Heart Rate Training Redefined, Part I — Fitness is in the Muscles

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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 vs. heart rate 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, 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 muscle fibers 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.

Therefore, focusing on direct measurements of work performed (power, speed, or pace) instead of heart rate, is both more efficient and effective: if we want to improve a thing, our body’s ability to perform more work, the best and most time efficient way to do that is to measure the thing we want to improve — power, speed, or pace — rather than heart rate, which is the body’s response, or symptom, to the increased workload.

That’s not to say that heart rate training is “wrong,” or of no value. Rather, it’s a cloudy, less precise measuring tool. But with the increasing affordability of powermeters for the bike and GPS units for the run, training with the objective metrics of power, speed, and pace is becoming more possible for more of us. In fact, we recommend that our athletes train with power, pace, and heart rate (if they can afford it), as the combination of objective and subjective metrics helps then learn more about their bodies, knowledge they’ll use on race day to solve problems.


But 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 increases (or the amount of blood pumped by the heart with each beat) 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, 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.

Improved Cardiovascular Fitness = Better Plumbing

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, from sprint to Ironman, you have plenty of cardiovascular capacity left, even if you are pushing very hard. Therefore, cardiovascular adaptations are not an important training consideration as far as performance is concerned.

To summarize the science-speak:

  • Most of what we call fitness is the expression of adaptations that occur in the muscles.
  • Contrary to popular/cultural belief, cardiovascular adaptations are a relatively small and an easy-to-achieve component of fitness.
  • By exercising at all intensities, but especially at higher intensities, we recruit all of our muscle fibers, forcing them all to adapt so they all become better and stronger at what they do.


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 become faster at threshold and all other intensities: you used to be able to ride one hour at 18mph and noodle at 16mph. Now you can hold 20mph for an hour and 18mph is the new noodle.

How do you develop the ability to ride 18mph for 6 hours or more on race day? Well, we can tell you that no amount of riding at 16mph is gonna get you there! You build the 18mph Noodle Fitness by riding often at 20mph.

You earn the right to ride faster on race day by riding faster in training.

Riding slowly for a long time does not produce faster speeds on race day. Actually…it does, but in our experience the amount of riding you need to at an Easy to Steady pace, to get faster, is about 13-16hrs per week — simply unrealistic for most real world, working age-groupers.

In our experience, 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 race build makes sense because it’s race-specific. However, traditional training tells you this is also the time to build your Fast, 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 over-training, injury and burnout.

The net is that “get faster” often 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. You try to combine Far with Fast (mixing in track and bike interval sessions) with your highest volume training of the year and end up sleeping under your desk at lunch.

Instead, we build your Fast in the OutSeason® when there is no requirement to also build Far.

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, not only from a training standpoint but also because it fits within your life as an age-group triathlete training in the winter. You need to keep your training short and sweet in order to preserve your head, family, job, and earn valuable Spousal Approval Units (SAUs) to be spent later when they really matter: closer to your race when you need the volume!



Coach P

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