Exercise Physiology

The Science of Performance

Currently endurance physiology promotes three physiological factors as being the most important in performance - VO2max, Lactate Threshold, and Running Economy. I call these three factors the Big 3. Pick up any book on running and there will be multiple references to the Big 3 sprinkled throughout the book, explaining what they are, why they are important, and how to go about improving them.

Of the Big 3 running economy is the least understood. Physiologists & Coaches have yet to figure out what determines a person's running economy or how to improve it. They believe running economy is important, but can't explain much about it. Exercise Physiologist Pete Pfitzinger writes, "An important determinant of marathon performance is running economy at marathon race paces. Although some evidence shows that economy improves with training, no one fully understands the secrets of improving running economy."(1)

Running economy is the term physiologists use when talking about the amount of oxygen being consumed while exercising at a given rate. Jack Daniels, PhD, writes, "Running economy refers to the amount of oxygen being consumed relative to the runner's body weight and the speed at which he or she is running."(2)

Improving running economy - i.e. using less oxygen while running at a given pace or running at a faster pace while consuming the same amount of oxygen as previously consumed at a slower pace - is seen as both important and very desirable. Jack Daniels, PhD, write, "This is a highly desirable result of training, because the runner can now race at a faster speed than before without an increase in energy expenditure."(2)

In short, then, running economy is promoted as being very important in performance but, to date, no one has really figured out much else about it.

A recent research study examining adaptations to marathon training found significant changes in oxygen consumption while running at a fixed pace - i.e. improved running economy. The improvements in running economy in this study were not accompanied by large, significant changes in aerobic capacity, in VO2max, as one would expect. Instead, significant adaptations occurred in the muscles themselves, in the muscles ability to contract & produce power. When I read the results of that study it stimulated some new thinking on my part about running economy. Let's delve into running economy, then, and see what we can make of it.

A Perspective on Running Economy

Let's begin by first taking a look at the big picture so that we have a historical perspective on why and how the idea of “running economy” came to be.

In the 1920s exercise physiologist A.V. Hill first proposed the theory that oxygen was the limiting factor in endurance performance. Basically, this theory holds that the reason you can't run faster is because at high running speeds you can't get enough oxygen to your working muscles to meet their energy needs. As a result, at fast running speeds your body had to increasingly rely on "anaerobic" energy production to meet its energy needs. Physiologists believed that the by-product of "anaerobic" energy production was lactic acid, which poisoned your muscles and caused fatigue.

The ability to breath in, absorb, transport, and utilize oxygen is know as VO2. The maximum rate at which oxygen can be breathed in, absorbed, transported, and utilized is known as VO2max

In essence, this VO2max theory (now termed the cardiovascular/anaerobic model) predicted that those who had the ability to intake and use more oxygen (i.e. had a higher VO2max) would always be faster than those who had lesser abilities to take in and use oxygen (i.e. a lower VO2max).

As a result of this theory, exercise physiologists starting measuring the VO2max of large numbers of endurance athletes, from casual runners all the way up to the elites. That's when one of the first serious challenges to the VO2max (cardiovascular/anaerobic) theory first surfaced. Scientists discovered that the fastest runners didn't necessarily have the highest VO2max. While all elite runners had high VO2max values compared to average runners, amongst elites those with the highest VO2max weren't always the fastest runners. For example, Steve Prefontaine had a 16% higher VO2max than did Frank Shorter, yet there was only a .2 second difference between their best 3 mile time. If VO2max were solely responsible for performance then Prefontaine should have been 16% faster than Shorter. Clearly, some other factor than VO2max was at work.

To account for this contradiction around the 1970s exercise physiologists advanced the idea of running economy. As Prof Tim Noakes writes, "How then to explain these anomalies? David Dill and his colleagues, David Costill, and Jack Daniels were probably the first scientists to suggest that there may be differences in the amount of oxygen athletes actually require when running at the same speed and that these differences in running economy could be a major factor explaining differences in running performance in athletes with similar VO2max values."(3)

So, basically, the idea of "running economy" was advanced as a way to account for one of the first contradictions in the cardiovascular/anaerobic model. According to the scientific method any time a theory is contradicted the theory must be abandoned or modified. The idea of “running economy” was a necessary adaptation to explain the contradiction in the cardiovascular/anaerobic model and to allow it to continue to be used as a model for explaining endurance performance. Since the 1970s, running economy has been accepted as a major factor (one of the Big 3) of the cardiovascular/anaerobic model and a major factor in endurance performance.

Oxygen Consumption versus Energy Consumption

The important thing to note about running economy is that it is focused on oxygen consumption (i.e. how much oxygen is being consumed as a given pace), not energy production. The problem with this focus is that running economy, at its essence, is about how much energy is being consumed, not how much oxygen is being consumed.

It is not possible to directly measure how much energy in the form of ATP is being burned in the body, but scientists figured out that they could indirectly calculate energy consumption by measuring how much oxygen a person is using. So, when scientists measure oxygen consumption they are doing so as a way of indirectly measuring energy consumption. But with running economy the focus isn't on energy consumption it is on oxygen consumption. When you read about running economy in a running book the general focus is on how much oxygen is being consumed, not how much energy is being burned. I believe the focus on oxygen consumption instead of energy consumption may be the primary reason running economy has been resistant to “revealing its secrets", as Pete Pfitzinger described it.

