First, endurance rides help build capillary density, which in turn allows our body to function more efficiently (oxygen and fuel delivery), improving aerobic performance. Intense riding will break down our capillaries, so the standard recommendation for the base phase of training is to spend no more than 20% of our ride time at a higher intensity. Recent studies also suggest that ANY time at a higher intensity will break down any increase in capillary density (as will resistance training during this phase). As I’ve come to learn, science isn’t an exact discipline, so we’re always learning new things about how our body works and responds to training.
The second physiological result of endurance riding is that at a lower intensity, our bodies burn a higher percentage of fat as fuel (as opposed to glycogen, the stored form of carbohydrate). Thus the popular term “fat zone” or fat-burning ride.” This can, of course, be deceiving to many people, because in reality, your body is burning more TOTAL CALORIES at a higher intensity level (although the percentage of fat as a contribution to fuel is lower). So relatively, you burn more TOTAL FAT at a higher intensity (since the caloric total is higher). But the key here is that our body’s first choice for fuel during higher intensity exercise is glycogen, and exercising at a lower intensity level helps your body become more efficient at burning fat for fuel. This is extremely important as you ride for longer durations (more than 90 minutes, such as in a century or road race), since our bodies’ glycogen stores are finite and it can be a challenge to replace all the calories we burn during a longer effort. By training your body to utilize fat as a fuel, you’re effectively stretching out your body’s glycogen stores to be used for a longer period of time.
Low intensity cycling also stimulates slow-twitch (type I) muscle fibers more predominantly that fast-twitch (type II) muscle fibers. These fibers then adapt by becoming more efficient at lower intensities with less fatigue. Because of this adaptation, we need to recruit fewer muscle fibers to maintain a given effort, and the fibers that are at work are more efficient. All good for cycling, because then we use less energy (fuel, oxygen) to ride!
My Summary: We are training to be more efficient. If you are not efficient at an aerobic pace, your inefficiency will only amplify as you try to increase your pace. We need to get your muscles, nutrition transfer/utilization, and your body’s lactic acid transfer system trained. All of that is physiologically taking place at this speed. While the spinning intervals are helping and training your form, (and giving you a slight bit of intensity) the primary growth is taking place internally.