Most of you reading this are already aware that fascia is viscoelastic, but what amused me the most is what gives it that property, and while devouring at least 25-30 odd research papers (from physics to biochemistry to material science… (yeah, I am crazy like that), I realised how deviated my perception was about viscoelasticity. So, here’s my attempt at making it simpler and easier for the rest of my fascia nerd family ????
I have tried to keep it to purely stringing facts together and less of any personal opinion (although there might be a few). I have also attached the links to a few of my favourite papers.
So, here goes…
According to Rheology (Greek: Rheos – flow/ logia – study of), most materials are viscoelastic & have both viscous & elastic components.
Materials with the highest viscous portion are called Viscous/Newtonian liquids & no matter how they are stressed, their viscosity never changes e.g water/ salad oil
Materials with the highest elastic portion are called Elastic/Hookean solids & they show the same level of stiffness unless they are destroyed e.g. rubber/ steel
WAIT…WHAT? Solids have higher elasticity?
Isn’t elasticity the amount a material can be stretched? Well, Yes and No! Yes, because it is the amount a material can stretch before it can deform & no, only because it doesn’t give the complete information about the material.
Let me explain…
Elasticity is the material’s ability to stretch when a force is applied and also its ability to return to its original state when the force is removed. It is a measure of the stiffness or resistance the material inherently possesses to avoid deformation when forces are applied on it.
E.g., when the intrinsic stiffness/ resistance to stretch is greater than the force that is applied, no change is seen in the material (steel), but when the force applied is greater than the intrinsic resistance/ stiffness, the material deforms and takes time to resume its original form (rubber). That would make rubber more deformable but steel more elastic!! (that blew my mind again as I typed !!!)
So, the stiffness of a material is used as a measure of the resistance to elastic deformation
|Young’s Elastic modulus =||Stress|
From the equation, it is clear that the lesser the strain experienced, the greater the elastic modulus (measure) of the material. For the same amount of stress, the strain (deformation) experienced by rubber will be way higher than steel
In purely elastic materials the applied energy/force for deformation is recovered & stored after each cycle without any loss (think of a thick steel spring).
Phew!! There… this was the hardest part of this article.
Now on to Viscosity.
Viscosity is a measure of a fluid’s resistance to gradual deformation/ flow by shear stresses (forces); it describes the internal friction of a moving fluid and also explains its thickness. The viscosity of a liquid depends on the chemical structure, morphology (shape & form) & the attracting forces between them.
E.g., Honey, a fluid with high viscosity results in a lot of internal friction between neighbouring particles (big sugar molecules in water) that are moving at different velocities & water, a fluid with low viscosity results in very low internal friction when it’s in motion.
Viscosity of a material is a function of the amount of force applied on the material (shear stress) and the amount of time it takes to deform (shear rate)
|Viscocity =||Shear Stress|
Higher the viscosity of the liquid, greater is the force that needs to be applied for it to flow (honey requires more force to flow than water). In purely viscous materials, all the applied energy/force is dissipated or lost by the internal friction as the system flows.
Viscosity & elasticity are just different ways a material resists deformation to a force.
But there are very few materials that are purely elastic or purely viscous; most fall under the viscoelastic/ Non-Newtonian category, i.e display both viscous and elastic behaviour.
When these materials are subjected to shearing forces (stress), they flow & deform over time (the viscous component) but they also show a time dependent recovery to their original form when the stress is removed (the elastic component)
Living tissues are all viscoelastic. Some tissues might even appear like a solid material, but when a shear force is applied, they don’t show a simple linear increase in their stiffness/ resistance to stretch (like a pure elastic material would), but rather a combination of complex mechanical behaviours such as, viscoelasticity, non-linear elasticity, viscoplasticity & poroelasticity.
If the force applied is beyond the viscoelastic material’s intrinsic resistance (a physiological upper limit for tissues), then the material will show a non-recoverable/ permanent deformation (viscoplasticity). E.g., When tendons are stretched slowly, they extend but also recoil back to their original form; but, if they are rapidly extended, they can further deform, stiffen and eventually rupture.
Fasciae (loose & dense connective tissue) also demonstrate viscoelastic properties.
But first; let’s see what are the components that contribute to this property? We know that all connective tissue basically comprises the (ECM) Extracellular matrix (i.e. ground substance & fibres) and cells. ECM is primarily responsible for viscoelasticity & other mechanical properties of a tissue.
Okay, now to the juicy part… Let’s break things down and see how the ECM contributes to the viscoelastic property of fascia.
