So many people are interested in doing exercise or changing their fitness, and there seem to be so many different ideas around. It can be tricky to know which one to believe. At the same time, the scientific studies might be a bit much to attempt to dig into. This article aims to provide the middle ground: a summary of, and introduction to the biological process that happen during exercise, which are responsible for altering your health and fitness. I focus on high intensity exercise, explaining some of the reasons why it is so great for health and fitness, and at the same time why aerobic, cardio exercise does not give you as much as most people think.

Aerobic or cardio exercise has gained a big reputation, and we have the association of needing to train at a moderate level for long time periods many times a week in order to get fit or stay healthy. Often when someone wants to get fit or make a change to their body, they think of going on a run, as if running is the best route towards it. Unfortunately, this is a very time-intensive myth.

You can have a huge effect on your fitness and body by doing shorter bursts of more demanding, higher intensity exercise – like sprinting instead of running, or using resistance/weights. In high intensity interval training (HIT), the structure is usually to do a series of short bursts of near-maximum effort, for example 5 repetitions of 30 second sprints, with 30 second rests in between, a couple of times per week. It can also be performed with various bodyweight or resistance exercises. The big practical bonus of this is that it doesn’t take much time! We’re talking about training for 20 minutes a couple of times per week, instead of training for a few hours per week.

Not only is high intensity training more beneficial, but relying on jogging or other low intensity, high volume training as the main form of exercise can actually have detrimental effects to health, which many people are not aware of. Aerobic/endurance training has its role, but it should definitely be combined with interval or strength training if you want to make a positive change to your health and fitness.

1. Tons of studies show how effective high intensity training is

We will look at the metabolic processes that are happening during high intensity training, which show why this way of training is so effective. But first let’s just establish that there is a lot of evidence these days showing that high intensity interval training is extremely effective.

A team from Japan’s National Institute of Fitness and Sport showed that a high intensity interval training program resulted in bigger gains in aerobic capacity (measured by VO2max: maximal oxygen uptake) than a program of steady state cycling. That study used 20 seconds of maximum effort alternating in repetition with 10 seconds of rest. In fact, many studies have shown this big effect of high intensity training on VO2max. In one study of high intensity interval training, the participants cycled at 80% of their max for 90 seconds, 6 times, with 3 minutes’ rest between, 3 times a week. After 6 weeks, in comparison to the group who did their normal training, the participants had significantly increased their VO2max, their anaerobic threshold, their work output, mean power, peak power and recovery rates as measured by lactate levels. Another cycling study comparing moderate intensity endurance training with high intensity interval training also showed greater increases in both aerobic and anaerobic capacity from the high intensity training. For middle distance running too, high intensity interval training has been shown to boost running performance. And studies for rowing have also showed this greater improvement in race time for high intensity interval training as compared to traditional, lower intensity training.

2. What is exercise anyway?

To set the context of comparing different kinds of exercise, we have to first think about what exercise is. People exercise for different reasons, but they must have something in common. I will define exercise as activity that creates a positive change in your health and fitness. And what is fitness? The broadest way we could define fitness is your physiological capacity to meet a challenge that takes you beyond your resting state.

exercise = activity that creates a positive adaptation to health and fitness

fitness = physiological capacity to meet a challenge beyond the resting state

So where does this positive change actually take place? The term “cardio” is often used to describe low intensity, steady-state training, and suggests that the training effects the cardiovascular system. Of course, exercise does creates changes in cardiovascular dynamics. But exercise simultaneously trains many systems and processes, and actually many of the most important changes happen in the muscles themselves. Exercise creates changes in neural recruitment patterns, meaning how your nerves recruit muscles for a given action, and also creates changes in muscle bioenergetics, and in muscle morphology and acid-base status.

On the topic of muscle bioenergetics, another popular term for steady state exercise is that it is “aerobic” exercise, which indicates that it’s trying to target the aerobic metabolic system in muscle cells. But unfortunately the popularity of this idea has also led to a misunderstanding of how to actually create changes in fitness. You don’t need to train aerobically to change your aerobic capacity. Since so many of the relevant changes are happening in the muscle cells’ process, let’s zoom in to inside the muscles to look at how high intensity training effects those processes.

