Your aerobic fitness is a reflection of your ability to take oxygen from the atmosphere and use it to produce energy for your muscle cells. Many factors influence aerobic fitness, including your lung efficiency, cardiac function, gender, age, training status and genetic makeup. Understanding the various components of aerobic fitness will help you train smarter to achieve optimal performance.
It's All About the Oxygen
By definition, aerobic fitness refers to your ability to transport and utilize oxygen, measured by VO2 max, the maximal amount of oxygen you can use during intense exercise. Exercise scientist Len Kravitz, PhD of the University of New Mexico explains that VO2 max is influenced by central and peripheral components. The central component involves the ability of your lungs, heart and vascular system to deliver oxygen to your muscles via your blood stream. The peripheral component involves the ability of your muscle cells to extract oxygen from your blood and use it to make ATP, the fundamental unit of energy. VO2 max values are lower in women, and decrease incrementally with age.
Priming the Pump
Your heart and lungs play a central role in aerobic fitness, with your heart being the prime limiting factor. While your lungs must function efficiently in order to transfer oxygen from the atmosphere to your bloodstream, they take a backseat to your heart, which must contract forcefully to eject oxygenated blood into your system to reach your cells, accounting for 70 to 85 percent of VO2 max. According to Kravitz, aerobic exercise training increases your total blood volume, heart muscle size and contractility, resulting in a greater volume of blood being ejected per each heart beat. Increased stroke volume means your heart does not have to beat as frequently at rest, resulting in a lower resting heart rate.
Regardless of how efficiently your heart pumps, aerobic fitness is also dependent on the ability of your muscle cells to extract oxygen from your blood and use it to make energy. Aerobic energy is produced in the mitochondria of your muscle cells, using carbohydrates and fats for fuel. Mitochondria are microscopic organelles that function as energy factories for aerobic metabolism. In response to repeated bouts of aerobic exercise, the density and number of mitochondria increases. High-intensity exercise that challenges your aerobic limit has a more profound effect on mitochondria adaptations and oxygen extraction than low- to moderate-intensity activity, according to Olympic coach David Joyce of "Peak Performance." During high-intensity exercise, you exceed your body's ability to produce aerobic energy, forcing it to tap into anaerobic pathways. However, anaerobic energy can only be sustained for a very limited time, usually less than two minutes. Perpetually exceeding your anaerobic threshold causes your muscles to adapt to perform aerobically at higher intensities.
While VO2 max is the ultimate measurement of aerobic fitness, it relies on a complex and invasive evaluation in a laboratory setting, conducted by trained technicians. According to "ACSM's Complete Guide to Fitness and Health," other methods may be used to measure your aerobic fitness. One way is to monitor your heart rate during maximal intensity exercise and translate it as a percent of your maximal heart rate, which you can calculate using the equation: 206.9 – (age in years × 0.67) = estimated maximal heart rate. The more fit you are, the higher your percent of max heart rate during exercise. Aerobic fitness classifications ranging from low to excellent are reflected by values of 60 to 90 percent of max heart rate, respectively.