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Metabolic Concepts In Return To Running

What happens when metabolic fitness and returning to running from an injury collide?

By on January 14, 2014 | Comments

Stay the CourseTreating injured runners can be a daunting task (already documented on these pages). In my five years in the business, I’ve made a lot of mistakes. But with more trials came fewer and fewer, and greater successes: healthier runners, faster runners. Patterns universal to all human movement emerged that now drive my ‘mechanics-first,’ motor-control-driven treatment emphasis. To any runner who walks–or limps–through the door seeking pain relief and better running, my word to each is the same: you will get better, and you will get faster. They go together.

In addition to and before optimizing mechanics, there is always a major focus on the Four Healthy Tissue Concepts:

  • Full Range of Motion. Of muscles, tendons, joints, and functional body segments.
  • Strong, Supportive Muscles. Have the strength and endurance to support healthy movement.
  • ‘Being Normal.’ Using tissues in neutral ranges, with normal motor control.
  • Pain Control. Eliminating inflammation and gradually and progressively increasing tissue tolerance without overdoing it.

This approach, while successful, sometimes isn’t enough. There are occasional cases that are more difficult. These folks–despite nailing the Four Concepts–continue to experience pain and dysfunction, and the inability to progress running. Previously, I would conclude these rare folks to be zombies–the true undead–and be forced to impale them with sharpened, wooden implements. But as I rained blows upon them, I realized that there had to be a better way!

There had to be a missing piece of the puzzle.

Too Fast for Your Own Good: A Mechanism for Chronic Pain

The Injured Zombie Scenario: A fit, previously healthy and pain-free runner suddenly experiences injury. It could be from an act of Trail Gods, or otherwise some imbalance of forces.

Prior to the injury, this runner ran an average of an hour a day at eight minutes-per-mile. One day a week she ran a hard workout consisting of a half-hour at six minutes-per-mile pace. Then she gets hurt. Because of the injury, she must stop running. She has some mechanical inefficiencies (as we all do) that may have contributed to her injury. These stride inefficiencies persist–if not magnify, due to compensations–for this runner. Pain persists, running is limited. One month off turns to two, then three.

She comes to see me with myriad issues related to the Four Concepts, but in the course of three to four weeks, we address them. Stride adjustments are made, and the runner is more efficient. And faster. With her new stride, she has difficulty running slower than 7:30 pace. Running this way, while more efficient, takes more energy in the learning process, and she has difficulty modulating this new efficiency to a sustainable pace. This is a very common problem in learning a new motor task.

We institute a return-to-running program. We choose a more conservative approach of run/walk intervals–one minute run, 30 to 60 seconds walk rest–for a total of 10 minutes. She runs. But she still has pain. We double check mechanics, tissue mobility, and strength. Everything checks out. She continues to run, and pain continues.

So what gives?

Delving further into the running, the runner notes that, while she is running only a minute at a time (and for only 10 to 20 total minutes), she describes her intensity as “pretty hard.” Further questioning reveals she is running at 7:30 to eight-minute pace, but sometimes faster. She notes that it “feels good” while doing so, but her injury “aches” the rest of the day.

Intensity Matters: Fuel Selection and Inflammatory Stress

The answer to the undead pain puzzle lies in intensity. Most runners and sports-med professionals agree that faster running is more stressful, yet most would cite that mechanical stress–higher speeds equal higher forces–as the primary reason.

But what about chemical stress?

With any activity, we have two primary fuel sources: fat and sugar. At most efforts, our bodies burn a mixture, but ultimately prefer to burn fat. We have lots of it, and we’re wired to ‘go slow all day’ with everything we do. However, as a part of our survival, we’re also equipped with the ability to go really hard for short periods of time. At any given time, we have about one to three hours of fuel to go at high intensity, and that fuel is sugar.

While sugar burning is a fast-acting, convenient, high-octane fuel, chronic use has its drawbacks. When the body shifts away from fat to sugar burning, the body responds by secreting inflammatory chemicals. The body, perceiving this high-intensity activity, releases these chemicals to preemptively repair any possible damage that may occur from such intense efforts.