3. Aerobic and anaerobic energy: how muscles burn sugar and fat

When people talk about “aerobic” exercise, the term aerobic actually refers to one of the processes by which muscles produce the energy they need to contract. But aerobic metabolism is only half of the energy situation, and we need to understand the whole thing in order to understand what different kinds of exercise do. Muscles produce energy, which they use to contract and create movement. They create this energy by breaking down glucose (sugar) and fats, a process that involves many steps and releases ATP (adenosine triphosphate), which is like your body’s energy currency.

Let’s look first at breaking down or metabolizing glucose. Glucose is a basic molecule of sugar, which is contained in sweet things, but also in carbohydrates like pasta and rice, in long chains. Glucose gets stored in your muscle cells as glycogen, which is a branched chain of glucose molecules joined together. In order to produce energy, a glucose gets cleaved off the glycogen storage, and goes through a series of chemical reactions to be transformed into a molecule called pyruvate. This process is called glycolysis, and happens in the main body of the cell, the cytoplasm. It happens without oxygen, so it’s known as anaerobic, and results in 2 molecules of ATP.

Anaerobic metabolism: Glucose Pyruvate + 2 ATP

By Dwong527 (Own work) [CC BY-SA 3.0 (], via Wikimedia Commons

By Dwong527 (Own work) [CC BY-SA 3.0 (], via Wikimedia Commons

The pyruvate then gets taken into the mitochondria, a unit inside your cell, via an enzyme. Inside the mitochondria, the pyruvate is transformed into a molecule called acetyl CoA, which then enters a long series of chemical transformations, known as the Kreb’s cycle or tricarboxylic acid (TCA) cycle. This process uses oxygen, so it is known as aerobic metabolism, and it releases a total of 36 ATP, which is a lot of energy. Actually, each round of the Kreb’s cycle only releases 1 ATP, but it also produces other molecules called NADH, FADH2 and GTP, which are electron carriers, and which are used by the oxidative phosphorylation pathway to produce ATP. There are two rounds of the Kreb’s cycle for each glucose molecule, since each glucose produces two pyruvate molecules.

Aerobic metabolism: Pyruvate → 36 ATP

By Narayanese, WikiUserPedia, YassineMrabet, TotoBaggins [GFDL ( or CC BY-SA 3.0 (], via Wikimedia Commons

By Narayanese, WikiUserPedia, YassineMrabet, TotoBaggins [GFDL ( or CC BY-SA 3.0 (], via Wikimedia Commons

Finally, fats also get metabolized. Triglycerides or “neutral fats” (both from fat storage and from diet) first get broken down into one glycerol and three fatty acids. Glycerol enters the glycolytic pathway that we saw above as anaerobic glucose metabolism, and so gets transformed into pyruvate. The fatty acids (and pyruvate) are converted into acetyl CoA, which enters the Kreb’s cycle in the mitochondria and undergoes aerobic metabolism as above. Since each fatty acid creates two acetyl CoA molecules, this means that in total seven acetyl CoA come from just one fat molecule. A single triglyceride will produce a massive 441 ATP, which shows why fat is such a rich source of energy.

4. Anaerobic vs aerobic exercise

Understanding these stages of metabolism is crucial for understanding what’s going on in different kinds of exercise. Whatever you are doing, both of these steps are always happening at the same time: anaerobic and aerobic metabolism. It is not possible to use only one or the other. So when you do “aerobic” training, you are also working anaerobically.

So what’s the difference? When you’re doing what people call aerobic activity, it means the aerobic phase can keep up with your energy needs. The aerobic phase also takes a while (about a minute) to really get going, and it is also very slow. This is because the process of getting the pyruvate into the mitochondria limited by the rate of work of the enzymes, and the tricarboxylic acid cycle also involves many steps with many different enzymes along the way. If you’re just walking along, this process can keep up. But if you start sprinting, it’s too slow to give you all the energy you need to continue.

