Where Does Human Energy Come From and Why Does It Work That Way?
An organism — such as your body — only interprets the supply and demand for energy in terms it understands — period. In evolutionary terms, each of us have built-in as standard equipment an astonishing range of energy production capabilities. But there is a catch. The problem is that modern society no longer accesses these capabilities so they wither away into a perpetual state of dormancy — use or lose. When these energy production losses pass certain thresholds, we become biologically fragile with manifestations as myriad health disorders. In response to this stealth dormancy and fragility, we desire to fill this energy-producing vacuum by artificial means. People say they want energy because they always feel tired but they don’t know how human energy is produced or where it comes from and why. This essay addresses this head-on. If quality of life is important to you, knowing and applying this wisdom should be on the shortest of lists.
There is nothing you can “take” to give you energy — stimulants only move the needle a tiny degree compared to what you can potentially produce naturally if you know how to intelligently manage the movement patterns of your body which, in the last 2000 years, has become known as “exercise”, “conditioning”, and “training.” [Explore origins of training: Eastern origins | Western origins].
So what is the range of human energy production? We often hear the terms strength, power, endurance, and stamina. These common terms have different meaning to different people in different contexts. Let’s examine how human energy is produced. This gets a little technical in spots but hang in there, it is worth it to know about human energy from the perspective of the human organism because it fuels the engine of your life. Two frames of reference are presented: biological anthropology and exercise physiology.
ATP (adenosine triphosphate) is the currency of biological energy. When people say “energy” from a Western scientific perspective they mean ATP*. The energy spectrum and its utility to the survival of the organism is defined by the dynamics between the means of ATP production and its demand under different conditions. Unfortunately, in modern society we do not experience any of these conditions in earnest. We have four processes (not three) for producing ATP. For simplicity’s sake, let’s say we have four “gears” but they smoothly overlap and shift like a Ferrari F1 transmission if we are properly “tuned”.
Let’s now examine each gear, in turn, in terms of ATP production and demand. The following descriptions are in the context of world-class human specimens (“athletes”) in their prime or our ancestors, not the unconditioned couch potato (“digital man”).
Understand: the human organism groks the lifestyle of athletes but is totally baffled by the lifestyle of digital man.
1st Gear
In 1st gear we are evolutionarily suited to travel a long time before fatiguing. Most of us today cannot go very fast in 1st gear before we are forced into 2nd gear. People that are world-class in 1st gear are ultra-marathoners (50+ miles); Ironman-distance triathletes; stage (e.g. Tour de France) and Race Across America cyclists; ultra-distance Nordic skiers; long-distance, open-ocean swimmers; explorers; and Himalayan sherpas. It takes years to build a high-performance 1st gear, much like putting rings on a Sequoia tree. There are very few world-class performers in their 20s; like a great scotch, world-class ultra-endurance literally takes years to mature.
Considering your ancestors in Paleolithic days, this was the mission-critical survival gear due to having to trek hours per day in search of food which may not be available for days at a stretch. Our ancestors forged 1st gear over the millennia in the unforgiving crucible of intermittent famine and consequent starvation and death. It is the underwriter of energy-efficient, long-duration movement.
No 1st gear used to mean certain death, now it means you are chronically tired while on the cadaverous pathway to serious illnesses of mind and body.
Homo sapiens are the best performing mammals on land for traveling long distances in vastly different environmental conditions, even superior to Arabian horses, camels or elephants. As recently as the 19th century, we had to hike over uneven terrain for several hours a day. Social hierarchy followed by fossil fuels subverted the necessity of a well-conditioned 1st gear. In today’s digital world, the majestic beauty of humanity’s 1st gear has been mothballed and erroneously replaced by lowly “cardio,” or even worse, “time saving” interval training. They are starkly different as we shall see.
The limiting factor in ATP production for 1st gear at the high-end of the range (meaning close to 2nd gear) is called maximal lipid power. This is the power you can produce when you are burning fat at the highest rate — not the greatest percentage of fat (like when asleep).
Simply: If you don’t burn much fat or cannot burn much fat then you will become fatter and fatter; “skinny fat” is another outcome.
When power demand exceeds your maximal lipid power, you are shifting into 2nd gear but the transition from 1st to 2nd is a broad range with respect to the rate of ATP production due to the relative contribution from carbohydrate use. Forget population statistics: on an individual basis there is a big, gray blurry area defining the baton hand-off between 1st and 2nd gear. Both 1st and 2nd gear are aerobic gears because they rely exclusively on the presence of oxygen. The physiological distinction between 1st and 2nd gears and the transition between them is a complex but a vital physiological topic and needs to be addressed to clarify what has been grotesquely distorted in fashionable training methods in vogue. Please stay with me.
