Metabolic Concepts In Return To Running

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

is a physical therapist, coach, and ultrarunner in Eugene, Oregon. 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

There are 13 comments

  1. jaxcharlie845

    Great info I haven't read before. Though I have practiced the super easy running only when coming back from injury, it's nice to see some science behind it!

  2. ClownRunner

    Top ten ways you know you've gone way too Anaerobic during a workout:

    (1) You drink 5 cokes and eat 4 cookies and 2 hamburgers immediately post-run;
    (2) You drink 2 cokes and eat 3 cookies and 1 hamburger an hour later;
    (3) You feel grumpy the rest of the day;
    (4) You feel grumpy the next day too;
    (5) You try to run fast again the next day to cure the anaerobic hangover;
    (6) You sign up for 3 races online because going anaerobic makes you feel powerful;
    (7) You regret signing up for 3 races online because a day later you feel injured;
    (8) Your order 2 new pair of shoes because you blame your shoes for the injury instead of the chemical imbalance;
    (9) You email Olive Oil Joe for advice and he tells you to eat nothing but Vespa and Coconut Oil and run slowly in the mountains for weeks until you feel better;
    (10) You vow never to make the same mistake but the temptation is too great once you feel better again. You end up watching Downton Abbey episodes with your wife and remember the good old days out on the trail mixing it up with the big dogs.

    (Not that I thought of #10 from experience, mind you…)

    1. 00joeuhan

      Mr Clownrunner-

      Thanks for the comment.

      This does, indeed, pry open the can of worms on aerobic vs anaerobic training and the concept of "anaerobic stress addiction". Indeed, few if any endorphins are released when you fat burn…because they're not necessary. Endorphins are released to protect you – for you to continue to frantically escape the lion that's chasing you, without succumbing to the intense pain of the escape. That said, they are addictive and pain-"relieving" (masking) to an injured, rehabbing runner.

      1. ClownRunner

        It's a different kind of endorphin. Back in 2008 I trained with Mike Wardian and Bryon Powell on the W&OD bike path in Arlington, VA. They had higher thresholds, but I could spike down to low 6's to hang on (easy pace for them)….I would get back home totally worked but feeling on an adrenaline high that was indeed addictive. But it ended up with me having plantar fasciitis, calf strains, hip pulls, you name it. Goin' too fast for my abilities. But wanting to keep up with the big dogs…

        30 milers in the mountains are a different animal….it's that slow build of milder endorphins that I crave now (and can't usually get because of family/job, etc.). Of course, it's not possible for that slow build of endorphins when you're compromised in any fashion….like when you show up at Bandera just over a week-long flu ;)

    1. 00joeuhan

      Maffetone, himself, admits that, with higher age, the formula becomes less precise. This is where individualized metabolic testing is helpful.

      But three months of aerobic, while requiring a lot of patience, *should* be the cornerstone to each and every runner's yearly training progression, in order to maximize fitness, and minimize injury and burnout.

  3. kjz

    Tempting to try this approach–wishing I had a PT similar to Joe here in town–any rec's in Denver metro, Joe? I'm on 2.5y with a nagging thing that will disappear for some months then return with a particularly flat/"fast" run… handles the super hilly/mountainous stuff just fine. I haven't tried any of the Maffetone training… and my success with PT (I am a PT) and ART, Massage, yoga, dry needling, chiropractic is great for awhile… until the problem returns again.

  4. smithmj77


    I can vouch for the Maffetone method to recover from injuries. A hip injury just kept hanging around even after visits to a good PT. It was better, but not normal. I read Maffetone's book and started using a heart rate monitor about a month ago. My last two workouts have been pain free. I've also lost 5-10 pounds. The slower pace annoys me a little bit, but I'm committed to sticking with it.

    Great article.