The anaerobic phase of glucose metabolism, on the other hand, is very fast and starts immediately. Although it only produces 2 ATP, it can happen so quickly that when you do more intense exercise, you are mainly relying on the anaerobic process. The aerobic step is still happening; it’s just too slow to give you all the energy you need to do more demanding activity, like sprinting, jumping or moving against weights/resistance. The proportion of energy provided by the aerobic system builds up over time, so that after around 75 seconds, the energy output is largely aerobically driven. The crossover point, when aerobic and anaerobic contributions to energy are roughly equal, has been shown to occur at around 600m or 75 seconds of near maximal running (i.e. running almost as fast as you can). High intensity interval training therefore often involves multiple repetitions of nearly all-out effort within the anaerobic zone, i.e. for somewhere between 20 and 75 seconds.


5. High intensity: training the anaerobic stage makes the aerobic stage get better

Using this difference between anaerobic and aerobic training, we can start looking at what happens when exercise directly targets the anaerobic phase of metabolism. In fact, training the anaerobic step, glycolysis, causes the second step to get better! When you train at a higher intensity and are relying on the quick, anaerobic glucose metabolism, then you actually stimulate your cell’s ability to do aerobic metabolism to get quicker. How can that be?

The first step to consider is that when you train at a high intensity, which means doing something very physically demanding that you couldn’t maintain for more than a couple of minutes, your muscle cells produce a very large amount of pyruvate: the end product of anaerobic glycolysis. You can imagine this pyruvate as building up a pressure that causes all of the steps after it to adapt to get better.

It stimulates adaptation in how fast the pyruvate gets taken into the mitochondria, which is an especially rate-limiting step, how fast it can go around the Kreb’s cycle, and how fast the electron transfer pathway creating ATP can work. These processes rely on a lot of oxidative enzymes, and high intensity training causes increases in these important enzymes, which means the aerobic process gets quicker. This would mean, for example that you would run your 5 km quicker, or you would go up the stairs without getting out of breath: the amount of work or output you can do while staying within your aerobic capacity is higher. You’re changing your anaerobic threshold.

Lots of research backs this up. In one study where participants did 8 intervals of 20 seconds at high intensity, with 10 seconds rest between, 4 times per week, the participants showed increases in several mitochondrial enzymes after the 4 week training period. Other studies have also shown this connection between high intensity interval training and increased mitochondrial oxidative enzymes. But while this increase in muscle oxidative capacity and endurance ability has been shown for not-so-trained people, in highly trained athletes, studies have shown that high intensity training does not affect those mitochondrial enzymes, suggesting that other factors must be responsible for the positive improvements. We’ll come to some other factors below, but let’s first stick with the enzymes.

Not only do the enzymes responsible for oxidation and glycolysis get increased, but the number of mitochondria also increase. This has been known to happen during long duration, low intensity training for some time, but has now been shown to happen through high intensity interval training as well. It happens through creating more AMP-activated protein kinase and p38-activated protein kinase, which has been suggested to be responsible for inducing the generation of more mitochondria, which means, again, that there is a greater capacity for aerobically metabolizing fats and sugars.

6. Lactate processing

The second reason why high intensity training is so great is all about lactate and your body’s ability to process it. High intensity training has been shown to improve lactate transport in muscle, and improve muscles’ ability to release lactate during contraction. So what’s lactate all about? When you do intense exercise and your muscle cells build up a lot of pyruvate, it doesn’t just stay like that. Under a lack of oxygen, pyruvate gets converted into lactate, or lactic acid in order to store it.

Lactate is what makes that burning feeling in your muscles like if you did a lot of sprints or weights or jumping. This burning feeling tells you “please stop doing this movement; I can’t handle it anymore!” It’s one way you could describe muscle fatigue.