The source of fat at very low energy demand comes from adipose stores like around the waist and travels to the muscle via the bloodstream. Energy also comes from short chain fatty acids as a byproduct of the bacterial fermentation of dietary fiber in your large bowel. As energy demand increases, progressively more fat stored within the working muscles (called intramuscular triglycerides or IMTG) is used. This occurs to limit the organism’s exposure prematurely to the hormone epinephrine (adrenaline), a catabolic hormone, which then ramps up after IMTG and glycogen stores are depleted. Depending on the capacity of IMTG stores, the concentration and organization of enzymes in the fat-burning pathway (the fatty-acid cycle) and availability of oxygen within the part of the cell where this occurs (mitochondria), fat burning at some point maxes out (maximal lipid power) and 2nd gear begins. IMTG can only produce ATP at one-third the rate of stored carbohydrate within the muscle (glycogen); 1st gear sacrifices the rate of power output for stored energy efficiency due to the fact that highly-trained 1st gear performers store up to twice as much IMTG as glycogen and theoretically enough overall adipose reserves to walk for over 250 hours.
Performance in 1st gear at the high-end of the range is defined by maximal lipid power and defines the transition to 2nd gear but the essence of 1st gear performance — and, consequently, the only means to accurately assess it in a fitness competition — is forcing a metabolic milieu where the maximal sustainable rate of ATP production are under conditions of depletion of IMTG and glycogen stores and prolonged for many hours.
Sustained energy production under hostile metabolic and environmental conditions is the underwriter of human survivability. In other words, seen through an evolutionary lens, we are designed to walk in an open-ended fashion. Digital man does not do this and suffers accordingly.
Ok, once your muscles’ internal energy storage depots are running on empty, what then? This is where survival response kicks in — it is called hitting the wall or bonking, the state where you are forced to slow down because the energy stores inside the muscle are nearly gone. Now fuel comes from adipose stores and from the breakdown of muscle to use the essential amino acid leucine and other amino acids. Leucine contributes around 3% of total energy for the first couple of hours and then increases over time. Skeletal muscle (the ones you can see) serves two crucial functions: (1) they are the motors for locomotion via contraction and relaxation (e.g. “weight lifting”) and; (2) along with body fat, are the fuel supply for energy demands under the environmental conditions described above (as muscle is used (catabolized) several amino acids are converted to glucose and others to ketones (leucine is one of these) for fuel including to support brain energy metabolism under conditions of liver glycogen depletion). Under hormonal control, epinephrine regulates energy production in this phase of fatigue by mobilizing fat stores and breaking down muscle for fuel. The pace you sustain at this point is what ultra-distance performance is all about: 1st gear is not about “cardio,” it is about fuel regulation and supply, not cardiovascular performance per se.
Performance in 1st gear also has two crucial roles played by the nervous system: motor control and willpower. The central nervous system when highly conditioned is capable of exhibiting creativity not only in terms of symbolic thinking like solving calculus but also very sophisticated body thinking in the form of complex and dynamic predictions of body movement and simulations of movement in real-time: as muscle fiber fatigues and can no longer contribute meaningful power output, the spinal cord, brain stem, motor cortex and cerebellum are forced to search and assemble motor commands that creatively recruit different motor patterns from the available motor pools targeting the appropriate muscles to maximize the efficiency of waning energy supply in the face of catastrophic fatigue. If a massive, multi-year, volume of ultra-endurance conditioning base is lacking, an untrained (“digital man”) nervous system is not up to the task and performance suffers — premature death is immanent given merciless migratory circumstances.
Additionally, the willpower of the mind plays a big role because your body in every way possible is signaling you to stop but physiologically humans can be conditioned to go long distances without food in this state as long as dehydration and electrolyte imbalances (and other environmental factors) are not life threatening. To even be considered as the world’s most-conditioned human, you must demonstrate high performance under the threat of these conditions because this capability is both our strongest asset relative to all other terrestrial mammals and — besides ingenuity and versatility of learning mental and physical skills — is our most valued survival asset.
Humans are the paragon of ultra-endurance amongst terrestrial mammals. We are only outperformed by cetaceans: dolphins, porpoises, and whales, the consummate fat burners.
