At first you can handle the extra effort, But soon your breathing rate increases dramatically -- almost to the point of panting. Then your legs start feeling heavy and even your arms begin to ache. You can't continue much longer. You must slow down,

What happened? Were you in the wrong gear? Did you misjudge your level of fitness? Or were you just having an off day? The answer, most probably, is none of the above. You simply crossed your anaerobic threshold.

AT, as it's commonly called, is your body's breaking point during exercise. Before reaching it, most of your energy is produced aerobically (with oxygen). But after exceeding it, a significant percent of your energy is produced anaerobically (without oxygen). Anaerobic balance. You feel this happening from such symptoms as breathlessness and muscle discomfort.

Unfortunately, the mechanisms governing AT are not fully understood. In fact, there’s disagreement among experts regarding its very definition. However most agree that once you exceed your AT, it's impossible to continue at such an intense level for long.

What are the practical implications of AT? By exercising at a level just below your AT, you’ll be able to sustain a fast pace longer. In one study, a group of cyclists was able to ride intensely for 50 minutes staying slightly below the respective ATs. However, not one was able to continue for more than 20 minutes after exceeding the threshold.

How can I measure my AT? The most reliable way to assess your AT is at a human performance laboratory equipped for V02 max testing. Such labs will find your breaking point either analyzing ventilation levels or by drawing blood during exercise and checking for lactic acid concentration. But it's also possible to estimate your AT at home although such methods rely on the less accurate, even controversial correlation between heart rate and AT.

One formula involves subtracting your age from 220, then taking 85% of it. The resulting heart rate, in beats per minute (bpm), is an estimation of your AT.

Another method, called Test Conconi, is more exact. To do it. you need speedometer, a heart rate monitor, a stopwatch, and someone to help you with the recording. Mark a circular course on a flat parking lot or attach your bike to indoor trainer. After warming up, set a fairly large gear that you won't spin out by the end of the test, and ride one minute at 10 mph. Record your heart rate. Continue to increase your speed 1 mph each minute until exhaustion. Record the corresponding heart rates at each 60-second interval. Afterward make a graph on which the horizontal axis is speed cubed (mph x mph x mph) and vertical axis is heartbeats per min. Plot each point from your test and connect the dots. The point where the line bends is an estimation of your AT.

Is anaerobic threshold the some as V02 ?. No, V02 max represents your aerobic power, or the maximum amount of oxygen you can extract from the air while exercising. Interestingly, you reach your AT long before you reach your VO2 max. Thus, AT is often measured as a percentage of VO2 max. For instance, if lactic acid starts to accumulate and you begin producing a significant amount of energy without oxygen at 50% of your V02 max, then this is your AT,

What are normal AT values? For untrained people, typical values are 40--70% of V02 max, with the average being 55%. As a cyclist, your threshold will be higher. Some researchers report ATs as high as 85% of V02 Max.

Can I improve my AT? Yes, Just as specific training can improve your V02 max. it can also raise your AT. Recent studies demonstrate that long, intense rides, as well as interval training, can raise your threshold. For instance, one study found that training rides done at 80% of VO2 max can improve a beginner's AT as much as 70% in just 9 weeks. Another study showed dramatic AT improvements doing 10, 2-minute intervals 3 days a week at 105% of V02 max. As you can see, intense riding is the key to increasing AT.

What does it all mean? Your AT represents the maximum level of riding intensity that you can sustain. While VO2 max is important, most people only reach it during the last few moments of intense exercise and can't sustain it. Thus, you rarely operate for long at your V02 max. For this reason, many physiologists consider AT to be a more important factor in determining cycling performance. The higher your AT, the harder you can ride for extended periods. Any improvement in your AT means a significant upgrading of performance-- and a better chance of staying with the pack.

Steve Johnson, Ph.D.