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Working The Right Side Of The VO2Max Equation
October 11, 2007
Research Says That Hitting 100 Percent Of VO2Max Is Key
As a runner's maximal aerobic capacity (VO2max) increases, his/her performances generally improve. A novice runner can often enhance VO2max by 20 to 25 percent with as little as 12 weeks of running training; an experienced runner might boost VO2max by 4 to 7 percent in the same time frame, given the right workouts. Each 1-percent advance in VO2max can be linked with a one-half to 1 percent upgrade in race performances.
That sounds great, but there's a lot of debate about how to raise VO2max to the greatest-possible extent. Some coaches and runners emphasize high mileage, while others look for high intensity. Many mentors and runners hit intervals at 5-K pace, while others look for even-higher-speed repeats in hopes of adding loft to VO2max.
The first step in resolving this aerobic-capacity controversy is to remember that VO2max is expressed by the following simple equation:
VO2max = HRmax X SVmax X (a-v O2 difference)max
In this equation, HRmax is maximal heart rate. SVmax is just maximal stroke volume (the greatest amount of blood which can be pumped out of the left side of the heart per beat).
(a-v O2 difference)max is "maximal arteriovenous oxygen difference," which is nothing more than the difference in the oxygen content of the blood coming into the muscles from the oxygen content of the venous blood flowing away from the muscles. An increase in the (a-v O2 difference)max means that the muscles are extracting more oxygen from incoming blood, thus driving oxygen-consumption rate (and VO2max) upward.
The equation reminds us that there are only three fundamental ways to increase VO2max - by upgrading maximal heart rate, by expanding stroke volume, and/or by enhancing the arteriovenous difference (i. e., by "working" the right side of the VO2max equation). There are no other possibilities. But we still need to know: Which of these three variables should we be focused on? Exactly how should a runner train to get the biggest VO2max take-off?
Science tells us that there is little difference in maximal heart rate between the very best and very slowest runners. Yet the best runners have high values of VO2max, and the slowest runners have poor VO2max readings. This means that expanding max heart rate is not the key way to boost VO2max. Gains in VO2max must be associated with expansions of stroke volume or advancements of the arteriovenous difference.
Research reveals that about 50 percent of the increase in VO2max which results from training is produced by an upswing in maximal stroke volume, with the other 50 percent coming from an uptick in the arteriovenous difference. Training can boost stroke volume in a variety of ways, but a key transformation is that plasma volume increases - so that the heart can fill with more blood between beats. This allows more blood to be ejected per beat (upping stroke volume).
Advances in the arteriovenous difference occur mainly because running stimulates an increase in the capillary density around muscle fibers in the legs. This aggrandizes blood flow to the leg muscles and decreases the distance across which oxygen must diffuse to get to the mitochondria inside muscle cells, where aerobic metabolism actually takes place. Upswings in capillary density exactly parallel increases in leg-muscle blood flow and whole-body VO2max.
But how can max stroke volume and arteriovenous difference be optimized? Back in the day, the answer was to run tons of miles, but research paints a quite-different picture. In one study, 12 individuals employed a training intensity of close to 100 percent of VO2max over a seven-week period, while 12 other subjects worked out at a moderate, "aerobic" intensity of 60 percent of VO2max (about 75 percent of max heart rate). The latter, "aerobic" group actually trained for considerably longer periods of time - but achieved a 38-percent lower increase in VO2max after seven weeks, compared with the 100-percenters. This result prompted the researchers to conclude that training at an intensity which elicits VO2max has the strongest, positive impact on VO2max expansion.
In a separate investigation carried out with experienced runners, one group ran about 100 kilometers per week at average intensities of 60 to 80 percent of VO2max, while a second group trained only 50 kilometers per week while emphasizing fast-paced intervals which ranged in distance from 60 to 1000 meters. After 14 weeks of training, the low-mileage, higher-intensity runners improved VO2max by 7 percent, while the high-mileage, "aerobic" runners failed to upgrade VO2max at all. Performance times improved by about 2.5 percent in the high-intensity group but failed to budge for the "aerobic" harriers.
As Jack Daniels used to say, improving a physiological system requires working at the limits of that system. In the case of stroke volume and arteriovenous difference, that translates into working at intensities close to 100 percent of VO2max. For your training, such an intensity can be estimated with the use of the six-minute test. Run as far as you can on the track in six minutes (or have the runners you are coaching do the same), calculate your average pace during this six-minute test, and you will instantly have a pace which will elicit 100 percent of VO2max when utilized during interval training. Begin with 200-meter intervals, and gradually work your way up to 800s or even 1000s. When you do this, VO2max will begin to take wings!
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