  5. stayvertical

    Damn, Joe. This is a masterpiece. I don't think many people really understand what a role the sugar vs fat equation plays in distance running- especially beyond 50 miles. Your explanation of the cortisol response is the simplest and most succinct I have ever seen. If people knew what it meant to their enjoyment and competitive desires in ultras, there would not be many bagel and pasta types left among us. In my mind, this issue is NEARLY AS IMPORTANT as the type and quantity of training that one does. When asked how top runners manage to run uphill at the end of 100's and finish strong, the real answer is that they have mostly burned fat all day and not polluted themselves with toxic anaerobic energy byproduct. The next question is how they were able to burn fat all day while still running so fast? That's an equation of training stimulus, metabolic and cardiovascular efficiency, proper pacing and fueling. For me, every bit of training and eating that I do is to pursue a more efficient fat burning response on raceday. It's not as hard as it sounds. That means using a heart monitor to SLOW ME DOWN for much of my training and eating lots of meat and veggies, but very little starch and sugars. This trains the body to burn fat around the clock. Not much of a sacrifice at all, if you ask me. Especially when you have experienced the effects of this approach late in a race. No going back.

    Great stuff man. Please keep it coming. We have so much to learn from you.


  6. misszippy1

    Agree, agree, agree! I did MAF training last summer for four months in order to switch my body over to a (mostly) fat burning machine. I am a believer. I also, however, switched over to some intense marathon training after that base building period and am now hurt. So…when I return, it will be MAF until the point when I am no longer seeing reductions in pace at my low HR. And then when I do add in intensity again, it will be far more gradually. In sum, I am a MAF believer and will use it as my foundation from now on.

    1. acruzcosta

      Miss, It is really good to hear good things about MAF. I started with on the end of November and I´m enjoying it. The only point to me is that I don´t know the right volume for run a marathon. I read his book, but I didn´t find it there.

  7. gjoyes

    Great article as usual, Joe, thanks for you sharing this with all of us.

    One clarifying question: if I use the 180 formula and a heart rate monitor to stay below my max aerobic pace, could I go into the anaerobic threshold cross training, say skate skiing, biking, or working in the gym? I'm just thinking it would be awfully nice to keep working on the VO2 max while staying injury free.

    1. 00joeuhan

      Stress is stress: whether it's various forms of exercise, or simply "life stress". The brain doesn't know any different, and treats it all the same. That said, to avoid "bathing" your sensitive tissues in inflammatory chemicals, stresses of all kinds must be mitigated/limited in the rehab process.

      Regarding VO2Max training: the marathon distance is 98% aerobic (and ultras? 99.9%?). That said, unless you're also training for mile/1500m training (which is 50/50), VO2Max "gear" plays little role in long distance performance and – in my opininon – wholly over-rated and unnecessary. That is not to say hard running isn't a vital part of ultra preparation; it is simply not a high priority, specifically, and has no role in the injury rehab process – other than destructive – and should be avoided.

  8. javieronn

    Dr. OOJ,

    What type of nutrition is required for a runner following a more aerobic-focused training plan? 'Stayvertical' commented above that he sticks to meats and veggies. Is this the gist of it? What about pre-workout fueling and post-workout refueling? Less carbs than usual? Can I replace all carbs with peanut butter and Nutella?

    1. 00joeuhan

      Thanks for the comment.

      As far as nutrition goes, you're entering territory ripe for debate (as with all areas of science). I will defer to Sunny Blende, who is far more knowledgeable than I am.

      This article appeared in Ultrarunning Magazine in 2013 and was just posted on their website:

      It describes the integration of both training and nutrition to emphasize fat-burning.

      1. javieronn

        Cheers! Thanks for the article. Also bought Maffetone's book to learn more about his method in-depth. Your column is consistently one of the best running resources on the web!

  9. @MattFielding

    I started the Maffetone approach in May after being injured on and off for 2 years. I also saw it as a good opportunity to completely change my running technique at the same time, switching to a barefoot style. I also changed to a high fat low carb lifestyle too. These 3 changes have completely changed my health. They all support each other.

    I got wife started on the Maffetone approach last week, She had to walk about 80% of a 5k route to keep her HR down under 140, but she was eager to get out and do it again a couple of days later. She's even convincing her mother to get a HRM and try it too.

