As a traditional yardstick to measure capacity for aerobic work, the VO2 Max is a considered by many endurance athletes to be the holy grail of fitness. While I completely disagree (for reasons given in an article posted HERE), I do concede that VO2 max is an important factor in determining performance in training and racing.
Since you could theoretically improve your VO2 max by 15-20% (and I've seen athletes do more than that), let's say you wanted to pull out all the stops and get your VO2 max as high as possible. Based on VO2 max being defined as the maximum amount of oxygen your body consume and utilize during exercise, you've basically got three viable strategies, and a few different approaches for each:
Strategy Numero Uno: Increase how much air you move into your lungs. This will increase oxygen availability at the pulmonary level. So how could you actually do this?
#1: Increase the force capacity of your inspiratory and expiratory muscles. There are a number of "breathing devices" out there on the market that claim to do this, such as the PowerLung trainer (http://www.powerlung.com/region/us/). The idea is that by making breathing harder, you're achieving the same effect for your respiratory muscles as doing dumbbell curls for your biceps or barbell squats for your hamstrings. Alternatively, you could attempt the poor man's version, and just breathe through a tiny twisty straw while doing hill repeats on your bike (or sitting in your car at a stoplight).
#2: "Open" the lungs with deep, diaphragmatic breathing. Yoga, breathing exercises, relaxation and a host of other techniques can technically give your lungs a little more space to expand, thus letting in more air and oxygen. Here's a link to an article on breathing techniques and exercises: http://www.everymantri.com/everyman_triathlon/2009/04/breathing-relaxati...
#3: Take something called "Cordyceps". In a double blind, placebo controlled study with 30 elderly volunteers, cordyceps sinesis herbal extract significantly improved the maximum amount of oxygen these people were able to assimilate. Yes, they were elderly. But other studies showed that cordyceps also increases cellular oxygen absorption by up to 40%, and improves functioning of the heart. There are many endurance supplements on the market that currently include cordyceps extract in their formula. Rhodiola root extract happens to also fall into the same family of herbs that are purported to improve VO2 max.
Strategy Numero Dos: Increase the blood volume pumped by a single stroke of your heart, thus increasing amount of blood and oxygen delivery to the muscles.
#3: Stay adequately hydrated and take electrolytes. These hydration strategies will ensure that you have enough fluid volume on board to actually supply your body with the blood-forming liquid. A dehydrated athlete will have a lower stroke volume, which means a higher heart rate for any given exercise intensity. That's why 10 hours into an Ironman, your heart rate is much higher than you would normally expect it to be.
#5: Train your heart. This should go without saying, but consistent cardiovascular exercise will improve VO2 max. However, this does not mean that a long three hour bike ride will do the trick, since you're never really "sucking air" during such an effort. Instead, very hard, very intense intervals of 1-5 minutes in duration, with a 1:3 or 1:4 work:rest ratio will stimulate a larger heart stroke volume, as well as a host of other VO2 max enhancing adaptations, like increased mitochondrial density and better capillary delivery to muscle tissue.
#6: Eat fish and flax. Studies have shown that Omega-3 fatty acids can increase stroke volume and cardiac output, most likely by stimulating your nervous system's parasympathetic "relaxation" component. You can take concentrated fish oil or flax oil compounds, or just have a nice piece of wild salmon every week, and put flax powder on your oatmeal every morning.
Strategy Numero Tres: Increase the muscle's ability to extract oxygen as it rushes past in the bloodstream, and the amount of oxygen available in that blood. This will enhance the actual utilization of the oxygen.
#7: Increase mitochondrial density. Mitochondria are the little powerhouses of the cell that allow for the conversion of oxygen into energy. The more mitochondria you have, the more oxygen you can utilize. Most research shows that, similar to increasing stroke volume, it is the short, highly intense efforts that improve mitochondrial density, and you really don't get much benefit once you've trained for more than one hour at a low intensity.
#8: Altitude training. Training at elevation both increases the number of oxygen-delivering red blood cells and makes those red blood cells more efficient at unloading oxygen to the tissues. To spare your body the rigors of constant altitude exposure, it is best to "train high" and "live low". Which is why you may want to re-consider that high altitude tent you've been eying. Your spouse will thank me later.
#9: Iron/ferritin supplementation. In order for those red blood cells to carry oxygen, they must have a crucial iron-containing protein called hemoglobin. Low iron means low hemoglobin. Which makes for some pretty crappy red blood cells. So take iron supplements, eat iron-rich foods, and look into supplementing with ferritin, a crucial iron-storing protein.
#10: Blood doping. Go ahead and call up your friendly neighboorhood illegal performance enhancing pharmacy. Ask for erythropoetin (EPO), which will stimulate the growth of new red blood cells, thus enhancing oxygen carrying capacity of the blood. Alternatively, you could just donate blood, freeze it, concentrate the red blood cells, then inject it back into your body about 30 days prior to your big competition. Don't try this at home. Just checking to make sure you're paying attention.
Just remember folks - increasing VO2 max involves more than just hopping on your bike and hammering. Strategies like electrolyte intake, omega 3, cordyceps and ferritin supplementation, occasional bouts of altitude exposure, staying hydrated, and practicing your breathing techniques can all add up to small but significant increases in VO2 max. Good luck!