In Part I we introduced you to our 5 Keys and discussed one through four. To review, the 5 Keys are:
1. Real-World Volume
2. Maximize Return on Time Invested
3. Fitness = Ability to Perform More Work
4. Intensity = Most Flexible Tool to Manipulate Training Load
5. The Best Predictor of Performance is Pace/Watts at Functional/Lactate Threshold
In this article we'll discuss why pace/speed/watts at functional or lactate threshold, not training volume, is the best predictor of performance at all race distances.
Let's begin with a quick review of Key No. 3: Fitness as the Ability to Perform more Work. As we discussed, the functional expression of your fitness is the ability to pedal your bike, run or swim at given speed—to perform the physics of moving the mass of your body through space.
In order to force your body to adapt, you need to make it perform more work. But let's talk about what happens inside your body as you ask it to perform more work—to ride, run or swim faster.
Slow- and Fast-Twitch Muscles
Your body begins by recruiting slow-twitch muscle fibers. These fibers:
- Are good at performing relatively low force contractions.
- They can go and go and go for a long time. They are fatigue resistant.
- They recover quickly.
- Primarily burn fat for fuel. At ~3,600 calories per one pound of fat, even the leanest triathlete has a nearly endless fatty gas tank.
- Muscles fibers only adapt when you recruit them and make them do more work than they are accustomed to. The more often you recruit your slow-twitch fibers (the more exercise you do), the better they become at doing slow-twitch stuff: They adapt to produce more force, go longer, and become more fuel efficient. Remember that your body is lazy and will only adapt to meet the load you place upon it. The more stress you place on this system, the more it is forced to adapt.
As the workload increases (you go faster and faster), your body recruits a more and more slow-twitch fibers to chip in. At some work rate (speed), you begin to run out of slow-twitch fibers to recruit and your body begins to call in fast-twitch fibers to do the work. Fast-twitch fibers:
- Perform high force contractions.
- Tire easily and take a long time to recover. One and done, or need to rest a good bit before being put back to work again.
- Burn glycogen for fuel. The well-trained, well-rested endurance athlete only has about 1,800 to 2,000 calories of glycogen stored in the muscles and in the liver, much more limited than our fat reserves.
- Same adaptive jazz as above: The more often you recruit fast-twitch fibers (force them to work), the better they become at doing fast-twitch stuff. Better still, some of them can begin to take on the characteristics of slow-twitch fibers, increasing the number of fat-burning (unlimited energy source) fibers you have available to swim, bike, or run.
- Produce lactate as a byproduct.
Which brings us to lactate threshold; a term you may have heard of. This is the exercise intensity at which lactate levels in the blood rise above a certain level, as more and more fast-twitch fibers (which produce lactate) are called upon to work.
It's called a "threshold" because we can exercise for a long time at or just under this intensity. However, above this intensity, lactate levels increase, we rapidly fatigue, and are eventually forced to slow down.