What is VO2 Max?
VO2 Max is a measure of the amount of oxygen a person (or animal) consumes when exercising at their maximum rate during a sustained exercise activity. It is expressed in units of mililiters of O2 per minute per kilogram of body mass (i.e., weight), or L/min x kg. If you take body weight out of the equation, it's just the total volume of oxygen used per minute, but the number has less utility in assessing fitness, because in addition to fitness, body mass itself has an impact on overall oxygen use. This is why VO2 Max is corrected or normalized to body mass by dividing L 02/min by weight in kg.
It is true that there are other factors related to athletic performance that prevent VO2 Max from being a very good way to compare different athletes for the purpose of predicting performance, as is frequently pointed out by exercises scientists and the more coaches and trainers with a background in physiology. Because all else is not equal among runners, a strong runner or one with a high lactate threshold and a VO2 Max of 50 can on any given day beat a weaker runner or one with a low lactate threshold and a VO2 Max of 60. Indeed, lactate threshold pace is a better predictor of finish order in a group of athletes in a race.
This is not to say, however that VO2 Max is unimportant on an individual basis for predicting performance and assessing changes in fitness and condition for individual athletes. This is because, if you are not comparing one runner to another, but instead comparing VO2 Max in a single runner at two different points in time (say, before and after a period of intense training), VO2 Max is a very good predictor of performance. So, the lesson here is that VO2 Max is important in determining performance of endurance athletes, but it not useful for comparing athletes one to another.
Improving VO2 Max
The straightforward answer is that an optimal combination of aerobic running and interval training at a pace a few seconds faster, or at higher intensity, than 5K pace will do the trick. Assuming that the runner's weight doesn't change during the training period, changes that result from this type of training can be directly attributed to improvements in muscular strength and running economy. Underlying these changes may be increase vascularization in the muscle tissues, and an increase in the metabolic machinery in muscle cells that are responsible for conversion of fuel (carbohydrates and fats) to energy.
Through weight management
The second way to improve VO2 Max is only important for people who are above their optimum race weight. Recall that body mass (weight) is in the denominator of the units that VO2 Max are expressed in, and in Junior High School (though the information doesn't stick if you don't use it later--I'm a geek, so I use it!), it that if you decrease the value of the denominator (the bottom number) in a fraction, the entire fraction becomes a larger quantity. So, for people who are carrying extra weight who want to improve both their speed and endurance, losing weight can be far highly effective.
It works like this: Let's say a person uses a total of 3600 ml of O2 per minute during maximal running exercise. This is determined largely by their physiology, so doesn't change much, even as their weight moves up and down a few pounds. It's the amount of oxygen used to do a particular amount of work (in the physics sense), although you don't need to deal in physics jargon to understand that if you wear an extra 10 lbs. around your waist, you're not going to be able to run as fast as you otherwise would--and that's the important point here.
Now, let's examine this a little more closely. Let's say this same person weight 72 kg. (about 160 lbs.). Our VO2 Max is calculated as follows:
VO2 Max = 3600 ml O2 / 1 min. * 72 kg. = 50 ml/min*kg
Now, Let's say the person reduces their caloric intake a modest amount for a month, and trains only enough to maintain their condition, losing 5 kg.. In that case, their VO2 Max still improves:
VO2 Max = 3600 ml O2 / 1 min. * 67 kg. = 53 ml/min*kg
So, Just from losing weight, we see this person's VO2 Max (and thus their ability to move their body mass further in a given amount of time by running) has improved. Further, since they were on a maintenance workout plan, all of the improvement can be attributed simply to weight loss. Further, weight fluctuates 2-4 lbs. during the day for a variety of reasons, which means that VO2 Max actually fluctuates as well, changing by around 1 unit. This might explain why some people feel like their training goes better early in the day. They may not being carrying the extra baggage of several daily meals and snacks and accompanying excess water weight early in the morning.
What's that? You don't believe a 2-4 lb. fluctuation can have much of a noticeable effect on your running performance? Consider this: Researchers have actually measured the effect and found that running pace changes in proportion to our body weight, almost perfectly. So, if I weight 160 lbs. and I gain 4 lbs. with no change in my strength and condition, I can expect my average training paces to decrease by 2.5% (4/160 = 0.025 = 2.5%). Thus, if my 5K pace is 7:30 to start with, losing 4 lbs. will improve my my 5K pace to about 7:18-7:19. In a 5K (3.1 mi.) race, that's a 33-36 second improvement!
Anyone seeking to improve their performance by losing weight should be careful, however. There are risks.
Know your limits
Of course, not everyone can afford to lose any more weight (there is such a thing as being underweight). If a person is already underweight, muscle tissue begins to be consumed by the body and performance can be adversely affected. So, there is a point of diminishing returns when it comes to weight loss, and each person has an optimum race weight based on their frame, bone structure, muscle mass, muscle types, and other genetically influenced aspects of physiology.
Though imperfect, body mass index (BMI) is a decent way to determine if you can afford to lose some weight to improve your VO2 Max and running performance. Most people can safely shoot for a BMI that approaches the low end of the "normal" range for BMI. Elite endurance athletes are often below this, but the average person may not have the knowledge, experience, or resources available to maintain such a low BMI and remain fully fit and healthy.
The moral of this story is that if you train right, and if you optimize your body weight, you can achieve significant gains in performance, but you should also be aware of and comfortable with your limits. The truth is that we are all probably capable of performing much better than we ever thought possible, if we can master this stuff, but many of us don't really have that level of ambition--and that's ok. It's still possible for those people to use these tactics to help reach whatever goals they set for themselves.
Improving VO2 Max can really improve individual performance, but once the optimum weight is reached, there are other tactics that, if previously neglected, can be more effective for further improving performance. They include strength training, and threshold or tempo training to improve lactate threshold. I've talked about these things in other posts and will likely post about them more in the future. The primary methods for improving strength for runners include a variety of hill training body weight exercises, and weights. Lactate threshold, or the pace/intensity at which lactate begins to build up in the muscles and blood, hindering performance and ultimately forcing us to slow or stop entirely, is best improved through threshold or tempo training--essentially running slightly higher than lactate threshold pace/intensity in 3-5 minute intervals (tempo intervals), or for longer sustained periods right at lactate threshold.
Serious athletes use all of the above training methods to improve and optimize their running performance.