[This article originally appeared in the
issue of Northwest Runner
Why are some distance runners so much better than others? A complete answer to this question might mention race tactics, motivation, nutrition, and many other factors. Clearly, though, how well you run depends first and foremost on what kind of body you have; some bodies are built for speed, and some are not. So how does the body of a top road racer differ from the body of a back-of-the-pack Jack? Again, there are many traits to consider, but exercise scientists have found that most major differences in endurance performance can be explained in terms of three variables: VO2max, lactate threshold, and running economy.
We will now examine the three parts of the model and discuss why each one is important to distance runners.
VO2max is an abbreviation. The V stands for volume, the O2 represents oxygen, and max is short for maximum; thus, VO2max is the maximum volume of oxygen that can be consumed in a given amount of time. As you may know, the body uses oxygen to break down food and release energy through a biochemical process called oxidative phosphorylation, which takes place in subcellular structures known as mitochondria. For the sake of completeness, it should be noted that the body can also generate energy in ways that don't require oxygen; however, for races lasting more than a couple of minutes, oxidative phosphorylation is the body's primary energy source.
VO2max is usually measured by putting someone on a treadmill or stationary bike, gradually increasing the difficulty of the exercise, and collecting the air that the person exhales as he/she exercises. The inhaled air (standard room air, about 21% oxygen by volume) can then be compared to the exhaled air (which contains less oxygen, since some of the oxygen has been taken in through the lungs) to find out how much oxygen the athlete has used at each stage of the test. The VO2max is simply the highest rate of oxygen consumption that the athlete achieves before falling off the treadmill or otherwise indicating that he/she is too tired to continue. It is normally around 40 milliliters of oxygen per kilogram of body mass per minute in active young adults but can exceed 65 ml/kg/min in elite endurance-trained women and 85 ml/kg/min in elite men (Saltin & Astrand, Journal of Applied Physiology 23: 353-8, 1967).
The exercise intensity at which someone reaches his/her VO2max is quite high -- usually a pace that can only be maintained for a few minutes. In contrast, long-distance races often last half an hour or more -- up to several hours in the case of some marathons. Thus, while VO2max "sets the upper limit for performance in endurance events" (Bassett & Howley, Medicine and Science in Sports and Exercise 29: 591-603, 1997), it is also important to know what percentage of the VO2max can be sustained for the duration of a lengthy race. This sustainable oxygen consumption approximately corresponds to the so-called "lactate threshold" (which is often misleadingly referred to as the "anaerobic threshold," and which is similar to but not the same as the "ventilatory threshold"). Lactate thresholds of untrained people usually fall between 40 and 60% of VO2max (Pollock et al., Medicine and Science in Sports and Exercise 30: 975-91, 1998), whereas they are often 75-85% of VO2max in elite marathoners (Joyner, Journal of Applied Physiology 70: 683-7, 1991).
The lactate threshold can be determined from a progressive treadmill test if blood samples are drawn at each stage of exercise. It is commonly defined as the percentage of VO2max at which the amount of lactic acid in the athlete's blood rises above a certain baseline value (Coyle, Exercise and Sport Sciences Reviews 23: 25-63, 1995). The threshold depends on the relative rates at which lactic acid is (1) produced and consumed within the muscles, (2) removed from the muscles by the blood, and (3) removed from the blood by other tissues and organs. Consequently, someone's threshold may be high or low for a variety or combination of different reasons. Irrespective of these details, however, the take-home message is that people cannot work above their thresholds for very long (presumably because lactic acid causes muscle fatigue, although this theory remains controversial). Therefore, among individuals with similar VO2maxes, those with high lactate thresholds have a clear advantage over those with low thresholds (Coyle et al., Journal of Applied Physiology 64: 2622-30, 1988). Furthermore, an exceptional lactate threshold can partially make up for an inferior VO2max (Coyle et al., Journal of Applied Physiology 54: 18-23, 1983; Allen et al., Journal of Applied Physiology 58: 1281-4, 1985).
Thus far, we've been discussing endurance performance mainly in terms of oxygen consumption. I've contended that one's maximal oxygen consumption (VO2max) and sustainable percentage of the max (lactate threshold) are both critical to distance running success. Now suppose I told you that I have a VO2max of 70 ml/kg/min and that I reach my lactate threshold at 85% of my VO2max. These are pretty good numbers, but you still wouldn't know for sure whether or not I'm a good runner because you wouldn't know whether or not I can efficiently convert oxygen into forward motion. This missing information is the final piece of our model, and it can be found in a measurement known as running economy.
It makes sense that running economy should impact distance-running performance, as shown in last month's infamous Audrey-versus-Blythe example. The point of the example was that, if two rival athletes have the same VO2max and the same lactate threshold, the one who is better at converting her oxygen into forward motion (i.e., the one who has a higher running economy) will usually reach the finish line first. This point is supported by a study (Conley & Krahenbuhl, Medicine and Science in Sports and Exercise 12: 357-60, 1980) showing that, among distance runners with similar VO2maxes, running economy is an excellent predictor of 10K race performance.
Now that we've seen how VO2max, lactate threshold, and running economy all affect distance running success, the next question to ask is: how should we train in order to improve these three things as much as possible? To find out, stay tuned for next month's article!