A primary chemical consequence of prolonged sugar-burning exercise is increased cortisol production. Cortisol is a stress hormone secreted in higher volumes with sugar-burning exercise; its primary role is to mobilize more sugar into the system, as well as other energy forms. In small doses, cortisol has positive effects. But with prolonged and excessive secretion, cortisol can, among other things, decrease tissue healing and dampen the immune system. There are other inflammatory byproducts from the sugar-burning process, but cortisol is the primary culprit.

So when a healing runner exercises at a high-enough intensity to burn predominantly sugar, the result is more than mechanical stress: they inadvertently flood their system with inflammatory, tissue-weakening chemicals. These chemicals attack the healing tissue (and often the sensitized nerve tissues around the injury). The result is ongoing pain and poor tissue healing. There’s the rub.

Aerobic and Anaerobic: A Tight Balancing Act

Most runners recognize that sugar-burning–high-intensity running (or any other physical activity)–must be done sparingly. But how, precisely, do we know when we’re fat versus sugar burning?

Aerobic fitness is defined by the maximum intensity and duration of exercise we can partake while using fat as a dominant fuel source. Fat burning requires a lot of oxygen and a lot of cellular mitochondria to process, but with more practice (training), the body can burn fat faster, and longer. But when the demands of activity out-strip what fat can provide, the body shifts to more and more sugar. But it is not an either-or, all-or-nothing scenario. Rather, it is a dynamic ratio of fat and sugar at various intensities.

However, for each of us, at any given fitness level, there is an intensity level at which our body shifts completely to sugar burning. This is referred to as the anaerobic threshold (AT). Beyond this point, we are burning pure sugar and the above-mentioned chemical stressors are most acute. Healthy runners are usually able to strike a balance of easy, fat-burning running with more intense exercise. They quickly learn what intensities are sustainable and stay within those limits.

But for injured runners, that system can be completely disrupted:

  • The fit, healthy runner might burn mostly fat at eight-minute pace.
  • The injured runner, due to lost fitness, has diminished fat-burning ability: eight-minute pace may now be entirely sugar-burning.
  • The fit, healthy runner, pre-injury, had consistent running mechanics.
  • The injured runner has learned a faster, more efficient stride, but the learning process drives her to run too fast–7:30 pace–beyond even her previous fat burning, and far beyond her current fitness.
  • Running fast (due to better mechanics, or impatience, and/or excitement to run again) burns sugar, creates chronic, systemic inflammation, and perpetuates pain and slows recovery.

It becomes a painful cycle that must somehow end.

Intensity Guidelines for Metabolic Stress Management

Phil Maffetone, DC, was one of the first sports medicine professionals to recognize the role of training intensity on injury incidence and recovery–and quantify it. Besides using it as a guide for optimal training, he devised and uses his ‘180 Formula’ to ensure athletes maximize fat burning and minimize sugar-burning and its accompanying stresses.

The 180 formula, simply put, is the optimal heart rate-based exercise intensity at which fat is still the dominant fuel source. The formula was derived from the thousands of respiratory metabolic tests he conducted with athletes over the past 30-plus years. The general rule: 180 minus your current age. This is an estimate based on the average of those laboratory results. And based on those thousands of results–and the various athletes, injured and healthy, he tested–he developed several caveats, including:

  • If you have or are recovering from a major illness (heart disease, any operation or hospital stay, etc.) or are on any regular medications, subtract an additional 10.
  • If you are injured, have regressed in training or competition, get more than two colds or bouts of flu per year, have allergies or asthma, or if you have been inconsistent or are just getting back into training, subtract an additional 5.
  • If you have been training consistently (at least four times weekly) for up to two years without any of the problems just mentioned, keep the number (180–age) the same.
  • If you have been training for more than two years without any of the problems listed above, and have made progress in competition without injury, add 5.

The 180 Formula represents a reliable estimate for those without access to a full testing apparatus. Well, I happen to have that apparatus. So I use it.

