What About Cardiovascular Fitness?
Cardiovascular fitness is actually adaptations occurring in three areas: blood volume, heart stroke volume, and an increase in blood capillaries.
Blood volume increases rapidly with an endurance training program—sometimes by as much as 8 percent in the first week. In general, trained athletes can have a blood volume that is 25 percent higher than untrained individuals.
While the initial increase is often blood plasma, the red blood cell mass eventually increases as well. This general adaptation occurs no matter what mode of endurance training is employed.
Heart stroke volume (or the amount of blood pumped by the heart with each beat) increases as you become more fit. While part of this is due to the increased blood volume, the left ventricle chamber also enlarges (holds more blood), resistance to blood flow decreases, and more blood is returned to the heart to be pumped.
All of these adaptations occur simultaneously, which means you are capable of pumping more blood at maximal exercise levels. These changes can occur rapidly, especially with high-intensity interval training.
Muscle capillaries increase. Capillaries are the small blood vessels that deliver blood containing oxygen and fuel to your muscles. Obviously, more capillaries mean greater blood delivery, which is a desirable adaptation.
During exercise, waste products and other substances enter the capillaries and trigger expansion. This occurs only in the muscles that are active during the exercise session, regardless of exercise intensity. It is true that longer sessions result in a stronger stimulus for capillary growth as the stimulus is present for a longer period of time.
However, short and hard sessions provide a stimulus for this type of adaptation as well and certainly do not reverse this adaptation, as some have claimed. In fact, the only way to increase capillary density in and around your fast-twitch fibers is to recruit them with harder efforts.
Think of these adaptations as reworking the plumbing in your body to deliver more blood, and therefore oxygen and nutrients, to working muscles. These adaptations happen very quickly and once they are complete it is still the muscles doing the work.
Therefore, the composition and work capacity of our muscles is the limiter, not the plumbing. At every distance that you might race as a triathlete you have plenty of cardiovascular capacity left, even if you are pushing very hard. For that reason, cardiovascular adaptations are not an important training consideration as far as performance is concerned.
Train Faster to Get Faster
So what does this science-speak mean for you?
- A rising tide lifts all boats." By training at threshold, we come faster at threshold. You used to be able to ride one hour at 18 mph and noodle at 16 mph. Now you can hold 20 mph for an hour and 18 mph is the new noodle. How can you develop the ability to ride 18 mph for more than six hours on race day? Build your 18 mph fitness by riding a lot at 20 mph or more.
- Farther + Faster = Disaster. Traditional training approaches tell you to build the aerobic engine, and then make it faster closer to the race. Our experience, with our own training and across hundreds of athletes, has told us that, yes, it makes sense that closer to the race we want to focus on race-specificity: effort, position, nutrition, etc.
We want to get very good at doing the stuff we will do on race day, so training at an aerobic intensity during the build phase makes sense because it's race-specific. However, traditional training tells you this is also the time to build speed—after you've earned the right by building a bigger engine. But this intensity on top of peak weekly training hours is a recipe for overtraining, injury and burnout.
The net is that "get faster" never happens under the traditional approach because, right when you're supposed to get faster, the race says you also have to go longer and something just has to give.
- Instead build speed, in the offseason, when there is no requirement to also build endurance.
- Finally, on a real-world level, we do this harder/faster work in the offseason because we have plenty of time to recover, we don't have to compete with races (build / taper / recover), for most of us it's cold and dark outside, and going long at the start of a long season would just make us nuts. In other words, it just makes sense.
As you consider your approach to offseason training, we suggest you think long and hard about...not going longer, but going harder. Not only will it rejuvenate your focus, it will set you up for a great season. Good luck!