2nd Gear
The rate of ATP production is the dominant factor determining which of the four gears you need. In the case of 1st gear at the high-end of the range, as the demand for more ATP production occurs beyond maximal lipid power, carbohydrate becomes the marginal source of fuel which means that further ATP production comes only from carbohydrate. As even more power is demanded there reaches a point where fat burning decreases at the expense of further carbohydrate use even though you are still 100% “aerobic.” This point is much higher in the highly conditioned than in the unconditioned but in all athletes fat burning ceases at 80 to 85% of maximal oxygen consumption. The cut-off percentage is higher running than cycling because at any given percentage of maximal oxygen consumption, there is greater recruitment of fast twitch oxidative muscle fibers (type IIA) cycling than running.
The marathon, Olympic distance triathlon, 50 km Nordic skiing, mountain bike marathon and other events in the 90 to 150 minute duration range are example of sports requiring a dominant 2nd gear. Total glycogen depletion and hitting the wall is a major physiological obstacle in 2nd gear events. The end of 2nd gear is approached when the metabolic byproduct lactate begins to accumulate in the blood (the lactate threshold) and the transition from 2nd to 3rd gear is VO2 max, or the the maximum (peak) rate of oxygen you can metabolically consume.
3rd Gear
3rd gear begins beyond this point and you now are in the realm of the anaerobic gears meaning greater amounts of ATP must be produced without oxygen as power demand increases from here.
To review the transitions, 1st gear terminates at the point you max out burning fat and 2nd gear terminates at the point you max out your capacity to use oxygen to oxidize carbohydrate.
Proper training changes the power output when these physiological events occur by shifting the transition points to the right relative to the oxygen consumption rate.
Does 3rd gear mean that 1st and 2nd gears and oxygen no longer contribute? No. Somewhere around the lactate threshold 1st gear fades out to zero. That means fat is out of the equation entirely. What happens now in this big, gray blurry area is the use of carbohydrate both aerobically and anaerobically. The way to understand this is that the anaerobic pathway (called anaerobic glycolysis) feeds into aerobic glycolysis (called the Kreb’s cycle) as a direct input, it elegantly plugs in.
“Aerobic” and “anaerobic” are not distinct means of ATP production the way most people believe; a large portion of the power continuum is the seamless integration of both metabolic chains identical to a bucket brigade but at the molecular level.
Thus, any “distinction” is but a human abstraction. In the intensity range defining the transition from 2nd to the first aspect of 3rd gear, 2nd gear is now going flat out whereas in the long transition from 1st to 2nd gear it was just ramping up.
As ATP demand increases while in the face of accelerating oxygen shortage, intracellular oxygen is metabolically regulated to exclusively fuel aerobic glycolysis in lieu of fat metabolism (β-oxidation) which means fat use falls off a cliff because it is the rate of ATP production that matters independent of fuel source when greater power is demanded. However, with ATP production now being demanded in excess of VO2max, the backend of glycolysis (the aerobic part of aerobic glycolysis) can no longer increase ATP production because it is now flat out. So the frontend (the anaerobic part) starts ramping up its outputs of ATP + pyruvate but now pyruvate cannot be fully metabolized by the backend because it is maxed so it accumulates and leaks out of the working muscle as lactate which can be used for fuel elsewhere. This metabolic behavior — the accumulation of lactic acid and pH effects on enzyme function as opposed to maximal pyruvate-lactate steady state utilization — defines the transition from the 1st to the 2nd aspect of 3rd gear.
If this intensity continues or increases, energy production declines rapidly because the conditions within the muscle becomes acidic and some of the enzymes in the metabolic chain shut-off below a certain pH level (around intramuscular pH = 6.2). Your limbs feel like lead, movement slows to a crawl, your face turns white and vomiting is not uncommon because there is a decrease in the pH of spinal fluid which triggers the vomiting reflex. This is the center of gravity of the 2nd aspect of 3rd gear. You cannot will yourself to continue, power output catastrophically declines despite your best efforts because metabolic enzymes fail to function and they are the ultimate engines of not just movement but life itself from amoeba to whale.
Imagine sprinting up a steep hill as fast as you can for as long as you can and what it feels like: these are the effects of 3rd gear.
The 1500 m run (the mile), 2000 meter Olympic rowing, 400 meter Olympic swimming (freestyle or medley), 5000 meter speed skating, and 4000 meter track cycling pursuit event are examples of 3rd gear sports at the low-end of the range (the 1st aspect of 3rd gear). A duration of 3 to 7 minutes defines events where power output is defined by maximal oxygen consumption and a very high lactate threshold, they are the primary performance determinants. However, it should be noted that in extremely conditioned 3rd gear athletes, such as the hour record in: cycling, Nordic skiing or ice skating, greater than 90% of VO2 max can be produced. These athletes really are 2nd gear athletes that can perform at very close to the low-end of 3rd gear intensity for long duration.
