Metabolic training: when and why should you add it?

When we talk about training, we usually think of muscle strength, building muscle mass and the work done during a session. We talk less often about the conditions the body has to provide so that we can do that work again and again.

Yet it isn’t always muscle strength that decides how much work you can do. Sometimes a muscle group struggles to recover between sets. At other times your muscles would be ready for the next exercise, but your general work capacity, your breathing and your cardiovascular system limit how far you can go. Different factors may lie behind these two situations, so they don’t necessarily call for the same method either.

This is where metabolic training comes in. Many people associate the term with sweaty, short-rest workouts or with fat burning. But metabolic training isn’t simply working with short rests and a high heart rate. The goal is to improve the energy-supply processes and the work capacity of the muscles – and of the whole body.

Kassem Hanson, founder and lead educator of N1 Education, teaches professionals biomechanics, individualised programming and nutrition. A key element of his approach to programming is that long-term progress doesn’t necessarily come from trying to develop the same quality all the time. It’s worth looking at what is currently limiting your performance, and which adaptation could help you get more out of yourself in the next training block.

This idea is especially interesting when it comes to metabolic training. If building muscle is your goal, it is of course important to give your muscles enough mechanical tension and do enough training volume. But what happens when it isn’t the muscle’s strength, but its energy supply or your whole body’s work capacity that limits the work you can do? Then you don’t necessarily need more sets or heavier weights. You may first need to develop the qualities that let you do more training work.

To understand how to do that, we first need to look at how muscles get their energy and how the body responds to training stress.

How do muscles get their energy?

Muscles need energy to contract. That energy is provided directly by a molecule called ATP. Cells only hold a small amount of ATP, however, so the body has to keep regenerating it. To do this it uses, among other things, glucose from the blood and glycogen stored in the muscles.

Glycogen is the stored form of carbohydrate. When you train, muscle cells draw energy from it, while mitochondria play an important part in energy production. They are the cells’ power plants, responsible for a large share of ATP regeneration.

So during training it’s not only the resistance you work against that matters, but also how efficiently your muscles can supply energy for repeated efforts. It follows that metabolic training can have two different directions: one targets the energy supply of a specific muscle group, the other the work capacity of the whole body.

In local metabolic training you load a given muscle group while deliberately not allowing full recovery between sets. Because of the shorter rests the muscle has to work again and again, and its energy-supply processes are heavily taxed as well.

One example is the Incomplete Rest Method. You do several sets with the same weight and short rests. The load is chosen so that the first sets still feel fairly easy and fatigue builds up gradually. The point is not to reach failure in the very first set, but for the difficulty to increase as the sets go on, with failure coming at most in the last one or two sets.

One possible example is an 8×8 protocol with 30-second rests. You pick a weight you could do roughly 12–15 reps with when fresh. The first sets are not maximal efforts, but because of the short rests the load becomes more and more challenging. This is one possible method, not a general prescription: the number of sets, the rest time and the weight have to be matched to your training level.

A muscle’s adaptation isn’t limited to producing force against greater resistance. With repeated loading, the processes involved in energy supply adapt too. Glucose uptake, the ability to refill glycogen stores and the systems involved in energy production can all improve. These adaptations can help the muscle tolerate repeated loading more efficiently.

Adequate glycogen availability is particularly important for sustaining performance during training. A 2025 systematic review and meta-analysis pooled the results of 20 studies and found that resistance training measurably reduces glycogen content in the trained muscles. The size of the drop was influenced by, among other things, the number of sets performed and the duration of the session [1].

Systemic metabolic training, by contrast, doesn’t focus on the energy supply of a single muscle group but on the work capacity of the whole body. Several muscle groups work one after another, so the cardiovascular system has to keep supplying the working muscles with oxygen and nutrients, while the body also has to deal with the metabolic by-products of intense work.

Lactate, for example, can be produced in larger amounts under these conditions. It’s important to be clear that lactate is not a waste product, and not simply a sign of oxygen shortage. It is a natural part of high energy turnover, and the body can use it in other tissues or convert it further.

That’s why, under systemic load, the limiting factor is often not the local fatigue of one muscle group but general fatigue and an increased demand for breathing. The goal is for the body to serve the greater, multi-muscle-group workload more efficiently.

What does this mean in practice?

The two approaches target different qualities, but both can improve the body’s adaptation to repeated physical stress. Local metabolic training can develop the energy supply and local work capacity of a given muscle group, while the systemic version puts the whole body’s capacity for work first.

Its relevance goes beyond the gym. Better local work capacity can help a muscle group tolerate more training volume. Better systemic conditioning can help the whole body cope more easily with longer or more intense physical activity. In everyday life this might mean that a long walk, climbing stairs or several physical tasks in a row feel relatively less demanding.

Improved tolerance of physical stress matters too. The body gradually adapts to regular, well-dosed loading, so a physical task that used to be demanding can become less of a physiological challenge over time. Kassem Hanson also discusses this in relation to systemic conditioning and the body’s stress response. When interpreting hormonal and nervous-system changes, however, caution is needed: improved tolerance of physical stress doesn’t automatically mean greater psychological stress tolerance.

In programming, all this can be especially useful because better energy supply and greater general work capacity can create better conditions for the next hypertrophy block. If local energy supply or your overall work capacity is limiting your performance, you may not be able to make full use of what a muscle-building programme has to offer.

In that case an earlier metabolic phase can target these limiting factors. The local version can help a muscle group tolerate repeated loading better, and the systemic version can help the whole body cope with higher training density involving several muscle groups. This can set up better conditions for doing the planned training work in the next hypertrophy phase.

This is the essence of building programmes on one another: you don’t have to develop the same quality in every period. In one phase the mechanical tension and training volume needed for muscle growth may take priority; in another you can develop the factors that support performance in the next muscle-building phase.

It’s important to note, though, that metabolic training is not a special method for speeding up muscle growth. A stronger pump, lactate accumulation or temporary swelling of muscle cells do not in themselves prove that you will build more muscle. For muscle building, adequate mechanical loading, a well-structured programme, recovery and proper nutrition remain what matters most.

So metabolic training isn’t necessary for everyone, and it doesn’t have to be part of every programme. It is most useful when you know exactly what is limiting your performance and want to target it. If the energy supply of a particular muscle group is the limit, the local version may be warranted. If your whole body’s work capacity is what caps your training, the systemic approach may be the better fit.

The point isn’t to get as tired as possible in every session, or to keep adding new methods to your programme. It’s to recognise what your body needs in order to keep progressing. Sometimes the next step isn’t even more muscle-building work, but developing the physiological processes that let you do more – and better-quality – work later on. That is the real role of metabolic training in long-term progress.