Envelope of Function: A Better Way To Approach Injury

Jimmy Picard   |
January 30, 2023
Injuries and the stories we are given

In 2008 I was fresh out of college and earnestly making the transition from collegiate track and cross country to road racing. That winter I was training for Boston and excited to participate in the fabled race. Training had been going well until I strained my calf during a hard interval session. Bummed, but hopeful for a quick recovery, I went to a well-respected PT who specialized in treating runners. After a brief assessment, I was told my calf strain was actually coming from my back. He theorized that I was excessively arching my back while running and this was pinching a nerve. This pinching was creating calf pain. I was put on a traction table and given some core strengthening exercises.

Not knowing any better and trusting this new narrative I dutifully followed the PT’s plan. I had regular traction treatments, during which I was strapped to a table while my upper and lower body were pulled in opposing directions attempting to relieve the pressure on the nerve. I was now worried about the arch in my back. I’d constantly check my posture and attempt to correct any arching in my back. I walked around “activating” my core muscles all day long. Any time I passed a mirror or window I’d glance over to see how I was doing. To my dismay, it always seemed the same.  

After 8 weeks my calf didn’t improve and I was forced to drop out.

In hindsight, I now know this was total rubbish. I strained my calf from overuse. There was nothing going on with my back. I had no objective signs or symptoms of a “pinched nerve.” I was training like a madman, not resting enough, and simply overdid it. But that story stuck with me for years – I felt flawed, weak, and worried about my arched back causing more problems.

A new narrative: There is a better and simpler way to explain most running-related injuries.

In 2005 Dr. Scott Dye introduced the idea of Tissue Homeostasis aka Envelope of Function (EOF).  The EOF model states that tissues have a certain limit of loading that they can tolerate. This limit we can call your tissues capacity. See the Fig. 1 below.

Fig. 1

The green line represents your tissue’s capacity to tolerate load. Below the line are activities that you can tolerate and do not result in overuse injuries. When you are healthy and life is going well you can easily walk the dog, go for a hike and even do some long runs in the mountains. Sure you might get some muscle soreness but overall your body tolerates these activities well.

This is your envelope of function. It’s important to note that it changes from day to day, week to week, and month to month. It bears repeating, your envelope of function (capacity) is constantly changing. Have a poor night of sleep or too many beers and that green line can shift downward. Overly stressed at work or home. Same thing, your capacity drops. The opposite is also true, stay consistent with healthy habits and smart training, and your capacity increases and your envelope grows bigger.

So when do injuries occur? What happened to me and my calf injury?

Injuries occur when the demands you place on the tissue exceed its capacity. Spend too much time above the capacity line and the tissue may not be able to adapt. My calf flared up during a hard interval session on the track, in track spikes, during a high-volume week, and at a time when I was overwhelmed with stress and not sleeping well. I exceeded the capacity of my calf by pushing too far outside my envelope.

What happens next is the most frustrating part of being injured: the capacity line lowers and your tolerance to load decreases. For me and my calf, I could go for walks and do some strength training but any running was off the table. I could no longer tolerate the demands of running. See Fig. 2.

Fig. 2

This helps illustrate one of my favorite concepts in rehab; most running-related injuries are due to a change in tolerance to the demands of running. Injury is less about specific areas being damaged, faulty movement patterns, or poor running form. Instead, the painful area is simply sensitive and not tolerating running at the moment. This gives rehab a target to aim for, calming the painful area down, building up the painful area, and restoring its capacity.

How do we restore capacity?

Accept where you are to get where you want to go.

Restoring capacity is no easy task and is the goal of all good rehab. The first step is to acknowledge and accept where you are. This means admitting to yourself that you are injured and need to do something. Personally, this is often much harder than it sounds. Here are a few helpful tips to recognize when you are actually injured and not just dealing with a little niggle.

  • Are you limping?
  • Do you have pain that doesn’t settle down within 24hrs?
  • Is the pain greater than 5 or 6/10?
  • Would describe the pain/symptoms as “unacceptable”?
  • Are your symptoms worsening?  

If you answered yes to any of these questions, it is time to admit you are dealing with an injury and it’s time to back off. Backing off doesn’t necessarily mean you have to stop running completely. Instead, find the amount of running that you can tolerate.

Load management – find the right amount through trial and error

Returning to Fig. 2, you want to focus on all the activities you can do that are within your EOF. In this case that could mean sticking with 20-30mins easy jogs, adding some cross-training and PT exercises. The key here is load management – find the dose of movement, running, and strength work that is tolerated well and does not exacerbate your symptoms. Most often this means we modify and adjust instead of eliminate and remove. That said, there are times when we do need to remove running altogether, as is the case with stress fractures.

If you are still with me, you have now accepted that you are injured and begun the process of determining the limits of your new envelope. It’s important to mention that this figuring out is a process of trial and error. Begin by taking your best guess at your current ability level and seeing how your body responds. If all goes well, great! Build from there.

If you overshot our mark and end up in more pain or worsening of your symptoms, there is no need to panic. Simply take an honest look at what you did and make adjustments. Maybe 30mins of running was just a little outside your EOF or daily PT was too much. Take your best guess at what was too much, modify it, and see how it goes. Continues with this process of trial and error until you’ve found your new EOF.

During these flare-ups, it can be tempting (easy, intuitive…) to think that you may have re-injured yourself or caused further damage but that is most likely not true. Remember, we are talking about sensitivity NOT damage. Stay optimistic and keep searching for the amount of activity that is tolerated well.

Build your Capacity

With your new EOF established, you can start to rebuild your capacity. This is the fun and familiar part. Similar to starting a new training block, you have a starting point from which you slowly and progressively build. Instead of a focus on building fitness your target is regaining capacity and building your envelope back up. And just like with training this means challenging the body, in this case, the injured area, with a stimulus that is close to its limits. Follow this with adequate rest to allow the tissue to adapt. Rinse and repeat! You are now on your way to recovery. A future blog will cover this in greater detail, for now, know that this process takes time and that there is no instant fix. Trust the process and be patient.

Stress + rest + time = growth

Key Points:

Most running-related injuries are due to overuse and a decrease in your ability to tolerate running and less about damaged tissues.

Look for simple explanations for your injury.

Accept where you are – injured runners need to modify their training and expectations.

Find your new EOF of build from there.

There are no quick fixes and it will take some time for your body to adapt and restore its capacity.

Good rehab doesn’t just restore your original capacity/EOF – it builds a bigger EOF than before!

Redefine PT & Performance
Run faster. Feel better.
Certified UESCA
Ultrarunning coach
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