4th Gear
So what about 4th gear? 4th gear is for production of extreme levels of force and is fueled by ATP that is present at rest within the muscle which includes the creatine phosphate pool. This is called the ATP-CP energy system and is anaerobic (meaning no oxygen required) and also alactic (does not produce lactate). This system produces ATP several-fold over 1st and 2nd gears but is extremely limited in capacity. In our Paleolithic days, this was a vital survival capability both offensively and defensively for strength, power, and explosive movement: 4th gear = fight or flight! As opposed to 1st and 2nd gear, this is using fast twitch glycolytic muscle (type-IIB fiber) instead of slow twitch. There is a third, mixed muscle type that is better suited for 3rd gear intensity, but just like there are long, broad gray transitions between the gears the same exists for muscle composition.
In fact, this extraordinary range of muscle structure reflects that function precisely: 1st gear muscle contributes next to nil for 4th gear tasks and 4th gear muscle contributes next to nil for 1st gear tasks. It is like the difference between a diesel engine (slow twitch) and a dragster engine (fast twitch). Very different tasks.
In modern societies and in many amateur training programs, 1st gear is completely forgotten and is tossed into the “aerobic metabolism” mental box and called “cardio.” Only 1st and 2nd gear athletes understand the vast differences between running the mile, the marathon, and a 100-mile ultra-endurance running race — all “aerobic events.”
Now let’s take a closer look at the range of 4th gear. At the high-end of the range is pure explosive power like the shot put and Olympic weightlifting. Maximal power is defined as how much force you can produce in a single contraction in the shortest time interval (faster generates more power) you can produce (i.e. power = force x velocity). It is fueled by ATP in the muscle at rest. After a few repetitions, the ATP is depleted and further energy is derived from the creatine phosphate pool (creatine phosphate + ADP + H⁺ ↔ creatine + ATP). Depending on its capacity and duration under load, it will be depleted in a matter of seconds and then the amount of force or power you can produce drops dramatically. This is the low-end of 4th gear; the transition from 4th to the 2nd aspect of 3rd gear is defined by the depletion of the creatine phosphate pool and the rapid ramp-up of anaerobic glycolysis (the frontend of glycolysis).
4th gear is a short gear in terms of time (less than 25 seconds, sometimes even just 12 seconds depending on the rate of contractions and is often called strength-endurance) but encompasses a large cross section of sports and human survival function.
Sprinting events fall into the 4th gear realm like the 40 to 100 meter dashes, 50 meter Olympic swimming events, and 200 meter track cycling time trial. Other 4th gear sports include Olympic gymnastics events like still rings. After the 4th gear transition into 3rd gear are many anaerobic glycolytic sports like 100 meter Olympic swimming events, 200 and 400 meter track sprinting, track cycling’s kilometer, and 500, 1000 and 1500 meter speed skating but these all have a rapidly increasing aerobic glycolytic contribution as a function of time if intramuscular pH is not in decline. All of these anaerobic glycolytic sports at the world-class level allocate the vast majority of training time to low heart-rate, long-duration, endurance capacity training (i.e. 1st and 2nd gear) despite the competitive event lasting only for around 60 seconds.
Conclusion
In a natural state, Homo sapiens are quite capable of producing prodigious amounts of energy over the lion’s share of our natural lifespan. Why we as a species have become energy paupers is because we no longer have to move our bodies in a manner aligned with our evolutionary expectations and design requirements to survive. Fossil fuels and machines now perform the work that our bodies require to maintain the minimum, structure-and-function to produce energy at a level to sustain vitality. The cost? Poor energy, poor quality of life. As a result, these structures and capabilities atrophy to the point of extreme dysfunction and come to roost by the second to fourth decades in the form of myriad common diseases which devastate the healthcare system with its exponential costs as society ages.
The use of technology needs to be reframed in terms of our evolutionary needs to maintain optimal function; technology should be a tool as a means to an end aligned with our health and well-being — not as an end in itself. What we are currently doing is tantamount to technology being wielded as a leveraged means to sabotage our quality of life. The World’s Fittest Humans addresses the proper role of exercise in a comprehensive fashion by showcasing what the potential of our species is at its breathtaking zenith.
Note: This essay was edited for brevity and only addresses the Western view of energy; for Eastern perspectives on energy (which is very different!) see: [Ji Feng (China) | Jōtara Musashi (Japan)].
James Autio | about.me/jamesautio
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