Painting Bridges: Chemical Stress In A Running Injury

Joe Uhan writes about the role of chemical stress in running injuries.

By on December 12, 2017 | Comments

Stay the CourseI use a lot of analogies in my clinical and coaching practices. No doubt, they’re entertaining for me, but I also feel they’re useful for making complicated processes easier to comprehend. What follows is one of my favorite analogies for what I believe is the principal factor in running injury.

“Why do we paint bridges?”

It sounds like a trick question, and I love asking it to my clients, especially my runner kids. And unless you’re a structural engineer, it’s a fun brainteaser. But before revealing the answer, let’s talk about steam engines. Yep, another analogy, so hop on board.

Get Going and Keep Going: Hormones that Drive High-Intensity Exercise

Hard running and our body’s fight-or-flight response are very similar. They are both fueled, in varying degrees, by powerful nervous and hormonal systems.

When we decide (either with the fire of a starter’s gun, or the firing of a real gun!) to take flight, two hormonal systems play a crucial role in getting and keeping us moving:

  1. Very-fast-acting ‘adrenaline’ (epinephrine, norepinephrine). These hormones instantaneously amp every part of our body to get us immediately moving to save our life. In athletic terms, this is the sprinter gear of our survival response. Go now!
  2. Less fast cortisol. This is another hormone secreted from the adrenal gland during a stress response. It helps keep us moving, supplementing the fast-acting flight systems with the energy they need to keep going. This is the marathon-gear survival. We are still going hard, more slowly, but for much longer.

For a more thorough primer on adrenal glands, their hormones, and function, consult Dr. Pam Smith’s piece on ‘adrenal fatigue.’

The Steam-Locomotive Analogy

A crude analogy of this high-intensity fight or flight: imagine the human body as a fire-fueled steam locomotive. In a resting state, the locomotive puffs along gently, burning a stable fuel of bulky slow-burning logs (in this analogy, fat) with relatively little necessity for fast- and hot-burning coal (glucose, carbohydrate).

But when called into flight–be it during a hard workout or race, or in a real survival situation–that locomotive’s engine must blast off with maximal energy to rapidly speed away.

The ‘conductor’ (the brain) immediately signals to the ‘engineers’ (the sympathetic nervous system and endocrine systems) to stoke the engines. They instantaneously message the ‘fireman’ to say, “We need fuel!”

The fireman’s first tool is adrenaline, which opens and activates all energy systems for full steam ahead. Fast-acting adrenaline, among other things, signals the consumption of any and all glucose for the getaway. Adrenaline powers the fireman’s shovel to fill the firebox with coal.

This rapid response, which happens in fractions of a second, creates enormous potential energy for action, which is paramount for our survival. That super-fast, intensely-hot-burning system powers immediate and swift action. Yet, this super-high-intensity response is inevitably short-lived. There’s only so much coal to shovel, ‘til it runs out. And like a race, we can’t sprint for very long.

But what if our flight is prolonged?

Glucose, that fast-burning coal, is in short supply. And those big ol’ logs (fat)? Too slow-burning to keep up the survival gear. In this case, the conductor has other tools. Through the engineers, the conductor signals the fireman to use cortisol. Through cortisol, the fireman is able to stoke the fire with alternative fuels. This, in effect, is the engine’s reserve energy system. While still powerful, as you’ll see, it is make-shift and wrought with potential negative effects.

Cortisol: The ‘Kitchen Sink’ Energy System

Physiologically, cortisol is a second-level stress hormone that provides additional back-up energy through catabolic energy liberation. In short, what cortisol does is break down other structures for use as fuel. It scrapes up alternative glucose from various sources, including:

  1. From glyocogen. Splits these long strains of carbohydrate into fast-acting glucose for energy.
  2. From fat. Breaks down triglycerides (fat storage) into free fatty acids, which eventually produces more energy.
  3. From muscle. Protein fibers in muscle are broken down into amino-acid building blocks, which are further reduced to produce more energy.
  4. From other byproducts. In the liver, other metabolic byproducts (lactate, glycerol) are also used to create energy.

As you can see, that’s a lot of breaking down! It’s akin to the fireman going through the train, ripping down wooden shelving (proteins), the upholstery (fat), and even the trash (lactate) and throwing it into the fire. In effect, cortisol powers the kitchen-sink fuel whereby the body breaks down essentially every physical structure for burning!

Through cortisol, we’re pretty much “cooking our garbage!” But its effects don’t end there. So severe are the orders from the engineer that most other non-essential operations are also halted. General upkeep and repairs (soft- and bony-tissue healing) and custodial and sanitation work (the immune system) are put on hold. This is to commit all resources for fueling the engine. And while this is a powerfully effective system for stoking a hot-burning engine–tearing apart the train to keep it going and ignoring repairs and sanitation–it is only a short-term option that, if too prolonged, will invariably result in catastrophe!