  10. sakimos

    Hi Joe. Thanks for great post. Maybe you could help me out or give me some insight?
    I have had a rough few years with regards to running injuries. The general gist goes something like thisStress fracture in left femur 3 years ago, Ongoing mysterious left "shin pain" the last 2.5 years that I have tried to just ignore (mri revealed nothing wrong), Ongoing left hip pain for last 2.5 years due to hip impingement and possible trauma when stress fracture occured. Posterior tibalius in right leg 6 months ago and just 6 weeks ago sprained or partial "tear" of meniscus in left leg. Generally, I haven't been feeling great in part due to life stress with work/life and/or stress due to not being able to enjoy running without pain for the last 3.5 years.

    It seems i have both issues: chronic inflammation and flaws in my four pillars of healthy tissue (even though i have been seeing a physio regularly). I live in Australia now, 28. I was once a sub 230 marathon and sub 15 5k guy 5 and would love to have better years ahead. Anyone you would recommend seeing? or any thought?


  11. @mikewhitenton

    Hey Joe,

    Incredible post and just at the right time as I'm starting out MAF. I headed to my normal (technical) trails (similar to Bandera) and couldn't get above a good hiking pace without hitting my MAF limit (135). I'm coming off about 6 months of no focused run training, but ran a handful of ultras before that. Is the logic that, eventually, I'll be able to do more than just hike at that heart rate… I just need to be patient? I'm good with that, but wanted to check with you (or anybody else out there) since it will be easier to be patient if I know I'm on the right track.


  12. RickHecht

    The advice to limit the level of high intensity exercise when recovering from an injury makes a lot of sense to me. As a physician whose research includes studies of stress and cortisol responses, however, the explanation that this should be done because “there are other inflammatory byproducts from the sugar-burning process, but cortisol is the primary culprit” is more puzzling to me- though maybe there is something in the medical literature I am missing. While the actions of cortisol are complex, the primary actions are anti-inflammatory. In the literature I am aware of, increased cortisol in the levels stimulated by exercise have been associated with more rapid wound healing- the opposite of what you seem to saying (see for example: Emery, et al. Exercise Accelerates Wound Healing Among Healthy Older Adults. J Gerontol A Biol Sci Med Sci.2005; 60: 1432-1436). Are there studies you know of that I am missing that implicate physiological levels of cortisol due to exercise as the cause of slow recovery from exercise related injuries?
    -Rick Hecht, MD, Professor of Medicine, UCSF

  13. Jazz_1618

    Dear Joe,
    Thanks for this great article and thank you irunfar for sharing it.
    I’m one of those injured zombie runners, or I was, until recently.
    I read the article, recognized myself in it, gave your advice a try, and for the first time in more than a year haven’t had any pain in my ankle. On top of that, my spirits and my confidence are up again. Long story short, you've helped me and I've learned a lot.
    I am very grateful.

  14. ggtri

    Great Article… particularly relevant as I am currently recovering from knee surgery. I would suspect the same concepts could be applied utilizing the Daniels' VDOT Easy Pacing as the target, dropping 2 or 3 VDOT points, depending on how long you were off from exercise. Thanks for the great information.

  15. Will

    Hi Joe,

    I’m not sure the MAF approach is sustainable for faster runners. My calculated MAF heart rate is 155bpm. I can go well under 6:00/mile and not go over 155bpm. If I ran 100+ mpw sub 6:00/mile I think my risk for injury would increase dramatically. Instead I often do easy runs around 7:00/mile, which is all sub 130bpm.

    One more example: I read somewhere that Galen Rupp did a 20 mile run (before the Olympic Trails) averaging 4:50/mile with 145bpm average. That would be BELOW his MAF heart rate.

    What is your take on this?

  16. Albert Shank

    Joe, I have lately begun to read your articles and have taken a great interest in their content. I have been following Maffetone training for years and have reaped the benefits of injury-free and burnout-free running. I see so many runners who just tear themselves to shreds and it’s so sad and unnecessary. The stuff I see from mostly ultrarunners is quite unbelievable (torn hamstrings, achilles, knee issues, blown lung [yes blown lung], back issues, stress fractures, heart problems, metabolic issues and so on). I usually push Maffetone on them, but now I also have your articles to back me up. Keep up the good work.

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