Endurance Training

Posted on | ~11mins
outdoors fitness

WARNING: I am absolutely no expert in endurance training, this is simply my notes mostly on Physiology from reading a few books. Take nothing you read here too seriously.

Ski touring photo

When I took up cycling (roughly 2014) and was frequently being dropped on my group rides, I bought a book, Cyclist’s Training Bible (CTB1) by Joe Friel. I read most of it at the time, but never took too much of it to heart and certainly never implemented a training plan from it.

More recently, under the guise of improving my fitness for ski touring, I started reading Training For The Uphill Athlete (Uphill2) and Training For The New Alpinism (Alpinism3) by Steve House / Scott Johnston. The information felt quite familiar so I did a quick re-read of CTB.

What follows is a squashed together summary of some of my the main things I took away from all these books. Mostly writing this so I can internalize it, if it’s helpful to you let me know.

No matter what I highly recommend buying the Uphill Athlete book, and perhaps the others if you are looking to up your fitness game though training.


Highly recommend reading one or all of the books or other resources on exercise physiology. Though, I include this rapid summary, mostly taken from Uphill and Alpinism, as a primer.

I found these sections of the books the most fascinating. Some of this stuff I recalled from high school biology, but having never studied physiology in depth a lot of this was new to me or at least providing “Ah moments” for putting it into context.

Systems involved in endurance activities

Energy Movement is done by contracting muscles, contracting muscles takes energy in the form of Adenosine Triphosphate (ATP). ATP is a heavy molecule4, so our bodies efficiently recycle it between ATP and Adenosine Diphosphate (ADP). Our bodies use (breakdown) a mix of fat (aerobically, Kreb cycle) and glucose (anaerobically, glycolysis) to run the ATP<->ADP recycling machine. “Fat stores, even in a lean, well-trained athlete, are virtually limitless (up to 100,000 calories of intramuscular fat). On the other hand, glycogen stores will rarely reach 2,000 calories even in the well trained."52

Muscle Structure Muscles are made up of fibers. These fibers exist on a spectrum, but can generally be classified into a couple categories: Slow-Twitch (ST) and Fast-Twitch (FT) 6. Muscle fibers are, grouped together into Motor Units (MU) that the nervous system can control as an individual block7 (MU contain fibers of all the same ST/FT classification).

ST fibers have higher concentration of mitochondria, this means ST fibers will produce more energy from fat via Kreb cycle (relatively slow process, more efficient, aerobic requiring oxygen, happens in mitochondria). Where as FT produce more energy from glycolysis (faster process, anaerobic not requiring oxygen, side effect is pyruvate produced).

To simplify ST fibers do aerobic metabolism and consume oxygen. FT fibers do anaerobic metabolism and produce side effect waste product.

Muscle metabolism Let’s discus lactic acid. Pyruvate, the side effect of glycolysis, has two pathways. First, and preferred for the athlete, is to be used by the mitochondria where aerobic metabolism will convert to more ATP. Second, a metabolic dead end, is to linger in the cell and become lactic acid and then immediately lactate and a hydrogen ion. That lactate can be used for energy in the heart, liver, and ST muscle fibers – our bodies are so efficient! However, the H+ ion will accumulate and cause an increase the acidity of the cell.

Note that we’re always doing both aerobic and anaerobic metabolism. At low intensities we are simply consuming all the pyruvate with aerobic metabolism and not accumulating lactate.

As intensity increases first you will see a measurable increase (1mMol/L) in lactate in the blood above baseline, this point is referred to Aerobic Threshold (AeT8).

As intensity further increases there is a point where the Aerobic Metabolism can no longer keep up and will see a run away lactate levels increase in the blood, the last intensity level where this can remain stable for an hour is referred to the Lactate Threshold (LT)9.

Above LT only minutes of work can be maintained before the body will force you to decrease intensity.

At the limit is what is “aerobic power”, or VO2-Max. This is an upper bound on an athlete’s potential, “the maximal volume of oxygen your body can process to produce movement”1. There will be some fractional utilization of this that is usable. “VO2 max is the product of the heart’s output and oxygen utilization of the skeletal muscles."2.

