So, what is aerobic threshold and why - as an endurance athlete - do I care?
The range of beats per minute that your heart can sustain is organized into zones. There are as many ways to calculate heart rate (HR) zones as there are training plans, but they are all based on a percentage of your maximum heart rate. For simplicity, we’ll consider a four zone range. Keep in mind that your heart rate can be affected by many factors like stress, hormones, sleep, and medication. Heart rate zones are are also highly personalized, they’re dependent on your genetics, age, and fitness. Your lowest intensity zone is about 50 - 60% of your maximum heart rate and should be reserved for recovery. Your body will feel at ease in this zone and you are able to breathe easily through your nose. In this range, about 85% of the calories that you burn come from fat. Since we all, no matter how fit you are, have ample fat stores, athletes are able to maintain work in this low intensity zone for hours. In part, this is because your body is using oxygen to break down, or oxidize, the fat that it needs for fuel.
As the intensity of work increases, either because workload or pace increases or the terrain steepens, your heart beats faster to meet the demand for oxygen from your muscles. You are now in your second heart rate zone, and working at about 60 - 75% of your maximum heart rate. As the demand on your body increases, less oxygen is available to oxidize fat for fuel and your body produces excess lactate. So, as your HR increases your body responds by looking for other sources of fuel, which it readily find in the form of glycogen (sugar), also known as carbohydrates. Lactate production will continue to rise, causing symptoms like muscle weakness, rapid breathing and nausea. Climbers will often say that they’re “bonking” when they reach this stage. The HR at which your body predominately uses glycogen for fuel and the amount of lactate in your blood rises above it’s baseline level, is your aerobic threshold (AeT). Because we don’t have an endless supply of carbohydrates, you are only able to sustain work above aerobic threshold for two hours or less.
Determining your aerobic threshold allows you to maximize your training time. The goal of an endurance athletes is to improve your speed and power at your AeT. This is accomplished by training for many, many hours at, or just below your AeT. This type of work often feels easy, especially to athletes that have ascribed to the “no pain, no gain” training motto. Stick with it and over time your AeT will increase.
Why do endurance athletes care? A higher aerobic threshold allows you to work at higher intensities for longer.
There are many methods to determine your AeT, competing a blood lactate test or gas analysis test at a metabolic testing lab is the most accurate. You can also estimate your AeT by gradually increasing your exercise intensity on a run or cycle until you notice that it becomes more difficult to breathe through your nose. To do this, warm up for 15 minutes with easy running or cycling on flat terrain. Gradually increase your effort by increasing speed or incline until you notice that you begin breathing through your mouth. Note the HR at which this breathing transition occurs, it is approximately your AeT. It’s usually helpful to repeat this assessment several times.
Like HR zones, AeT is a personal number. Having a higher AeT than your climbing partner doesn’t mean that you are more fit than them.
Now that you know your AeT, it should become the foundation upon which you build your cardiovascular training. Plan to spend several training sessions per week, especially early in your training regimen, at this heart rate. If trail running is your preferred mode of cardio, begin with three forty-five minute runs per week and build up to one two-hour run and two one-hour runs weekly.
This type of training isn’t glamorous. But, it is the proven method to build your cardiovascular capacity. Over time, your aerobic threshold will increase which will allow you to work at higher intensities for longer periods of time.