As a cycling coach, I am a huge advocate of performance testing. Periodic field tests provide a very effective way to evaluate the efficacy of your training regimen. They also supply you with the data you need to create training zones based on lactate threshold heart rate (LTHR) and functional threshold power (FTP).
Lactate threshold is the highest intensity you can maintain for 60 minutes. Any increase in intensity beyond this threshold level necessitates a reduction in effort because your body starts to produce lactic acid more quickly than it can remove it. The higher your lactate threshold as a percentage of aerobic capacity (VO2 max), the faster you can ride a bicycle.
Lactate threshold is the best predictor of race performance for many cycling events and unlike aerobic capacity, it is highly trainable. This is why most training zones for competitive athletes are based on lactate threshold and/or functional threshold power, which is the highest average power, measured in watts, you can maintain for one hour.
Generally speaking, there are two ways to determine lactate threshold. The first, and more accurate method, is through laboratory testing on a cycling ergometer. During the test, the resistance on the ergometer is increased at specified intervals (e.g., every three minutes). This process is repeated until the cyclist is exhausted (the test takes about 30 to 45 minutes). Blood samples are taken at each interval and examined with a lactate analyzer. The point where blood lactate increases significantly corresponds to lactate threshold (although there is debate about which point most accurately defines threshold level).
The second method is the field test, which may be slightly less accurate, but is effective nonetheless. A field test allows you to accurately estimate your LTHR and evaluate your cycling performance.
My favorite field test is the 30-minute time trial. After a thorough warm-up, simply ride as hard as possible for the entire 30 minutes. Your average heart rate for the last 20 minutes of the time trial will be a close estimate of LTHR. For example, if your average heart rate for the last 20 minutes is 170, this is a good estimate of your LTHR. If you have a power meter, your average power for the 30-minute test is a close estimate of your FTP. You can use these numbers to create training zones, which allow you to train at varying levels of intensity for different workouts and greatly increase the quality of your training program.