Every winter, we all go into our ‘base’ training mode, but based on my experience over the years, very few people actually understand the purpose and specifics of base training. For most, base training is simply unstructured workouts and more often than not, a reason to take it nice and easy when you don’t feel like hitting it hard. Interval workouts are still done and the aggressive groups rides are still a regular part of the weekly equation. Ask someone about their base training and you will get some vague answer about focusing on aerobic activity or low intensity and decreasing the hard efforts.
I see the base phase of training much different. I think base building is the most important phase for endurance athletes. It is a very focused training period in which you train almost 100% of the time in a very narrow window of aerobic effort. This window doesn’t include easy aerobic training nor does it include anything at anaerobic threshold or above. Basically (outside of warm up and warm down) you keep your effort right at or just below your aerobic threshold(AT). For those who use a heart rate monitor this means constantly keeping your heart rate in a range of 10 bpm where your upper ceiling is your aerobic threshold and you never get to your anaerobic (or lactate) threshold . So lets say your AT is 155bpm, then you want to train between 145-155 for your entire workout. Note: this doesn’t mean that this is your average for the workout, this means that if your heart rate hits 156, you slow down. Likewise, if you heart rate drops to 144 you must pick it up. For those who train without a heart rate monitor, this is roughly your Ironman race pace.