How about this for a week’s worth of mountain fun?

  • Days 1–2: Approach several hours on foot from Chamonix, France, to the north face of the Grand Jorasses. Climb the Colton-MacIntyre route (ED, VI, 1,200m) and then descend a long, difficult route into Italy for a total time of 27 hours while consuming only two gels and a half liter of water.
  • Day 3: Pace a friend in a 50K trail race.
  • Day 4: Win the Chamonix Vertical K race.
  • Days 5–6: Travel to Alaska.
  • Day 7: Win the Mountain Marathon in Seward, Alaska, setting a new course record.

Sounds unlikely if not downright impossible, right? Believe it or not, Kilian Jornet did all of this in one week in 2015. Yes, his legend is well deserved. Astonishing as this and many of his other feats may be, there are sound and comprehensible reasons as to why Kilian has kept all of us amazed by his exploits over the years.

On top of any genetic gifts he may have, what Kilian has developed over the past 20-plus years of consistent and heavy training is a very high aerobic capacity along with a high degree of fat adaptation. This gives him what exercise scientists call extremely high efficiency. The good news for the rest of us, genetically gifted or not, is that we too can develop higher efficiency. In fact, it is one of the most trainable aspects of endurance.


How can you tell if you are a fat-burning beast who can go for hours and hours without eating or if you are fueled mainly by carbs and need to be pounding a gel an hour? By understanding your metabolic response to exercise. How much fat versus carb you burn at any given intensity can and should directly influence how you structure your training.

Just as in your car, efficiency in human performance refers to the amount of energy it takes to move from point A to point B. It can pertain to the energy required to run at a 6-minutes-per-mile pace or to front-point up a steep couloir at 500 meters per hour. Efficiency is comprised of two components: metabolic efficiencies and biomechanical efficiencies. These combine to paint a picture of the overall energy cost of locomotion in terms of both the quality and quantity of the fuel.

Metabolic Efficiency

Metabolic efficiency is a measure of the conversion of the fuel sources, fat and carbohydrates, into the intermediate energy storage molecule ATP, which is needed to power muscle contraction. Remember that ATP molecule from the “What Enables Endurance” article? If not, you might want to refresh your memory with a quick reread. Aerobic metabolism, which uses the energy stored in the molecular bonds of fat, produces 16 times as much ATP, gram for gram, as anaerobic (glycolytic) metabolism, which breaks down glucose and glycogen to make ATP. Since a gram of fat contains about twice the calories as a gram of carbs, the overall efficiency difference is about 8 times greater with fat than carbs. A gram of fat will produce about 8 times the amount of ATP as a gram of carbs. Since ATP fuels muscle contraction and thereby your movement, this means a great deal more work getting done gram for gram when using fats. What if you could get an 800 percent increase in fuel efficiency with your car?

Biomechanical Efficiency

Biomechanical efficiency is a measure of the amount of energy (in calories regardless of fuel source) needed to propel you at a certain speed. Watch world-class runners who seemingly float along, barely touching the ground, in comparison with the average weekend plodder. See how effortlessly elite climbers move across difficult terrain. These are examples of high biomechanical efficiency. It costs them less (in many cases MUCH less) energy to perform their events than it does the untrained. As interesting and important as biomechanical efficiency is, we will save that topic for another time.

Both of these qualities play major roles in endurance sports. And both are among the most trainable of the physical attributes that, when combined, are strong determinants of endurance performance. The remainder of this article focuses on the testing of metabolic efficiency.

Steve House and Kilian Jornet on the summit of Taschhorn, Switzerland
Steve House and Kilian Jornet on the summit of Taschhorn in Switzerland

Metabolic Efficiency Testing

Metabolic Efficiency Testing (MET) is done to measure the ratio of fat and carbs being used to fuel your exercise. The gold standard for determining your personal metabolic response to exercise is the Gas Exchange Test (GET). An MET requires the same equipment but a different protocol. It uses a very gradual ramping up of intensity in order to capture an accurate picture of your muscles’ metabolic response to exercise, especially in the low-intensity aerobic domain.

Any GET requires sophisticated lab machinery costing many thousands of dollars along with trained technicians to administer the test. This equipment measures and compares the amount and concentration of CO2 in your exhaled breath to the inspired oxygen to determine the relative percentages of fat versus carb being used to fuel your exercise. These tests typically cost in the range of $175–$250. Not every physiological testing lab can or will do these tests. In fact, it can be frustratingly difficult to find a place to do this type of test. To prepare you for this search, please read this article.

Knowing your actual metabolic response to exercise—whether you are a fat-burning aerobic monster like Kilian or a carb-guzzling anaerobic beast because you do CrossFit four days each week—will give you some of the most valuable information you can get as to how you need to structure your training. Ignorance may be bliss but it may also be what is holding you back.

While a properly done MET will give you the most information, the difficulty in finding a place that can do this test means that you may need to use one of the other methods we discuss: blood lactate testing or the heart rate drift test.

In “Getting Tested Part 2,” we delve into how to interpret the results of an MET and use the results to plan your training.

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