Endurance training benefits:
Athletes who train in Zone 2 for 6-12 weeks will train their bodies to use oxygen more efficiently, burn fat as fuel, improve capillary density, increase mitochondria size and numbers, and give themselves a break from the high intensity workouts the season demands. Many people have asked if this training model is worth the time citing several studies showing high intensity interval training throughout the year to be a better model.
High Intensity Interval Training benefits:
Athletes who chose to train at the high intensity throughout the year will not have to spend the long hours training. The time commitment for base building is deterring factor for many amateur athletes. Studies have shown a marked improvement during the season when this model was used as compared to a base building control group. Many people will cite "specificity is king", meaning you compete at a high intensity, you should train at high intensity.
Choosing the right model:
I am a big proponent of base building, but not every year for everybody. First of all it is important for new athletes to build an aerobic base. Lifetime athletes have built that base up over many years and can afford to skip a season of endurance rides. The studies showing benefits of high intensity intervals used lifelong, elite athletes.
Second group are those who may be returning following an injury. A back injury decreased my cycling training for a five year period. This last season was the first I felt I could actually train. The problem was I had lost a good deal of my base since I couldn't be on the bike for more than an hour and a half. My season, therefore was inconsistent. I am putting together an early base building period for myself in order to re-establish my aerobic fitness.
Third group are those with a shift in goals. A base building period will likely be needed to go from a crit racer to a road or stage racer.
Lastly those who perform the high intensity work need to look at the quality of the intervals. As a strength and conditioning coach, I have found there needs to be a specific order of fitness. I can't teach speed drills to someone who becomes exhausted after the first two drills. Strength and conditioning need to be established before improving speed. Strength and conditioning can't be improved if the athlete doesn't have the endurance to complete the workouts. The same concept holds true for endurance athletes, if you can't complete the prescribed intervals, what in your fitness is lacking?