Physiological Fitness - Aerobic Capacity

Exercise physiologists generally agree that there are only three things you can improve to become physiologically more fit for endurance sports performance: aerobic capacity, lactate threshold and economy. Ultimately, these are the reasons you train. So what are they and how do you improve them? I'll discuss aerobic capacity now and come back to lactate threshold and economy in a few days.

Also referred to as VO2 max, aerobic capacity is your ability to use oxygen to produce energy. The more oxygen your body can process the more energy you can produce and the greater your output (power or pace). It’s common to find that the fastest athletes in a race have the highest aerobic capacities of the entrants. The farther down the race results you go typically the lower the athletes’ aerobic capacities. But don’t take this to mean that knowing your VO2 max tells you how fast you will go or how well you will do compared with others in your race category. The two other physiological factors – lactate threshold and economy – also play a major role in race outcomes. One of these by itself does not constitute all of what it takes to race fast.

Aerobic capacity is literally at the heart of success in endurance sport. Improvements in aerobic capacity have largely to do with how much blood (which contains oxygen) the heart pumps out to the working muscles with every beat. This is called “stroke volume” and has a lot to do with how much aerobic capacity you have. A purpose of training is to improve your stroke volume. There are basically two ways to do this. The first is to focus on the volume of your training. The heart responds to lots of time spent at higher-than-resting intensity (above about 50 percent of VO2 max) by becoming more efficient and effective which ultimately means pumping more blood per beat.

The other way to improve aerobic capacity is by doing high-intensity intervals, especially those done at about the power or pace associated with your VO2 max. At that intensity your heart rate is approaching maximum, so these are very hard efforts. This method will produce a higher stroke volume sooner than by relying only on volume. Most experienced athletes employ both strategies.

There are other physiological contributors to aerobic capacity such as aerobic enzymes found in the muscles, blood vessel diameter and ability to dilate, blood volume and related hematocrit (red blood cells). Many athletes seem to believe their lungs are the deciding factor when it comes to aerobic capacity. Training produces insignificant changes in lung volume.

Body weight also has a lot to do with aerobic capacity. The formula for determining VO2 max is expressed in terms of milliliters of oxygen consumed per kilogram of body weight per minute. What this means is that as you lose body weight, especially fat as opposed to sport-specific muscle, your VO2 max increases. And most of us have experienced this at both ends of the weight spectrum. When we have gained weight it’s harder to run or ride a bike uphill. Conversely, when body weight has been low the effort of exercise is decreased at any given power or pace. This is clearly the affect of body weight on aerobic capacity.

Aerobic capacity is largely dependent on who your parents were. Research (Bouchard, 1986) has shown that identical twins have nearly identical aerobic capacities. While genetics probably sets the boundaries for the upper limit of your VO2 max, proper training can take you to near the upper limit. But also bear in mind that there are two other physiological factors that contribute to endurance performance. I'll come back to them soon (I hope).