The Physical Stress Theory was proposed in 2012 to help guide physical therapy rehabilitation. It basically says that any change in the stress applied to any tissue in your body will have a predictable response, and that applied stress is a combination of three things: time, intensity, and direction. Translating this into a more practical illustration, if you apply pressure on your knee in different directions at a low intensity but increase the time under pressure, you essentially increased the stress to that knee. If, on the other hand, you apply that pressure for a shorter time with the same low intensity but always in the same direction instead of different ones, you again increased the stress. And so on, you get the idea. This theory proposes then, that when exposed to any change in stress whatever the combination of time, intensity and direction is, be it higher or lower stress, the tissues respond with one of 5 possibilities: decreased stress tolerance (atrophy), maintenance, increased stress tolerance (hypertrophy), injury, or death.1  

If the stress level that the tissues are submitted to is lower than the maintenance level, then the tissues will adapt by lowering their maintenance level threshold. This means that any low stress will help maintain the tissues the way they are, but this also means that a higher stress will go over that maintenance threshold into the increased stress tolerance and if too high, into the injury threshold. 

When you are in a rehabilitation process, the amount of stress you apply to your tissues (muscles, bones, tendons, ligaments, cartilages, lungs, heart, etc.) will cause these thresholds to change.  If you apply too much too soon (the intensity and time we talked about earlier), your tissues may get injured (they cross that injury threshold). But if you apply too little stress, and this is important, they will lower their thresholds and be injured by a lower stress. The trick in rehab is to maintain the stress levels so they are not too much nor too little.  This way we can get tissues to gradually increase their tolerance without venturing into the injury zone.

The reason I mentioned before that applying too little stress and lowering that injury threshold is important is because it gets easier to be injured if you let this happen. And this is an unfortunately too common occurrence when people try to rehabilitate. For centuries whenever a person gets injured, they would and still get told by physicians to rest. Then friends, neighbors, colleagues, coaches, etc. might also add things like “hey you are doing too much, you’ll get injured”, or “you shouldn’t be exercising, your back is hurt”. And in fact, people might do something while recovering from an injury and feel a significant increase in pain. But in reality, it’s not that whatever they did actually injured them, it’s that most probably they did too much of it too soon (the time and intensity part of the equation we talked about before).  

If that same person would have done some light physical activity first, and then gradually increased the difficulty as the days went by, then the body would have had the opportunity to adapt. The threshold for increased tolerance and injury would have risen. Then they do it again, at a higher intensity, raising those thresholds again. But what most people do is avoid the activity altogether thinking it hurt them more, whilst lowering their injury and tolerance thresholds. When these thresholds are lower the chance of getting injured is higher. This is when they enter a vicious cycle of avoiding physical activity thinking it hurts, getting weaker lowering their tolerance, then finding an even lighter easier activity that now also hurts so they avoid doing that too, lowering those thresholds even more, and so on, and so on.

The takeaway from this Physical Stress Theory is that when rehabilitating we need to strive to increase our tolerance and injury thresholds so we can do more, and it gets more difficult to get injured. Avoiding movement is detrimental to any rehabilitation plan and should be discouraged. As long as we take into account doing the adequate amount of intensity, time, and direction in which we stress the body we can recover from a plethora of injuries. As physical therapists that is what we do, we figure out along with the patient what is the best form of exercise and how to progress it safely to get you moving again.


  1. Mueller MJ, Maluf KS. Tissue Adaptation to Physical Stress: A Proposed “Physical Stress Theory” to Guide Physical Therapist Practice, Education, and Research. Phys Ther 2002;82:383-403.