Aerobic training is any training that is performed that utilizes aerobic energy.
Training primarily in an aerobic zone most efficiently improves aerobic capacity, trains the body to utilize fat for energy, reduces injury, improves recovery, and better prepares you for endurance activities.
Your performance during any type of endurance event is directly related to your aerobic capacity. An endurance event is one that utilizes aerobic energy as the major energy source; this can be any event from a 2 mile run to an ultramarthon or ironman triathlon. It is important to understand that even during shorter events, such as a 2 miler or 5k, although your heart rate is above the aerobic zone, aerobic energy is the major energy source. Anaerobic energy only contributes a small amount of energy in these events, therefore a greater aerobic capacity allows you to produce more aerobic energy which enhances performance in ANY endurance event.
The best way to improve your aerobic capacity is to train in your aerobic zone. By training in this zone, you are specifically and exclusively training your aerobic system, thus maximizing the benefit of the training on your aerobic capacity. Enhancing your aerobic capacity will make you run faster. If you are doing a lot of speed workouts and not focusing on improving your aerobic capacity, you will not significantly improve your performance. This may be hard for a lot of athletes to understand and believe, but it is true - the best way to improve your speed and performance is through aerobic training.
Many athletes do not utilize a heart rate monitor and do a majority of their workouts in the anaerobic zone. Many athletes do use a heart rate monitor yet still train above the aerobic zone. By spending a lot of time training at an anaerobic intensity level, these athletes are not gaining the maximum benefits on their aerobic capacity.
How can you become faster without doing all those speed workouts? The increased speed and performance comes from a larger aerobic capacity. When you begin aerobic training you may feel like you are running slowly to maintain your heart rate in the aerobic zone. However, you can be assured that you are working your aerobic system. Over time, as you build your aerobic capacity, you will improve your ability to run faster. If you start out running 8:30 per mile pace, over time you will improve to 8:15 and then to 8:00 per mile at the same heart rate zone. This increase in aerobic capacity will significantly improve your race paces as well; if you previously ran a 6:30 per minute pace during a 10k, you will soon notice that at the same level of exertion you are running 6:15 per mile or better.
During aerobic training, your body learns to more efficiently utilize fat for energy. When you are training in an anaerobic zone (above the aerobic zone) your body is essentially utilizing more energy than can be produced using oxygen, it is utilizing anaerobic energy. Anaerobic energy in general is derived from the consumption of glycogen (carbohydrates). Additionally, the anaerobic system does not produce energy as efficiently as the aerobic system.
Aerobic energy is the energy that your body produces when the oxygen supply matches the energy requirements. This form of energy is produced utilizing glycogen and fat. During prolonged aerobic training (typically greater than 45 minutes) the body learns to more efficiently utilize fat for energy. It is therefore easy to understand the benefits of this in regards to endurance events. The more efficiently your body utilizes fat, the better you will perform.
To further emphasize this, a discussion on the calories utilized and consumed during an ironman distance triathlon is useful. There are several different theories on the calories consumed during an ironman triathlon, roughly we will estimate 700 calories during the swim, 4,000 calories during the bike, and 3,000 calories during the run (check this data) for a total of 7,700 calories. A good carbo load will provide the liver and muscles with about 1,500 calories of stored glycogen. If during the ironman one was able to consume 300 calories per hour (which is pretty much the max that you can digest during this type of activity, and most will agree this a difficult feat to accomplish), one would consume 3,300 calories during an 11 hour ironman. Add this to your glycogen stores for a total of 4,800 calories consumed. This falls significantly short (about 2,900 calories) of the energy required to complete an ironman triathlon. So, where does the extra energy come from? The main source of this extra energy is fat that is utilized for energy.
I know a female triathlete who has finished IM Hawaii three times, with finish times just over 12 hours, who consumes around 1,000 calories during the entire event. She finishes strong and healthy, frequently seen dancing in the crowd after crossing the line. Her body's ability to utilize fat for energy is INCREDIBLE! She achieves these fat-burning abilities from consistent aerobic training.
Additionally, for those interested in burning fat and losing weight, aerobic training provides significantly more fat-burning than anaerobic training does.
Aerobic training results in less stress to the body and joints thus reducing training-related injuries. During aerobic training, the lower intensity level of the training results in less stress to the body's structure (i.e. bones and joints) as well as the immune system. Stress reactions, stress fractures, knee tendonopathies, foot and ankle tendonopathies, frequent infections, and fatigue are common training-related injuries that are most frequently caused by overtraining. Athletes who are committed to aerobic training suffer far less injuries than those who stress their body more regularly. If you are an endurance athlete that experiences any of these problems, you should seriously consider examining your training schedule and doing more aerobic training and less anaerobic training.
Along the same lines as reducing injury, the lower stress on your body from training at a lower intensity level significantly enhances recovery. By not over-stressing your body from frequent anaerobic work-outs, your recovery times will be shorter. Shorter recovery times will not only allow you to train better and longer, but it will also significantly reduce the fatigue that endurance athletes often experience during training cycles. Most frequently this fatigue is related to over-training, inadequate recovery, and insufficient sleep. By decreasing the stress on the body, aerobic training reduces fatigue and enhances recovery.
The biggest challenge for an athlete starting aerobic training is the feeling that they are not getting a good enough work-out. The lower intensity level results in less post-exercise fatigue and muscle soreness. However, this does not mean you are not maximizing your training, rather you are training smarter. Imagine still having energy after a 100 mile bike ride or a 20 mile run.
You have to be patient when you begin a program of aerobic training. Initially it will seem that you are running quite slowly to maintain your heart rate in the aerobic zone. Stick with the program and be patient. You will not notice these changes overnight, it will take approximately 4 to 6 weeks of consistent training to notice improvements in your performance. However, it works and it works well; the result being improved performance with less injuries and fatigue.