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 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.
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.