Limiting Factor in Distance Running
Flickr Photo: Sheffield Tiger
Most runners are familiar with VO2 max, lactate threshold and running economy but aerobic threshold is a measure that isn't as widely known. Aerobic threshold is the maximum level at which you're still working in your aerobic zone.
Aerobic threshold is an intensity you could run at for hours on end. It is the critical threshold for runs lasting 2 hours or longer. Aerobic threshold is relatively equal to your marathon race pace (or just slightly below it).
Unlike running near or beyond your lactate threshold level, you don't buildup excessive levels of lactate and acidosis if you stay below your aerobic threshold. Your body is able to clear the lactate and acidosis from your muscles at this intensity.
Running below your aerobic threshold you will be using fat as your primary energy source (though carbohydrate in the form of muscle glycogen will contribute some energy as well). At intensities above it, carbohydrate will contribute a greater and greater percentage of energy until you reach your lactate threshold beyond which carbohydrate provides 100% of your energy.
Testing Aerobic Threshold
Your aerobic threshold is measured in the same test as your VO2 max and lactate threshold test. When your lactate begins to accumulate exponentially you've reached your aerobic threshold.
The graph below shows my results from a recent VO2 max and lactate threshold test at the Peak Centre in Vancouver.
My aerobic threshold occurred at a speed of 11.9 kph and a heart rate of 171 bpm. You can see the lactate level at the second blue point and the heart rate at the second red point on the graph. The first point shows my lactate level and heart rate at the start of the test, after I had done a 10 minute warm-up running at 10 kph.
Aerobic threshold can be expressed in terms of pace, speed, heart rate or percentage of your VO2 max or maximum heart rate.
Training Aerobic Threshold
The most direct method of training aerobic threshold is to run below it at a continuous pace for 30 minutes or longer. This is your standard long, slower pace distance running.
One of the biggest mistakes runners make is running their long runs, recovery runs and easy runs at too hard an intensity. All these runs should be done below your aerobic threshold. However many runners run these sessions slightly above their aerobic threshold in what is often called the "junk zone". They don't get the full benefit of aerobic training nor do they get the full benefit of anaerobic training runs (such as threshold and VO2 max intensity runs). Thus the term junk mileage.
Another way to improve your aerobic threshold is to increase your VO2 max. This works because VO2 max is your ultimate ceiling. If you can raise your upper limits then your lower limits should also go up. Read more the section on VO2 max training to understand how to train it.
You could also improve your aerobic threshold by improving your running economy. By becoming more efficient you will use less oxygen at a given pace. That means that at the oxygen consumption of your current aerobic threshold you should be able to run faster.
Aerobic Threshold, Lactate Threshold & VO2 Max
Aerobic threshold can reach an upper limit after which improvements are unlikely. In such cases training time is better spent developing other aspects such as lactate threshold, VO2 max or running economy and maintain your current aerobic threshold. The chart below shows the relationship and upper limits of aerobic threshold and lactate threshold as a percentage of speed at VO2 max.