Implications of Changes in Running Economy

Now that we have both a historical and an energy perspective on running economy, let's examine the implications of changes in running economy from the standpoint of conventional wisdom.

Conventional physiology wisdom teaches that improving running economy - i.e. consuming less oxygen at any given pace - is highly desirable. Why is it highly desirable? The answer is because running economy is a major factor in performance and the more economical you are the better your performance can be.

But, wait a minute. If running economy is really just a proxy for energy consumption, if it is just a way of measuring energy consumption, then when running economy improves what is really happening is that energy consumption has decreased. The amount of energy it takes to run at any given pace has gone down so oxygen consumption at that pace has decreased, and this is expressed as an improvement in running economy. Improved running economy isn't about oxygen consumption at all; it's about decreased energy consumption. That's what Jack Daniels meant when he wrote, "...the runner can now race at a faster speed than before without an increase in energy expenditure."

Conventional training & physiology wisdom teaches that training improves the aerobic capacity of the body and that these changes result in improved performance. Changes such as increases in VO2max and muscle mitochondrial density increase the body's ability to absorb and utilize oxygen, which translates into improved performance. While these changes really do take place in the body, the net result isn't that the body produces more energy aerobically and less "anerobically" while running at a given pace. Instead, the body actually burns less energy and less oxygen at any given pace - in other words, all those aerobic adaptations that are taking place are not being put to use while running at that pace because the body's need for oxygen at any given pace has actually gone down. The "aerobic base building" training that runners engage in is actually "energy conservation" training.

The bottom line is this: running economy isn't about oxygen usage at all, it's about energy usage. One of the major training adaptations taking place in the body is that less energy is required to run at a given pace. This important fact has been obscured by a misplaced focus on changes in oxygen consumption. Nor has the 30+ years of focus on oxygen consumption been particularly enlightening either.

You should be jumping out of your chair about now asking the question, "Why does energy consumption go down with training and what are the implications?" Let's look at that now.

The New Running Economy

Logic and reasoning have shown us that focusing on oxygen consumption at a given pace is of little value and just serves to divert attention from other changes taking place in the body. Instead, our focus should be on running economy as a measure of energy consumption. So, I propose that the term running economy should be used in accordance with its actual meaning – a reference to energy consumption. By using running economy in accordance with its definition, then, an improved running economy means energy usage has decreased. The result of focusing on energy consumption instead of oxygen usage is that some of the basic “secrets” of running economy that have remained hidden for 30+ years become self-evident.

Why does energy consumption go down with training?

There are three primary reasons energy consumption goes down with training:
– increased efficiency
– decreased resistance
– increased power.

Increased efficiency: The more your repeat a movement pattern the more efficient your body becomes at that activity. With increased efficiency comes decreased muscle activation because fewer muscle fibers are needed to perform that activity.

Decreased resistance: The amount of resistance you are fighting while running is determined by your body weight – the more you weigh the more force that gravity is exerting on your body and the more resistance you have to overcome to move. Losing weight results in decreased resistance; decreased resistance results in less energy required to move. Lance Armstrong dropped 15 lbs between 1992 and his first victory in the Tour de France, resulting in decreased resistance while riding and a 10% increase in power output. Research on Lance indicated that increased power output is the most likely physiological explanation for him catapulting from “also ran” to multi-Tour de France winner.

Increased Power: When muscle fibers increase their power output, then fewer muscle fibers are required to be activated in order to perform an action. The research study on adaptations to marathon training showed that this is the major adaptation that occurs with training – muscles dramatically increase their power output. As a result, fewer muscle fibers are required to run at any pace.

The reason energy consumption decreases with training is because of some combination of these three factors. All three factors combine to produce improved running economy, however, they do not all contribute equally to improved running economy.

– Increased efficiency likely occurs during the beginning stages of training, but is not likely to significantly change in experienced runners and, thus, is not likely to be a major source of decreased energy consumption (improved running economy) in experienced runners.

– If you lose a significant amount of weight then decreased resistance will be a major source of improved running economy. But, in those cases where little to no weight is lost during a training period, then resistance won’t decrease.

– Research has shown that muscle power output increases large amounts with training. Subjects in a marathon training program increased their absolute power output more than 50% and their relative power output by as much as 100%. Studies on master runners showed their slow twitch muscle fibers produced twice as much power as their sedentary peers. Even elite athletes can improve their power output; Lance Armstrong increased his power output an additional 8-9% after he was already a professional bicycle racer.

Increased muscle power, then, is the primary explanation for changes in running economy (i.e. decreased energy consumption). When muscle power goes up energy consumption goes down.

On a practical basis, what all this really means is that basically anything that increases muscle power will improve running economy (i.e. will decrease energy usage) and will have a positive effect on performance. This explains why strength training, an anaerobic activity if there ever was one, improves running economy. Strength is a component of power and increases in strength generally result in increased power output. The increased power output results in a decrease in energy consumption (i.e. improved running economy).


Reference:
1. Pftizinger P, Douglas S., Advanced Marathoning, Human Kinetics, 2001, pg 29

2. Daniels, J., Daniels Running Formula, Human Kinetics, 1998, pg 41

3. Noakes, T., Lore of Running, 4th Edition, Human Kinetics, 2001, page 47