it won't show us a neat alignment of fibres along the line of force.

Sure enough when we actually look closely, particularly under the microscope, that’s what we find. When connective tissue is placed under tension, the fibres that become visible don’t obligingly follow the direction of pull. Some diverge, some appear to run obliquely, others seem to slide away from the applied force altogether. In many cases, the most prominent fibres aren’t those aligned with the load, but those managing the redistribution of that load elsewhere. (Chaudhry et al., 2008).

Crucially, if the tissue is released from tension, rehydrated, and then placed under tension again, the visible arrangement will have changed. The pattern we see isn’t fixed, nor it is not reliably reproducible. This isn't because the tissue is disorganised or damaged, but because what we are observing is a temporary mechanical response, not a permanent deformation or new structural arrangement. (Findley & Schleip, 2007).

If fascia is a sheet stretchable along a single vector, this makes little sense. It does however if fascia is understood as a hydrated, multi-layered connective system that manages force predominantly through shear - the sliding and relative movement of tissues against one another — rather than simple linear stretch.

Shear allows force to be dissipated across large areas, rather than concentrated along a single line. It allows tissues to adapt without tearing, to change shape without failing and to accommodate the complex, three-dimensional movements we associate with human function. It also explains why fascia can appear to “change direction” under load, and why attempts to follow fibres with the hands so often feel ambiguous or inconsistent.

What appears under tension therefore, is not a blueprint being revealed, but an emergent pattern that reflects how force is being managed in that moment, taking into account the tissue’s hydration, history, and surrounding constraints. The mistake is assuming that this momentary pattern represents an underlying anatomical truth, rather than a singular, contextual solution.(Chaudhry et al., 2008).

This is where many simplified fascia models tend to overreach and risk becoming illogical. By treating visible fibre orientation as fixed and directional, they mistake a transient mechanical response for a stable structure. In doing so, they encourage the idea that fascia can be followed, released, or corrected along preferred lines, when in reality its behaviour is far more conditional.

Seen this way, shear is not a secondary or obscure phenomenon, but instead is central to how connective tissue works. What we as therapists feel with our hands or see in patterns of movement, are not lines waiting to be followed, but an anisotropic system responding to load in that moment.

Fig 4. This arrangement is for this moment only. Release the tension and pull it again and the directions will all have changed.

References

Benjamin, M. (2009). The fascia of the limbs and back – a review. Journal of Anatomy, 214(1), 1–18.

Chaudhry, H., Schleip, R., Ji, Z., Bukiet, B., Maney, M., & Findley, T. (2008). Three-dimensional mathematical model for deformation of human fasciae. Journal of Applied Physiology, 105(4), 1221–1230.

Findley, T. W., & Schleip, R. (2007). Fascial plasticity – a new neurobiological explanation. Journal of Bodywork and Movement Therapies, 11(1), 11–19.

Schleip, R., Findley, T. W., Chaitow, L., & Huijing, P. A. (2012). Fascia: The tensional network of the human body. Edinburgh: Elsevier.

Yahia, L. H. (1993). Biomechanical properties of the human lumbar spine ligaments. Journal of Biomedical Engineering, 15(5), 425–429.