I have previously wrote a blog about what exercise zones are and how to use them in training here. I would recommend reading it before reading this blog. This blog will cover the physiological reasons training zones exist.
How We Use Energy to Exercise
Recall from biology that a cell gets it’s energy from the ATP cycle. This cycle starts with a adenosine triphosphate molecule, which stores potential energy in its chemical bonds. To access this energy a cell breaks the ATP molecule apart into ADP and a phosphate group. This breaking of a chemical bond releases energy that the cell can use. Your body then finds a way to recycle the ADP and P back into ATP. The method used to recycle the ADP depends on the conditions it is operating under. These different methods operate at different exercise intensities, with different efficiencies which accumulate fatigue at different rates. These different energy sources is the fundamental reason exercise zones exist.
ATP Recycling
There are three main ways ADP is recycled, for the use of cycling. These are the aerobic (oxidative phosphorylation), anerobic (anaerobic glycolysis), and sprint (creatine phosphate cycle) energy systems. First I will explain the sprint energy system. This one relies on ambient creatine phosphate molecules in your cells. These molecules float around waiting to be used. When you decide to dip into your sprint energy reserves, your cells immediately start taking these creatine phosphate molecules and splitting them, using the phosphate to recycle ADP to ATP, and sending the creatine to your mitochondria to begin recycling it back to creatine phosphate. The process of recycling creatine is slow, taking around two or three minutes, and once you use all the free creatine phosphate you can no longer sprint(15-30 seconds). Having more creatine phosphate sitting in your cells gives you a bigger sprint capacity, so many people interested in fast twitch muscle performance supplement creatine. Recycling of creatine also requires oxygen, which is why you keep breathing hard after a sprint. In order to recover for a sprint to be able to sprint again you need to first catch your breath to supply oxygen, then allow the creatine to recycle. This is why sprint training requires at least 2 minutes of rest between sets, most often 5 minutes.
The second energy system is anerobic glycolysis. This system uses ambient glycogen from your muscles and recycles ADP to ATP without oxygen. This is useful as getting oxygen to the muscle is a big limiter in hard efforts. This energy system lets riders go harder than their lungs can handle, ideally to win a race. This system does not come without a cost however. The reactions(18) that occur anaerobically to recycle ATP produce the byproduct of lactate and a hydrogen ion. These are colloquially known as lactic acid, however lactic acid doesn’t really exist in the blood stream. The burning feeling is because the muscle and blood stream become more acidic, but this is due to the hydrogen ion, not lactic acid. Each hydrogen ion is paired with a lactate molecule, meaning the more you need to anaerobically recycle ATP, the more lactate and H+ ions grow, meaning the burning gets worse and worse. This is the main fatigue sign of this energy source, eventually forcing the rider to ease the intensity. It is also very important to point out that this method of cycling ATP is very inefficient. For each glucose molecule only 2 ATP molecules are recycled. This means that you get less energy per carbohydrate you are able to consume. While essential for producing high powers, the anerobic glycolysis system is very inefficient, hurts a lot to use, and only lasts 2-3 minutes.
The third main energy stream is the aerobic system, or oxidative phosphorylation. As you probably guessed, this system requires oxygen. This system is much slower than the anerobic system. This can take hundreds of reactions as opposed to eighteen for the anerobic system. This just means it takes a lot longer to process, so the “ceiling”, or maximum power this system can sustain is lower. It simply cannot produced ATP fast enough to keep up at very high intensities. The trade off is that the oxidative system is far more efficient. For one glucose molecule, a cell can produce 36 ATP. This is far more energy efficient than the other systems. This is essential for a sport that requires a lot of work over a long period. If you can be more efficient with you energy, you can save some to go faster later. The oxidative system can also use fats as a fuel source. This occurs at lower intensities, fats are more efficient and can be used for longer. At higher intensities, carbohydrates are utilized more as they are faster to process into energy. This is where the aerobic system further splits into two energy streams.
Once again at lower intensities our body can process energy more efficiently. This is true for fat burning. Fat is much more energy dense. It is also more prevalent in the body. It is always better to burn fat as opposed to glucose. Even burning glucose through the aerobic system still depletes the muscle of glycogen. This is why it is ideal to burn fat as much as possible, as fat has a much larger stored bank of energy than glycogen in the muscle. Muscle glycogen can be replaced through absorbing carbohydrates through the ride, but again it is always more beneficial to burn fat that is within the body and save glycogen for late race selections.
How Energy Systems Combine
Each of these energy systems don’t turn completely on and off as intensity changes. The aerobic system is the first to be turned on and used. More specifically the fat burning oxidative is the primary energy source at lower intensities. As higher intensities are used, the energy system begins to shift towards carbohydrate oxidation. This shift occurs at what is referred to as fatmax. Fatmax is the intensity where the maximum rate of fat burn sits. Any higher intensity carbohydrates take over, any lower and not enough energy is required to burn a lot of fat. Fatmax changes from rider to rider, with highly trained riders having a higher fatmax than others. The proper testing procedure is to get a lab test which measures your breath composition to determine where your fatmax(and also VO2) are. In general most people’s fatmax tends to be around 60% of VO2(5 minute power), which tends to be near or below the first exercise threshold. This is the ideal endurance training intensity, just below the first threshold.
Above the first threshold the aerobic system burns primarily glucose in the muscle. At this intensity lactate starts to rise. Both aerobic and anerobic energy systems are used here, with the primary source being aerobic. At the second threshold is where energy demands fully max out the capabilities of the aerobic system. This begins to rely heavily on energy from anerobic glycolysis. This increases acidity and discomfort, and is also far less energy efficient. This area is referred to as W’ or anerobic work capacity. Read about my blog on Critical Power and W’ here.
Above the anerobic energy system is the sprint capacity. When both aerobic and anerobic energy streams aren’t enough, sprint capacity begins to produce power. This is easily depleted and difficult to regain. This suggests a third exercise threshold, one that marks the end of the lactate energy stream and the sprint capacity. For what it’s worth Intervals.icu calculates this boundary between anerobic(glycolytic) and sprint energy(PCr). I don’t know how they do this. I have messaged some forms and will edit this with an answer eventually. The intervals.icu fields are pretty useful, it makes it easy to see what energy system(above second threshold) you are using. You can both see how much of an energy system is being recruited, as well as the strain score you are putting on that energy system. If one knew their fatmax they could do something similar for power under the second threshold.
It should be noted that these energy systems should be treated like buckets with ascending priority. As intensity goes up, your body will pull as much energy from the most efficient energy stream as possible. For example if you are doing 300 watts and your ftp is 280, all 280 watts of aerobic energy will be used before 20 watts worth of anerobic energy will be used. This is of course a rough guideline, your body doesn’t follow this rule exactly but it’s pretty close.
Conclusion
In conclusion this is important to know and track so that you are able to quantify your training. Again read my blog about training zones to understand the importance of tracking intensity. This info is simply interesting to understand why training zones are set where they are. It all comes back to energy streams and which ones are being used.
Leave a Reply