Additional terms to research

  • mitochondrial mass – “biogenesis response by the mitochondria begins to occur within minutes of the training stimulus and ceases within about twelve hours after the stress of training is removed” 3
  • aerobic enzymes – inside the mitochondria, increase order of minutes, but decrease after weeks of not training.


As mentioned above muscles are sub-divided into controllable motor units. ST-MU have a lower activation threshold, so as intensity increases they are the first called into action. FT fibers take more activation energy and will be reserved for high intensity work. This will become important in training because in order to train a muscle you must activate them.

Additional terms to research:

  • Autonomic Nervous System (ANS):
    • ANS is divided into the sympathetic (SNS) and the parasympathetic nervous systems (PNS). “A simplified explanation of these is that the SNS is the system responsible for speeding up the body’s processes, such as increased heart rate and respiration and the shutting down of digestion. […] The PNS does the opposite and can be thought of being responsible for the rest and digest response. It slows many body processes in preparation for rest”3
    • controls “unconscious activities that go on in our bodies such as heart rate, respiration, digestion, and the release of hormones. It also offers a critical feedback system to let the brain know what is going on throughout the body.”
    • Heart Rate Variability (HRV)
      • Can be used as a window into these two systems
      • “It turns out that well-trained athletes have relatively high HRV when resting. Sedentary, unfit people as well as tired but fit people have lower HRV at rest. When an athlete’s HRV is low, it is an excellent indicator that the autonomic nervous system is out of balance due to fatigue."3


Basically, you have the lungs you need, they are not the bottleneck and they are not really trainable. We can ignore them.

This feels like a good place to also mention that AeT/LT have alternative names and effects to the ventilatory systems. AeT maps to VT1 and LT to VT2.

VT1 is the point which breathing begins to increase and you will no longer be able to maintain breathing solely through your nose.

VT2 is the point at which conversation stops and you have to breath noticeably deeper and more rapidly.

If the lungs are not the limiter to O2 delivery, what is? Answer: The heart’s pumping capacity.


The heart has two knobs; the rate it beats and the volume of each beat. The rate increases rapidly in response to demand from muscles, the stroke volume can be increased over time from training.

A couple other cardiovascular points. First is the amount of hemoglobin (Hgb) available to transfer O2 around the body. If you are not doping with EPO, this is directly related to the blood volume. Blood volume can increase with training. Second is the capillary density in muscles; “The closer the capillaries are to the working cells, the less distance the oxygen needs to diffuse to get to the mitochondria."3

Additional terms to research:

  • Myoglobin (O2 holding protein in muscle fibers)
  • Capillary bed density

What is trainable?

Dr. Seiler recent talk on this

  • What is trainable / not trainable
    • Metabolic Markers we can move
      • Aerobic Capacity10 (VO2-Max) – while not super relevant for endurance, VO2max is somewhat trainable, the biggest factor is covered by heart stroke volume.
      • Anaerobic Threshold (LT) very trainable
      • Aerobic Threshold (AeT) very trainable (slowly) (“will move higher as measured by both speed of movement and heart rate”2)
    • Movement economy
      • Sport specific efficiency / skill
      • metabolic economy: essentially switching to burning fat at higher intensity (covered by raising AeT)
    • O2 Delivery
      • Lungs not the limiter and not trainable
      • Heart stroke volume
      • Blood volume (10% increase possible with training)
      • Capillary bed density
    • O2 Utilization
      • Mitochondrial mass
      • Aerobic enzymes
    • Fat adaptation
      • Consuming less carbohydrate calories and training in fasted state can slightly improve adaptation to producing energy from fat.
    • FT to ST conversion
      • By increasing mitochondrial mass and other adaptations FT fibers can start to behave more like ST fibers
    • Recruitment pool availability
      • size of the available motor unit pool and the endurance of those fibers.

Zone systems

While all of these books use a more traditional zone system with 5 or 7 zones. All of the training prescriptions, however, are around based around the two primary metabolic markers (AeT/LT).

