Heart Rate Training Redefined - Part I: Fitness Is in Your Muscles

At Endurance Nation, we focus exclusively on age-group triathlon training that leads to measurable, quantifiable results. At the foundation of our approach to training is the idea that fitness is in the muscles, not the cardiovascular system.

In other words, running with a higher heart rate doesn't make you faster, it just means you have a higher heart rate while running. Running faster by working harder is what makes you faster.

Fitness is in the muscles, not the cardiovascular system.

If you want to run, bike or swim faster next racing season, you will need to run, bike, or swim faster in training. By putting the focus on the muscles (which we control) vs. heart rate (which is out of our control) we add a new level of awareness and agency to our training. Let's take a closer look at how muscles power our exercise.

The Role of Muscles During Exercise

We control the force of muscle contractions primarily by recruiting and using more motor units. Small-force contractions use fewer motor units. In order to produce more force, we begin to recruit more motor units. Motor units are called into action based on what is called the "size principle."

According to the size principal, low-force contractions recruit small, mainly slow-twitch motor units. Higher force contractions recruit mainly large, fast-twitch motor units. As the force requirement increases, the motor units recruited get larger and more fast twitch.

For example, high force activities such as jumping or sprinting will recruit primarily large, fast-twitch motor units. Lower force activities such as a slow jog will recruit primarily smaller, slow-twitch motor units. As we move from jogging slowly to running faster, we begin to recruit larger, fast-twitch motor units.

Here's a real-world example to make things clearer: Imagine that you are doing a graded exercise test on a treadmill or exercise bike. During a graded exercise test, the exercise intensity is increased every minute going from very light to maximal effort. At the start, you begin to engage slow-twitch muscle fibers, using primarily fat as fuel and no lactate is produced.

As the intensity increases, you begin to engage some intermediate fibers and just a few fast-twitch fibers (as well as the already active slow-twitch fibers). You have activated a few fast-twitch fibers, producing some lactate. However, because you are not producing much lactate, and your slow-twitch fibers can consume lactate as a source of fuel, lactate levels do not increase significantly. As the intensity increases further, you begin to activate additional fast-twitch fibers. These fibers produce lactate and consume primarily carbohydrates.

Eventually you will reach an exercise intensity that results in more lactate production than your slow-twitch fibers can clear. This point is called the lactate threshold and it is marked by a rise in the level of blood lactate.

Lactate accumulation can contribute to fatigue because it can reduce the capability of muscles to produce force (by inhibiting anaerobic glycolytic pathways). Therefore, we can exercise at intensities just below our lactate threshold for quite a while, but we fatigue quickly at intensities above our lactate threshold.

A few critical things to point out:

  • Aerobic vs. Anaerobic exercise. Note that the body moves in and out of different energy-producing systems as more and different types of motor units are recruited to do more work. This does not happen at some definitive line, such as under this line equals 100 percent aerobic, but cross that line and it's 100 percent anaerobic.

  • At lower intensities we are only training the muscles we are recruiting. By exercising at higher intensities, we recruit a higher percentage of both our slow- and fast-twitch fibers, forcing them all to adapt.

    At higher intensity levels we get all of the "go longer" adaptations we want from the slow-twitch fibers, while also accruing the "get faster" adaptations from our fast-twitch fibers. If you want to ride fast, you have to ride fast. If all you do is ride slow, you'll get very good at riding very slowly!

  • Lactate threshold is nothing more than a reasonably identifiable blood marker at which we can infer that we've recruited nearly 100 percent of our slow-twitch fibers, as well as lots of fast-twitch fibers. We sit on this threshold because it is just that: a threshold.

    You can sit right there for a long time, forcing lots of good changes in your muscles. Too far under LT and you don't recruit enough motor units. Too far above LT and you can't sit there long enough. Just at or just under LT is optimal.

  • Increased fitness can be defined as the ability to perform more work: swimming, biking or running farther and faster. Heart rate is simply the body's response to this increased workload, it is not a direct measurement.

    Focusing on direct measurements of work performed (power, speed, or pace) instead of heart rate, is both more efficient and effective. Our attention is placed on the root cause of increased fitness: increased workload.
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