Even for a writer known for drawn-out analogy, that’s quite a yarn. The most important takes-away are these:

  • Cortisol is a second-level hormone that is activated by moderate- to high-intensity stress. These can be both real–like races, hard workouts, long runs, heavy strength training, and any moderate- to high-intensity exercise including non-impact cross training–or perceived–such as psychological or emotional stress (work, relationship, family stress).
  • Cortisol helps prioritize survival by mobilizing fast-acting fuels, producing an ongoing (and, on the short-term, inexhaustible) source of fuel.
  • In that process, cortisol breaks down key structural elements (muscles) for fuel.
  • To save resources for exertion, it curtails any tissue healing (muscle, bone) and immune-system function.

Based on these points, it is clear that, like many physiological processes, cortisol is a powerful, vital, and useful in the short-term but dangerous if utilized for too long. This, in effect, is the chemical stress of exercise.

Mediators of Cortisol: Balanced Stress and Rest

The good news is that cortisol can be limited in both frequency and intensity of use. Consistent and lower-intensity exercise helps down-modulate cortisol. In effect, by doing a lot of lower-intensity exercise, the body learns to more judiciously use its slow (logs) and fast (coal) fuel without resorting to tearing apart the train’s passenger seats for fuel.

Another huge mediator of cortisol is sleep. Sleep, in this crude analogy, has a way of calming the nerves of the conductor and engineer. Physiologically, the nervous system controls exercise and consumes by far the most fuel during heavy exertion (indeed, the brain itself consumes 25% of blood flow at all times). Thus, if the brain (conductor) and nerves (engineers) are properly rested, the conductor will find it far less necessary to dump cortisol into the system, thus preventing that panicked burn-it-all response. Sleep increases restoration and resilience throughout the body. More sleep, less stress.

After sleep, balanced training is the most important mediator of cortisol. As I tell my high-school kids, training is convincing the brain that what you’re doing is safe. In this way, we do large volumes of easy running to train our internal systems to do more running with less internal stress. This includes promoting fat-burning fuel (slower burning, but more abundant and less stressful) and decreasing quantities of destructive kitchen-sink fuel strategies.

This is the driving force behind the maxim of keeping the easy days easy. According the at least one study, cortisol secretion is negligible at exertion levels at or below 40 to 50% maximum oxygen uptake (VO2Max). At this level, blood cortisol levels raise only 5%. But with higher intensities, cortisol levels increase sharply: +39% cortisol at 60% VO2Max, and +83% cortisol at 80% VO2max.

About 50% VO2max equates to 60 to 70% maximum heart rate. Interestingly, this intensity level also correlates with Phil Maffetone’s “Maximum Aerobic Function” heart rate, described in this article on the importance of low-intensity exercise in returning to running after injury. In essence, by exercising at a low intensity, you are training the brain that running is safe, limiting the destructive chemical breakdown that occurs from too much cortisol.

That said, bouts of very high-intensity exercise can have the same effect! That, too, is brain training. Doing (and then surviving) short bouts of very hard exercise also trains the brain that we don’t necessarily need to burn the kitchen sink.

However, the most important element in stress and rest is the balance. Cortisol has its power not necessarily in circulating concentration, but in duration. The longer the body is ‘bathed’ in cortisol, the more pervasive and powerful its effects. Therefore, the key to mitigating cortisol is to blast very hard workouts–which temporarily create very high cortisol levels–then immediately follow with very low-intensity exercise, during which cortisol is destroyed.

Stress and rest. Stress and rest.

Easy is Not the Absence of Hard: The Importance of Having a Metric for Easy Training

Because cortisol increases markedly with even moderate-intensity exercise, keeping base-training efforts truly easy is vitally important because it limits the production and concentration of cortisol.

Conversely, if we start to ‘grind’–to run or cross train at medium or medium-hard efforts–cortisol increases sharply, yet seldom are any specific training objectives addressed. For many runners, this may be the difference between 30 to 60 seconds per mile on an easy day. This is a double-barreled negative effect:

  1. Cortisol is elevated (50 to 75%, versus 5%), ‘bathing’ tissue in corrosive cortisol for prolonged periods.
  2. Unless this is race-specific pace, no specific training objectives are being addressed. (Read: it isn’t a workout.)

High cortisol for nothing is a quick way to sabotage training and health! Thus, simply running medium-hard too often is an insidious pathway toward bonafide overtraining. Therefore, having a metric for easy is crucial in limiting cortisol production. This could be using a heart-rate monitor or more intuitively with mindful breath control. A meaningful metric for ultramarathon runners would be running at all-day pace.

Keeping easy days easy limits cortisol. Its relative absence allows for unfettered structural (muscle, tendon, bone) restoration, optimal immune function, and maximal readiness for those meaningful hard workouts and races.

