The Difference Between Stress and Strain - Jules Mitchell Yoga

Every profession has its jargon. In yoga, we insert Sanskrit words in the middle of an English sentence, speak in metaphors that defy the laws of physics, and use anatomical analogies that would baffle anyone in the medical industry. But we understand each other. We agree that using a word like pranayama is more suitable than breathing (the weak English translation) because it has complexity, many subparts, and multiple layers of meaning.

Our jargon is poetic. It’s also hard to quantify.

When we use a biomechanical term, like stress, as jargon, we imply multiple layers of meaning. Said another way, we speak vaguely.

It’s not necessarily problematic to speak vaguely about quantifiable variables. But if we promise quantifiable outcomes we should absolutely speak more accurately. For example, “Breathe in life, breathe out stress” is a harmless metaphor (other than to a yoga teacher’s public image, perhaps). “Stress your ligaments to make them stronger” is different as it proposes a specific result from a specific action.

The statement about the ligaments is not wrong. But its accuracy is compromised when we aren’t totally clear what we mean by stress and how we define stronger.

Colloquially (and yogically), stress has many rich layers of meaning although it is generally used to mean a response to a negative stimulus. There is eustress (good stress) and distress (bad stress), so all stimuli are not actually negative. Additionally, a stimulus could fall into just about any category: chemical, thermal, emotional, physiological, psychological, etc.


Biomechanically speaking, stress is an applied load. It is measured as a unit of force. In the lab, we usually measure it in Newtons (kg*m/s2). In the gym, we lift units of mass (kg or lb). Either way, stress is equivalent to load. While you can stress biological tissues in non-mechanical ways (thermally or chemically, e.g.), you probably don’t stress your ligaments emotionally. Or maybe you do. What do I know?

The implication that stress makes ligaments stronger leaves us with a contextually narrow definition of both stress and strong. Connective tissues adapt to load by increasing their capacity to withstand load (i.e. get stronger). (Keep in mind not all stress is eustress –  see my previous progressive overload blog). Therefore, we are defining stress as load and strength as greater load bearing capacity.

Enter the stress-strain curve

Stress Strain Curve

I’ve blogged about the stress-strain curve before (when my narrative was immature and somewhat fear-mongering – yet I leave my old work up to show how I’ve evolved as I’ve continued to research).

The y-axis represents stress, and the x-axis represents strain, where strain is defined as deformation. In this hypothetical graph, strain is measured in units of length (mm). So for each unit of load, you can plot the associated unit of deformation. Notice stress and strain are not the same things!

Asana example

Malasana.  Squat Pose if I am going to avoid yoga jargon.

Biomechanics of malasana (garland pose in yoga)(image by Dreamstime)

When you do malasana, the patellar ligament (just to pick an arbitrary structure) of the knee is loaded. Load is measured in Newtons (simplified as roughly body mass x gravity). It also undergoes a temporary deformation (strain) as it is stretched over the flexing joint. To increase the load on the patellar ligament, you wouldn’t have to flex the knee joint more (not possible since it’s already in maximum flexion), you’d have to apply more load (maybe hold a sandbag overhead in malasana).

Passive stretching is a low load activity. To increase load, you need either

  • Gravity to accelerate
  • Greater muscle contractions
  • External load

Apply this to the previous statement “stressing ligaments makes them stronger” and you see that passive stretching does not increase stress on the tissue. It may increase strain as range of motion increases. But it doesn’t increase stress.

And no, guruji standing on someone is not an increased load. Well, technically it is, but the purpose was to increase strain. So, I guess if your teacher stands on you, your best bet is to resist further range of motion.

If you go to the research, you’ll see that load is overwhelmingly the variable researched and applied clinically [2, 5]. How I teach is informed by an overview of the body of literature in biomechanics and load management. This blog is the product of a hundred conversations I’ve had in recent years with teachers who are curious about biomechanics and tissue behavior. I am not criticizing passive stretching nor yoga. I just want to shed light on concepts in the yoga teacher educational circuit that need some clarity.

All of that said, I will say there is some new evidence that looks at the effects of strain on tissue and the results look promising. [1, 3, 4] But the outcomes so far mostly measure fibroblast activity and complex histochemical outcomes, not strength. We’re now just discovering the complexities of tissue quality as a combination of composition, architecture, and behavior –  all which could all be affected by variables other than load.  I would love for that research to continue so that in a few years I can write a blog with yet another evolved narrative.



[1] Cao, T., Hicks, M., Zein-Hammoud, M., & Standley, P. (2015). Duration and Magnitude of Myofascial Release in 3-Dimensional Bioengineered Tendons: Effects on Wound Healing. The Journal of the American Osteopathic Association, 115(2), 72–82.

[2] Gabbett, T. J. (2016). The training-injury prevention paradox : should athletes be training smarter and harder ? British Journal of Sports Medicine, 0, 1–9.

[3] Langevin, H. M., Bouffard, N. A., Badger, G. J., Iatridis, J. C., & Howe, A. K. (2005). Dynamic fibroblast cytoskeletal response to subcutaneous tissue stretch ex vivo and in vivo. American Journal of Physiology. Cell Physiology, 288(3), C747–C756.

[4] Langevin, H. M., Storch, K. N., Snapp, R. R., Bouffard, N. a, Gary, J., Howe, A. K., & Taatjes, D. J. (2011). Tissue Stretch Induces Nuclear Remodeling in Connective Tissue Fibroblasts. Cell, 133(4), 405–415.

[5] Schwellnus, M., Soligard, T., Alonso, J.-M., Bahr, R., Clarsen, B., Dijkstra, H. P., … Engebretsen, L. (2016). How much is too much? (Part 2) International Olympic Committee consensus statement on load in sport and risk of illness. British Journal of Sports Medicine, 50(17), 1043–1052.