After you’ve finished exercising and are resting, the lactate gradually gets turned back into pyruvate, which then has the positive effects on the mitochondrial enzymes that we saw above. This is also the reason why people say that high intensity interval training makes you keep on burning more energy in the hours after you finished training: because the lactate gets gradually transformed into pyruvate and sent through the aerobic metabolism of the mitochondria, which is like keeping on burning fuel in your body.

The lactate also has another important role in understanding fitness: it goes on a big journey through your body. The built-up lactate drains out of your muscle cells into your blood, and ends up reaching the liver. There it gets transformed back into pyruvate, and the pyruvate goes through a process called gluconeogenesis to get turned back into glucose. Amazing! The glucose then gets taken into your blood and ends up back in the muscle cells, where it can be used again to make more energy. This whole process is known as the Cori cycle.

You can also view the Cori cycle as a measure of fitness, since it indicates your body’s ability to process lactate. Exercise causes changes to the enzymes and transporters involved in the Cori cycle. If the cycle is functioning more efficiently, then you can have a higher output before you get to the point of your muscles being clogged up with lactate and tiring.

Studies have shown that a program of interval training lowers the levels of lactate present in athletes’ blood after training. So if you were running up the stairs for example, then your muscles don’t tire so quickly. It also means you can recover more quickly between actions. You could see this for example on a football pitch in someone’s ability to sprint and recover, and sprint again, and again. High intensity training has been shown to improve performance in doing repeated sprints, i.e. to recover from exercise, more significantly than continuous moderate exercise. But exercising at a low intensity does not cause such a big change in this cycle, because it does not involve creating such a build-up of lactate in the first place.

7. Burning fat

The third aspect to consider about high intensity exercise concerns the metabolism of fats and glycogen (glucose storage). When you do high intensity exercise, this higher level of muscular exertion triggers the release of some hormones, including adrenalin and glucagon. This triggers the glycogen stores in your muscles to get cleaved off into glucose, which get metabolized. The hormones trigger more of the storage to be released, so much more glucose gets broken down.

The hormones released from high intensity exertion also trigger the release of hormone-sensitive lipase, which causes fatty acids to be released from your fat stores. They get carried along to your muscle cells by a protein called albumin, where they get taken into the mitochondria and burned in the mitochondria to produce energy.

Activating hormone-sensitive lipase has a big effect on fat burning. There is an association that low intensity exercise burns fat, but actually most forms of exercise don’t burn that many calories (above the basic metabolic rate) during the actual exercise itself. Hormone-sensitive lipase has an amplification effect on mobilizing fat, which does not occur in low intensity, aerobic exercise, like jogging or walking. Studies have shown that high intensity training has a greater effect on oxidizing fat relative to carbohydrates, as compared to lower intensity, steady state training.

8. Resetting insulin sensitivity

Breaking down a huge amount glucose, though breaking down stored glycogen, also brings us to the fourth aspect of high intensity training. Studies have shown that this allows your cells to become more sensitive to insulin. Many people lose their insulin sensitivity because we’re walking around pretty much full of glucose, because many people eat a lot of carbohydrates (which are chains of glucose), exercise at a lower intensity or are more inactive.

Insulin is a hormone that works to allow your muscles to take up glucose from the blood in order to store it. If the cells are already full of glucose, because hardly any of it is being used up, then no more glucose will go in from the blood. The cells then become desensitized or resistance to glucose. This makes your body produce more insulin, which makes the cells get inflamed. The body reacts to this with LDL cholesterol, which puts you at greater risk for cardiovascular diseases. High levels of blood glucose levels are connected with being overweight and increase the risk of diabetes, and are a central feature of various forms of cardiovascular disease.

If you are doing high intensity resistance or interval exercise, where you are using a lot of glucose, and release tons of glucose from your glycogen stores, then there is such a big change in the glucose levels in your muscles that insulin is able to get active again during the recovery period after you’ve been exercising. It brings more glucose back into the muscle cells to replenish them. In other words, high intensity training increases the transport of glucose from your blood into your muscle cells. It therefore helps to re-sensitize your muscle cells to insulin, which is a great thing from your body’s perspective. For one thing, it lowers the risk of diabetes created by insulin resistance.