Let me try to explain this in a few steps:
1. The molecules/components of the ECM first begin to disperse/ flow/ organise in the direction of the pulling force & demonstrate a time dependent deformation. Similar to the sugar molecules in honey, the ECM has many more large macromolecules of varying sizes, that are attracted to each other, move at different velocities creating an internal friction (i.e an intrinsic resistance to freely flow unlike water) & also loses some amount of the applied energy in the process (how the energy dissipation and the internal friction occur are surely for another article… we’ll stick to the basics for now). Higher the shear stress, higher the rate at which it will deform. This is the viscous component.
2. As the shear stress/force continues, the fibres of the ECM now come into play. They uncrimp (collagen)/unfold (elastin) & are rearranged in parallel. Since this is still a time related deformation, it is considered as a component that contributes to the viscous behaviour of fibres.
Picture from cen.aes.org
3. If the tissue experiences a greater stress/ force, the uncrimped/ straightened collagen fibres start to align and assemble closer to each other, becoming stiffer (as a reaction to resist & deal with the applied force). This behaviour which appears similar to a linear-elastic material can extend until the fibres/ tissue reach their physiological upper limit/ maximum strength.
4. If the force is removed before the tissue/ fibres reach their physiological upper limit, then the matrix & the fibres recover and go back to their pre-load/ stress form and will also restore some of the applied energy back in the process. This is also the elastic component of the ECM.
But if the force continues to increase beyond the physiological upper limit of the fibres/ tissue, the strong bonds that form the cross links and hold the stiffened fibres together, break. This is called the point of failure, where the fibres experience a non-recoverable plastic deformation.
Picture from researchgate.net
Thus, when a shear force is applied, fasciae show a combination of, a time dependent viscous deformation & a recoverable elastic strain i.e, viscoelastic behaviour.
An altered balance of viscous & elastic properties in the ECM could result in an impaired mechanical & physiological functioning of the tissue/ organ. For e.g., an increase in the viscosity of fascia could result in densification & an increase in the elastic modulus (stiffness) due to fibre congregation could lead to fibrosis.
Under normal physiological conditions, tissues maintain homeostasis by a tightly controlled synthesis and degradation of the molecules of ECM (hyaluronan/ proteoglycans/collagen/elastin etc.). The cells of the ECM are observed to be constantly working on a feedback regulatory mechanism in order to maintain this balance.
But in pathological conditions such as arthritis/ osteoporosis, the net degradation of ECM components is observed & in conditions such as fibrosis, motor neuron injuries & muscle stiffness, a net increase in ECM synthesis is observed.
Most importantly, immobility & trauma to the fasciae are shown to increase synthesis of ECM resulting in an increase in their viscoelastic behaviour. Imagine if this situation were to occur at the myofasciae, the fascicles would end up sticking together because of the increased thickness of the matrix (imagine many molecules competing for the same space), resulting in an increased friction between the layers & a reduced gliding during movement.
This knowledge of the viscoelastic property of fasciae is so critical in providing a rationale for various manual and movement therapies. For e.g., the sensation of tightness/restriction that we experience when we keep stretching the life out of our hamstrings, is actually a message that is being conveyed to us that the fasciae/fibres around the area have reached their physiological upper limit and begging us not to push it any further. If they are stretched beyond this upper limit, then the strong bonds that are holding the already stretched & stiffened collagen fibres could break, and for the cells to replace these damaged fibrils with new ones could sometimes take up to a year!!
Not just hamstrings, but any muscular area not complying to give an optimal range of motion (specific to the individual) requires a detailed understanding of the role of extra-muscular fasciae & myofascial chains in muscle function.
I better stop now, I feel like I have almost written a book (a tiny one…haha!). I hope you enjoyed reading this as much as I enjoyed writing this! Looking forward to hearing all your questions/ comments/ feedback. Let me know what else you would like me to write in detail about.
I also offer a series of courses where I teach this and many more amazing aspects of fascial anatomy and behaviour. Get in touch with me if you wish to host a course. All courses are online currently.
Here are the links to a few papers (please email me if you need the links of all the papers i have read) https://pubs.acs.org/doi/pdf/10.1021/acs.macromol.0c02709 https://tel.archives-ouvertes.fr/tel-00365542/document https://onlinelibrary.wiley.com/doi/full/10.1002/adhm.201901259 https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5786325/ https://www.researchgate.net/publication/281308346_Viscoelastic_Properties_of_Hyaluronan_in_Physiological_Conditions