And life stress counts! Because fight or flight is an automatic response to perceived threats, high cortisol levels can result in the absence of heavy exertion. Any ire-raising situations can cause the body to secrete and maintain high levels of blood cortisol levels. The longer they circulate in our system, the longer and more pervasively they ‘bathe’ the tissues in chemical stress. The body perceives stress (and in effect, high cortisol), expressed as elevated heart rate. This piece on the inter-relationship between physical exertion and non-exercise stressors explains why such life stress matters. High stress of any kind equals high cortisol, elevated internal strain, and impaired training and recovery.

Chemical Stress is the Preeminent Reason for Orthopedic Running Injury

Several years ago, I saw a cable news program expressing outrage about how “millions of dollars are being spent to paint a bridge!” They were incredulous that so much money was used for such a seemingly superfluous project.

“You know why we paint bridges?” I ask my student-athletes? The answer is to prevent corrosion. Chemical stress weakens physical structures. And weakened structures fail under load. Unpainted bridges rust. And rusty bridges collapse.

Chemically weakened tissues fail under otherwise normal stresses. This concept is so often overlooked by everyone from the orthopedic specialist, to the coach, to the athlete. But it is a compelling reason for chronic and repetitive injury.

The hallmark signs of chemically sensitized, over-trained runners include:

  • Failure to heal from an injury in a normal time frame. Most soft tissue (and bone) injuries heal in two to six weeks. Persistent pain and failure to heal often results when an athlete cross trains too intensely. Chemical stress is irrespective to impact, so your super-hard pool-running session floods your system with just as much cortisol as a hard run. Poor life-stress management can also prevent healing.
  • Pervasive injury sensitivity. These runners hurt one tissue (say, the left Achilles), recover, only to hurt another area (say, right hip) in quick succession. Cortisol saturation is systemic: all tissues are corroded, weakened, and prone to failure.
  • Unusual (or severe) non-traumatic injuries. The demise of tissues typically thought of as extremely tough and resilient is often chemically driven. Non-traumatic fractures to thick, sturdy bones like the femur, pelvis, or vertebrae in runners are often chemically mediated. This can also occur with major tendon ruptures like the Achilles and patellar tendons.
  • Impaired immune system. Chemically sensitized, over-trained runners tend to frequently get sick.
  • Impaired weight management. Chronically high cortisol impairs fat metabolism, blocks peripheral sugar uptake (in non-fight-or-flight cells), and, over time, promotes fat accumulation and weight gain. This is a prime reason that runners can run a lot, yet fail to manage let alone decrease body fat. (For a compelling and detailed analysis on the hormonal theory of obesity, check out Obesity Code: Unlocking the Secrets of Weight Loss by Jason Fung, MD.)

Thus, the keys to sustainable training, injury prevention, and optimal health are inherently stress management:

  • Balanced training. Have a metric for (true, low-intensity, cortisol-limiting) easy training. Then go hard, but allow optimal easy days to recover.
  • Life stress management. Limit cortisol production from non-running sources like family, relationship, and work stress.
  • Promote rest. Sleep is everyone’s most potent performance-enhancing strategy.

I strongly feel that the sustainable solution will improve multiple areas of our lives. By balancing chemical stress, injury risk will drastically decrease, fitness will rapidly increase, body fat will drop, and energy and mood will improve.

Call for Comments (from Meghan)

  • Alright, two extended analogies, one article, what do you think about Joe’s argument as a whole for the role chemical stress plays in our running injuries?
  • The ever-elusive balance of stress and rest, how do you find it? How have you successfully found it in the past? And when have you failed to strike this balance?


Hill EE1, Zack E, Battaglini C, Viru M, Viru A, Hackney AC. Exercise and circulating cortisol levels: the intensity threshold effect. J Endocrinol Invest. 2008 Jul;31(7):587-91.

NseAbasi N. Etim, Emem I. Evans, Edem E. A. Offiong, Mary E. Williams. Stress and the neuroendocrine system: implications for animal well-being. American Journal of Biology and Life Sciences. 2013; 1(1): 20-26

Relationship between Percent HR Max and Percent VO2 Max – NCSF. Obtained 12/9/2017 from

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Joe Uhan

Joe Uhan is a physical therapist, coach, and ultrarunner in Auburn, California. He is a Minnesota native and has been a competitive runner for over 20 years. He has a Master’s Degree in Kinesiology, a Doctorate in Physical Therapy, and is a USATF Level II Certified Coach. Joe ran his first ultra at Autumn Leaves 50 Mile in October 2010, was 4th place at the 2015 USATF 100k Trail Championships (and 3rd in 2012), second at the 2014 Waldo 100k, and finished M9 at the 2012 Western States 100. Joe owns and operates Uhan Performance Physiotherapy in Eugene, Oregon, and offers online coaching and running analysis at