Reducing the level of muscle glucose has a second effect on fat storage as well. When there is too much glucose inside the cells and they have become insensitive to insulin, the glucose gets transformed (via pyruvate) into fatty acids, which are then taken to be stored as fats. With low intensity training, not only does it not significantly tap into fat stores (as above), but it also doesn’t significantly reduce the amount of glucose inside the cells, and this excess glucose is therefore converted into new fat stores. In addition, when there are high levels of insulin, this inhibits hormone-sensitive lipase and therefore inhibits the mobilization of fat stores.

In other words, for anyone doing exercise with the intention of managing their weight, it’s really vital to access the glucose stores, and high intensity training is an effective way to do that. Since low intensity training does not release such meaningful levels of glucose, it does not have such a significant effect on reducing insulin resistance.

9. High intensity training increases muscle mass

The fifth great thing about high intensity training is that it help you to increase muscle mass, which has several positive effects. Some studies suggest that this effect on muscle mass may even be at the core of why high intensity training has such a big positive effect on endurance performance. We could look at this from two angles.

The first angle is that training at a higher intensity taps into your fast twitch muscle cells. There are various types of muscle fibres, ranging from fast- to intermediate- to slow-twitch. The slow ones recover the most quickly; they are slow to fatigue, but produce less force than the fast twitch fibres. Your body recruits the slow twitch ones preferentially when you are moving, and is kind of reluctant to access the fast-twitch ones, and will only do that if there is a much more demanding situation. So in low intensity exercise, like going jogging or cycling at a rate that you can maintain for 30-60 minutes, you are only recruiting the slow twitch muscle fibres.

Your body will only call up the fast twitch fibres if a movement is beyond a certain threshold of intensity, for example by moving against a resistance/weight or by moving very quickly, and when it happens over a time period that is long enough to sequentially recruit the different muscle fibres without time for the slow twitch fibres to recover along the way. This could mean doing something intensively for between 30 seconds and 2 minutes, depending on the movement and the effect you want. The movement should be of a high enough intensity that continuing it for longer would be somewhere between difficult and impossible.

Specifically, low intensity movement is defined as that below the first ventilatory threshold, and high intensity movement is that above the second ventilatory threshold. You can think of it as low intensity training being when you can still breathe comfortably, like when you could maintain a conversation, and high intensity as when your breathing becomes labored and heavy, as if you can’t get enough air in and your heart is about to jump out of your chest. This kind of breathing is an indication that the fast twitch fibres have been activated.

High intensity training has been shown to increase the activation of motor units (which are groups of muscle fibres). A major effect of tapping into the fast-twitch muscles hinges on them having the most glycogen stores. Using those fibres means you use a radically higher amount of glucose from your stores, which therefore all of the positive effects we have already seen about using up glucose: restoring insulin sensitivity and triggering the release of fat stores. Low intensity exercise does not tap into the fast-twitch muscle fibres, and therefore does not empty the muscle cells of such meaningful levels of glucose. In fact, endurance exercise is known to trigger the transformation of fast twitch fibres into slow twitch, which is a shame because it’s an irreversible process.

The second angle on high intensity training’s effect on muscles is about building muscle mass, which is in general a great thing. Increasing muscle mass through resistance training has been linked to reducing various cardiovascular disease factors. Increasing muscle mass has also been shown to be an effective way of reducing weight in various studies.

Increased muscle mass also has the benefit that muscle is metabolically expensive. It costs your body a lot of energy to keep muscle fibres running, so that if you increase your muscle mass, then your daily calorie use increases even while you’re sitting on the sofa. Another benefit of having more muscle mass is that each movement you do puts less strain on your cardiovascular system, because each of your motor units (muscle fibre groups) has greater strength, so your cardiovascular system has to support less motor units to create the same strength output. It’s like taking care of your cardiovascular system. Muscle mass also increases through heavy resistance exercises, through stimulating the proteins responsible for muscle contraction and through causes muscle cells to increase in size.