After reading the books, I stumbled on Dr. Stephen Seiler, who promotes a 3-zone system based on three primary physiological anchor points (AeT/LT/VO2-Max); in my mind this seems simpler to reason about.

Anyway, once AeT and LT are established the Uphill Athlete’s online HR zone calculator can do the math for you. Linked from that page are several methods for determining AeT/LT.

Training goals for endurance

  1. Aerobic base: The first goal of endurance training should be to raise AeT (in terms of Speed and HR). A good goal for this is to get within 10% of LT, alternatively looking at heart rate drift / aerobic threshold coupling11. This aerobic base serves to: Increase the speed you can go on primarily fat fuel. Increase the ability to vacuum up pyruvate and prevent lactic acid buildup.
  2. Raise LT: LT is the point where we are operating a lot of FT fibers and cannot maintain the intensity for long. By raising this threshold in terms of HR or speed we will be moving faster and going longer at the higher intensity before we reach this threashold.
  3. Muscular Endurance: Ability to sustain high intensity over time. This is more related to strength training than endurance, but should not be ignored.

Additional terms to research:

  • Overreaching
  • Periodization
  • Progressive. Gradual.
  • Polarized training: Spending most time under AeT 80%, almost none between AeT and LT, and some time (20%) over LT. Common training methodology in highly trained athletes.
  • Rest period
  • (Max) Strength training
  • Recovery: REM sleep human growth hormone (HGH) release.
  • HRV / resting HR ^ can be early signal of overtraining or illness and training should be backed down.
  • Tracking intensity
    • RPE: Borg scale
    • Rating in training log from Uphill Athlete: “A – means you felt like Superman and had plenty in reserve.        B – means it was a good workout. You completed the task with no problem.        C – means you did the workout but felt flat or off.        D – means you could not finish the planned workout or had to reduce it.        F – means you could not train that day due to fatigue or illness."2


I got tired of making notes 🤣 However, I have found myself referencing this private doc several times over the months since I wrote it down. Clicking publish so I can share with friends … And again, cannot recommend those three books enough a TON of great information in them.

  1. Cyclist’s Training Bible. Looks like there is a new edition, but that is not what I read. ↩︎

  2. Training For The Uphill Athlete  ↩︎

  3. Training For The New Alpinism  ↩︎

  4. Uphill states that if we didn’t recycle it we would burn through about 60-70kg ATP in a day 😳 ↩︎

  5. Note: Subcutaneous fat is more expensive to access than intramuscular or intra-organ fat (of which our bodies have plenty of for many hours of work). I think this means you are not immediately burning away that beer belly when we talk about using fat for energy. ↩︎

  6. Sometimes FT is broken into FTa and FTb, however, that distinction does not seem terribly important. Mostly the distinction is that FTa behave a bit more like ST fibers, “fatigue-resistant fast twitch”. Muscle fibers, however, exist more along a continuum rather than as discrete types. The interesting distinction, however, is that with training muscle fibers can convert classes and thus developing FTa to gain mitochondria and become more like ST is trainable. ↩︎

  7. While FT fibers in a MU always fire together, there, it turns out, is some asynchrony involved in ST-MU. This means within an ST-MU some fibers can be resting and others working at a given moment. ↩︎

  8. The exhalation of CO2 serves to bring your body’s pH back into range, and thus this point in increasing intensity can be noticed by a sharp increase in ventilation rate/depth. This is sometimes called Ventilatory Threshold (VT1). ↩︎

  9. LT also goes by Anaerobic Threshold (AnT), Functional Threshold (FTP on a bike), Maximum Lactate Steady State, Second Ventilatory Threshold (VT2) – breathing becomes noticeably deeper and more labored. ↩︎

  10. CTB uses “aerobic capacity” for VO2Max, confusingly Uphill uses “aerobic capacity” for AeT, but since we have a better term for that “aerobic threshold” I will stick with Capacity for VO2Max. ↩︎

  11. “When you can do an aerobic threshold ride for two hours while your heart rate and power remain coupled, you can consider your aerobic threshold fitness fully developed” - CTB. Similar can be calculated from running AeT from heart rate drift↩︎