We can compare the human body to a hybrid car, which is able to generate energy and movement from two sources of fuel. In the case of humans as well as cars, the machine will operate using a single source, but it won’t operate as it was designed. It’s got to be able to use the right fuel at the right time, that is, in the right circumstances. We are naturally capable of doing so, but modern lifestyle is making it sometimes difficult, other times plain impossible. The two sources of fuel for the human body are carbohydrates and fats. Proteins can be a source of fuel in rare cases, such as prolonged fasting when sources of carbohydrates have been severely depleted. For the purpose of this discussion, we can consider proteins as a fuel source by avoiding to use them as such. Muscle mass is a necessity, and when the body starts to consume its own healthy tissues, things are going in the wrong direction. It can safely use rogue, senescent, that have become cells unfit, discard them and make way for new cells, but it needs to leave the good tissues alone.
The role of carbohydrates in endurance training
Carbohydrates we eat are digested and stored in organs, like the liver and our muscles, in a form called glycogen. When considering the analogy with a hybrid car, glycogen is electricity, that is, the first fuel the car is going to try and use. If electricity doesn’t supply enough power, the car will activate the gas engine, which today has more energy reserves, although they are less efficient. Another analogy with glycogen comes from the inside of a computer: it has various levels of data storage, one being volatile memory, and another being the hard drive. Volatile memory, in the form of cache, doesn’t have as much capacity as the drive, but it’s very quick to access and more efficient. The drive, and some of its even slower forms, called ‘cold storage’, can store more data at lower costs, but data is slower to retrieve. Glycogen in the body similar to memory in the computer: available in limited quantities and providing quick access to data. When we engage in sports, the body will initially tap mostly into glycogen, especially if this involves high intensity exercises. Glycogen provides energy no matter how slow or how fast we run, unlike fats, which provide energy at lower intensities.
What defines endurance training?
A definition of endurance is a physical activity that involves elevating the heart rate for over 45 minutes. The premise behind this approximate duration is that during the first few minutes, the body will use the widely available glycogen. As time goes by, glycogen stores start depleting, and at a point that depends on everyone’s personal metabolic profile, we start using fats as a fuel source. This is a first way to consider the exercise, and with the objective to train ourselves to be able to use fat as a fuel source. This is central to endurance training, and one of the first benefits: if we never have to tap into fat burning, either through time-restricted eating, or intermittent fasting, or through some kind of endurance sport, our body is always full of glycogen, full of “sugar”, so it becomes acquainted to only using it as energy, and never burning fat. This state of constant abundance does not help cure pre-diabetes, type 2 diabetes, excess weight, and other illnesses of modernity. On the other hand, engaging into endurance sports helps clear up glycogen reserves every now and then, so that we when refeed at the next meal, any extra calories we are not going to use can be put into new glycogen stores, instead of being converted to body fat. Glycogen stores are a buffer for energy intake and usage, because it is so easy for the body to replenish and deplete them.
Alternative definition of endurance and how to get faster
Another way of considering endurance is looking at it as a percentage of VO2Max, our maximum level of oxygen intake. If VO2Max represents 100% of oxygen intake, this means that for very short sprints, like running one hundred meters, you can generate energy beyond 100% of VO2Max, because the body has little bit of extra oxygen it can use in addition to what’s being breathed in and out. Above that distance, we move to high intensity, between 80% and 100% of VO2Max. At this level of intensity, the body produces lactate as a byproduct, some kind of metabolic waste that slows down the athlete. Lactate is a soft poison, in the way that it accumulates in the muscles and rapidly deteriorates performance to the point that the athlete must slow down below anaerobic level, that is, below lactate accumulation threshold. Then, in the 70% range and below, we find aerobic activity: a more comfortable zone, that of endurance, which we can stay in for much longer time periods.
This indicates that in order to practice endurance successfully, you must train yourself first for gradually longer sessions, for your muscles and organs in general to become acquainted and comfortable exercising for extended periods. This groundwork allows your cells, for example, to become increasingly efficient at transitioning from burning glycogen to burning fatty acids, i.e. body fat. This is in part how you can avoid putting on weight and become better at losing weight, by training your cells to use fat as a fuel source.
We get better at math by doing math, but in the case of endurance we get better at it by doing speed work
Another indication from the fact that endurance can be defined as a percentage of maximum effort, is that by raising your performance at high intensity, you benefit by second order effect on the endurance. In other words, sprinting is high intensity. Muscle-specific workouts are high intensity: heavy loads, fast repeats, and short duration. These exercises raise your VO2Max and the lactate threshold, such that now, the percentage of that top can be much higher: your endurance performance has improved.