Muscles are not the only tissue that adapt when you train intensely. Ligaments and tendons also adapt and get stronger, and high intensity strength training has also been shown to be an efficient way of improving bone density, which is a good way to reduce the risk of bone fractures or osteoporosis.

10. Aerobic exercise increases injury, disease and atrophy

Our sixth and final reason for doing high intensity exercise centers on the link between aerobic exercise and injury, disease and loss of muscle mass. When you do lower intensity exercise, or what people call cardio or aerobic exercise, it tends to be of a much longer duration. You might go on a 30-60 minute run multiple times per week, for example. The repetition that is involved over this longer time, greatly increases the chance of injury to your body in the long run, through all of the wear and tear. If you fast-forward to 15-20 years after doing regular jogging or running, then you can start to see a lot of knee pain, hip and lower back injuries, or bone spurs in the shoulder joints. The repetition takes its toll on muscles and joints.

For this reason, it is arguable whether going running regularly should be considered as exercise, when we look back at our first definition of exercise being something that creates a positive adaptation in not only fitness, but also health. Doing regular jogging or endurance training is overall likely to be detrimental to your health. Studies have shown that 60% of runners are injured in an average year. Scientific literature makes the case that long-distance runners are much more likely to develop cardiovascular disease, that a longer time of aerobic intensity decreases oxidation (which is a deterrent of cardiovascular disease), and that cardiovascular risk for ultra-endurance runners is especially great. Long-distance running has also been shown to increase the risk of cancer, as in one study where after a 30km race, runners showed increases in various markers of cell inflammation, including some that are linked with cancer (TNF-alpha). Marathon and ultra-marathon running has been shown to create various biochemical effects that contribute to liver and gallbladder disorders, kidney dysfunction and other tissue damage.

Although many people think it is a good idea for fitness, aerobic exercise actually causes the loss of skeletal muscles: your muscles get smaller, or undergo atrophy. If exercise is carried out for a long time, like in endurance events, then glycogen depletes and protein from the muscle tissue is used to maintain glucose homeostasis. Muscle tissue is broken down into amino acids and converted to glucose. Even without doing marathons, it is possible to do activity of an intensity that is of sufficient volume to cause large amounts of tissue destruction. This happens because the continued training is perceived by the body as a stress; there is a state of continual high demand on the body, without adequate time for recovering. In order to conserve energy, both muscles and fat are together broken down, since muscles are metabolically expensive.

A reduction in muscle mass has been shown to lead to a higher prevalence of obesity, insulin resistance, type 2 diabetes, dyslimipdemia and hypertension. This means that the body has adapted in order to use less energy in its resting state. In terms of survival, this is very clever. But when you have more muscles with a greater storage capacity, then your body is constantly using more energy or burning more calories. Another effect of the muscles becoming smaller is that they have a lower capacity to store glycogen, so they become full of glucose and glycogen much more easily, so they are much more likely to become desensitized to insulin and to have to shunt off excess glucose for fat storage. This is what happens through sustained low intensity exercise, which ends up in your muscles reducing in size.

Some last words

I hope this has explained some of the reasons why high intensity training is so useful. To summarize, although “aerobic” training targets the processes happening in mitochondria, we cannot separate the anaerobic and aerobic phases of metabolism, so it would be better to train in a way that targets both phases. We can see that when we do train both parts, there are a whole range of metabolic changes that happen, all of which have a huge impact on both your health and your fitness. And although “cardio” training has a good reputation behind it, it should be clear that the center of your metabolic health is your muscular system, not your cardiovascular system. If you want to make some kind of positive change in your body, your fitness and your health, then you need to think in terms of training your muscular system. This is the starting point, that creates all of the changes elsewhere.

So, I hope this article helps to shift your way of approaching exercise. Remember, high intensity training is very short! For some examples of how to train like this, you can explore some of my youtube links: