The term "running economy" gets used a lot but what does it actually mean? We explore the science with Professor Andrew Jones who has measured and guided great athletes such as Eliud Kipchoge, in his attempt to break the two-hour marathon, and Paula Radcliffe, in her long-standing marathon record.
How can we define “running economy”? Can we highlight some of the key physiological and mechanical attributes that limit our economy?
Running economy refers to the oxygen (O2) cost of running. It is determined by measuring the concentration and volume of O2 breathed in and out every minute, with the difference between the two representing the rate of oxygen uptake required to meet the demands of the exercise that is being done. Running economy can be expressed in terms of the volume (in ml) of O2 used per kg of body mass per minute (i.e., ml/kg/min) or, alternatively, as the volume of O2 used per kg of body mass per kilometer of distance run (i.e., ml/kg/km). The former is used when the runner is running at a set speed (e.g., 16 km/h) and the latter is used when O2 cost is expressed independently of running speed. It is important that running economy is measured at speeds where the runner is in a ‘steady-state’. Running economy can also be expressed as an ‘energy cost’ (kcal/kg/km) which takes into account the proportion of fat and carbohydrate that is being used by the runner. The importance of running economy can be understood with reference to the following example: imagine two runners with the same body mass of 75 kg and the same VO2 max of 60 ml/kg/min but with different running economy (one has an ‘average’ O2 cost of 200 ml/kg/km and the other has ‘good’ running economy of 185 ml/kg/km). If these runners ran together at the same speed of 16 km/h, the first runner would be operating at 89% of their VO2 max while the second runner would only be using 82%; the former would very likely find this speed to be challenging and would fatigue more rapidly than the latter. Various factors influence running economy including physiology (proportion of slow-twitch vs fast-twitch muscle fibres), anatomy (various limb lengths and girths as well as body mass), and biomechanics (the way people move, i.e. their running technique).
At the elite level, what are some of the numbers we see in regards to their economy and how does that compare to the weekend racer?
There is quite a lot of inter-individual variability in running economy at all levels of performance. However, elite runners tend to have better running economy (i.e., have a lower O2 cost of running) - typically 170-190 ml/kg/km - compared to recreational runners (typically 190-220 ml/kg/km). It is unclear whether elite runners naturally have better running economy than ‘mere mortals’ or whether their running economy is so good due to their training - but it’s probably a bit of both.
How much does running technique play in being economical?
A lot. If you measure the O2 cost of cycling at a particular power output, there is little if any difference between the world’s best riders and people who are untrained, whereas in running the difference in O2 cost is considerable. This likely reflects the fact that cycling technique is much more ‘fixed’ than is running technique.
Is there anything that we can do to improve our running economy?
Run! There is evidence that running economy can and does improve with consistent practice. Running an appropriate mileage consistently and over the long-term, running regularly, and running at a variety of different speeds (including the speed(s) you intend to race at) are all important. Some forms of strength and plyometric training have been shown to be helpful too, possibly by enhancing the ‘stretch-shortening cycle’. In terms of nutrition, it costs less O2 to ‘burn’ carbohydrate compared to fat so making sure that you have consumed enough carbohydrate and have high muscle glycogen levels before a race, and that you ‘top up’ with sports drinks and carbohydrate gels during longer races, can help to keep O2 cost low. Dietary nitrate (e.g., via beetroot juice consumption) can also lower the O2 cost of exercise by making muscle contraction more efficient. Finally, the new ‘super shoes’ are effective in part because they improve running economy.
How does our economy change over time? I.e. Mile 1 of a marathon vs mile 26.
Running economy deteriorates during long, fatiguing exercise. This is partly as a consequence of an inevitable shift in substrate utilisation towards greater fat use as muscle glycogen becomes depleted. Fatigue and/or muscle damage will also lead to biomechanical changes which will likely alter the O2 cost of running. Running performance has traditionally been explained in terms of three physiological variables: VO2 max, running economy and lactate threshold; however, a fourth factor might be the extent to which these three variables deterioriate with time during competition. All-time greats like Kipchoge and Radcliffe might be better than others in part because they are more resilient or fatigue-resistant. More research is needed in this area.
Can all these ways of improving our running economy compound together or do they start to cancel each other out? (dietary nitrates improve by x, plyometric training improves by y, running shoes improve by n. Is it x+y+n = 20% improvement or the more things you add the less effective they get?
Theoretically, they ‘should’ be additive...however, to my knowledge, this has never been tested. While there are some effective nutritional and mechanical ‘ergogenic aids’, it is important not to rely on them and to realise that regular and consistent long-term training is the key lever to improving running economy; the other things are the ‘icing on the cake’.
On a chemical or mechanical level, what is causing the “fatigue” that affects running economy as the race goes on?
If our biomechanics (i.e., our running technique) changes due to fatigue then this might make it more energetically costly to run at a certain speed and the O2 cost will therefore increase. Also, as we deplete our glycogen stores, we have to rely more on fat to generate ATP and this requires more mitochondrial O2 consumption. Moreover, as the glycogen in our slow-twitch muscle fibres runs out, we likely have to progressively recruit more of our fast-twitch muscle fibres and these may have worse efficiency. Selective loss of glycogen might also make the muscle contraction process more costly (e.g., the energy cost of ion pumping might increase). Any micro-trauma or damage to fibres as a consequence of repeated impacts might also render those fibres less capable of generating force while they still require O” to recover and repair. Finally, losing economy is a ‘vicious circle’ because when the leg muscles require more O2, this has to be delivered by increasing heart rate and breathing rate and these are O2-consuming processes!
Andrew Jones PhD is Professor of Applied Physiology in the Department of Sport and Health Sciences at the University of Exeter. Listen to episode 7 of the Run Talk podcast with Professor Andrew Jones to learn more about the science of speed and endurance, and hear about what makes Kipchoge and Radcliffe different from the rest.
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