Metabolic Testing

At our clinic, we use the iMett system for metabolic testing equipment. It uses a mask and tubing to capture exhaled gases and analyzes the ratios of carbon dioxide produced while the athlete runs progressively harder on a standard treadmill progression. Measuring exhaled carbon dioxide, correlated with heart rate, allows us to determine the precise effort level and heart rate at which runners stop burning fat and switch to anaerobic, sugar-burning.

Treadmill Test

A runner undergoes metabolic treadmill testing using the iMETT system. All images courtesy of Joe Uhan.

It also measures maximal oxygen uptake (VO2Max), but that value is far beyond max fat burning and is of little use to us as runners. (This is fodder for a future column.)

iMett Raw Data

A sample data set. The “%FAT” column represents the decreasing fat utilization with increasing intensity (and heart rate, noted under the “HR(bpm)” column).

After the short test is concluded, we have a precise value of:

  • Anaerobic threshold–the absolute point (heart rate) where fat burning ceases, and
  • Max aerobic (fat burning) zone–the point approximately 10 to 20 beats below anaerobic threshold, which represents a moderate exercise intensity that still burns a substantial fraction of fat.
iMett Ranges

‘Aerobic’ effort range is calculated from the collected data.

The maximum aerobic intensity–this neat, tidy, heart-rate range–becomes the injured runner’s maximum exercise intensity.

Using Metabolic Limits to Guide Running Progression

As standard practice, all of my injured runners returning to running are advised to stay within their maximum aerobic range: either estimated using the 180 Formula, or directly measured.

For the sensitive, ‘problem healers,’ it is a mandate: all are tested, all are strongly encouraged to obtain and use heart-rate monitors, and all running must stay under that max aerobic intensity. For how long? It depends on the runner.

Numbers don’t lie. When a runners sees the measurement of their own body, it is difficult to deny. Our injured zombie runner above, who ran 8-minute pace pre-injury — and ran at time faster when she returned to running – was now limited to running only 9:30 pace: the fastest she could run while maintaining her Max Aerobic heart rate. She ran at that pace consistently, and over the course of the next several weeks, that pace gradually increased as she became more fit and efficient. But more importantly, her pain disappeared.

Every ‘problem runner’–those who continued to have pain despite all else being normal–that stayed discipline to their max aerobic intensity got better. Pain disappeared. They got better, and in time, they got stronger and faster.

Guidelines for Injured Athletes Returning to Running

The following are a collection of tips for everyone returning to running after prolonged injury:

  • Be sure you and your sports-medicine professional thoroughly address the Four Concepts of Healthy Tissue prior to resuming running.
  • Perform a metabolic test to determine your anaerobic threshold and max aerobic effort (based on heart rate). The test is submaximal (not going to 100% effort, VO2Max testing), so it is only moderately stressful, even for a healing runner.
  • If you don’t have access to testing, calculate your max aerobic range using the 180 Formula.
  • Consistently monitor your heart rate during each run, ideally using a heart-rate monitor. If you don’t have one, get one: they are as critical to your running as your stopwatch.
  • Spend 5 to 10 minutes gradually warming up to your max aerobic heart rate at the beginning of each run or exercise bout.
  • Spend 5 to 10 minutes gradually cooling down from each run/exercise bout.
  • Do not exceed your max aerobic heart rate at any point in the run. Confounding factors such as stress, poor sleep and nutrition, and environment will drive up heart rate: some days your run will be two minutes per mile slower. Regardless of how aberrant the reading, and how easy it feels, maintain effort at/below.
  • Do not exceed your max aerobic heart rate on any run for:
  1. Short-term injured (under three months): at least one month
  2. Chronic injured (over three months): at least three months
  • Consider cross training in order to more feasibly stay at/below your max aerobic pace.

This approach takes a lot of discipline, but the payoff is tremendous: tissues are allowed to heal without interference from harmful inflammatory processes, and you are immediately–and robustly–forming the foundation of a strong aerobic base critical for strong and healthy running!

Best of all, it is a true cure for those Injured Zombies.

 Call for Comments (from Meghan)

  • Might you be one of those runners who’ve gone through a complete rehab process after an injury but who aren’t yet healed? If so, what’s your situation?
  • Have you trained with Maffetone’s 180 Formula? What have you learned from it?
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