The Anaerobic System

Your body needs energy every time you move. Whether you’re standing up from a chair, lifting a heavy barbell, sprinting for a train or pushing through the final few seconds of a hard conditioning session, your muscles need a continuous supply of energy.

That energy ultimately comes from adenosine triphosphate, better known as ATP. Because the amount of ATP stored directly within your muscles is tiny, your body has several ways of continually replenishing it.

The anaerobic energy systems are particularly important when you need to produce a lot of force or power quickly. They help fuel heavy strength training, sprinting, jumping and other forms of high-intensity exercise where the demand for energy rises faster than your aerobic system can comfortably supply it.

Understanding this doesn’t mean you need a degree in exercise physiology. But knowing roughly where your energy comes from can help make sense of the way you train, why different exercises feel so different and why recovery between hard efforts matters.

Anaerobic Doesn’t Mean Exercise Without Oxygen

The word anaerobic literally means “without oxygen”, but that can give a slightly misleading impression of what happens during exercise.

Your aerobic and anaerobic energy systems aren’t separate engines that switch on and off depending on the exercise you’re doing. They’re working together all the time. What changes is their relative contribution.

During lower-intensity activity, your aerobic system can provide most of the ATP you need. As intensity increases and your muscles need energy much more rapidly, the anaerobic systems make a greater contribution.

That’s why walking feels very different from sprinting, despite both involving many of the same muscles.

The Three Energy Systems

There are three main pathways your body uses to replenish ATP:

the ATP-PC or phosphagen system
the anaerobic glycolytic system
the aerobic or oxidative system

The first two are generally described as the anaerobic systems.

None operates completely independently. Even during an all-out sprint, your aerobic system is contributing something. Likewise, during a long run, anaerobic metabolism still contributes when you accelerate, climb a hill or produce a finishing sprint.

The difference is which system is doing most of the work at a particular intensity.

The ATP-PC System

If you suddenly need to produce maximum force, you don’t have time to wait for slower energy-producing processes.

Your muscles contain a very small amount of stored ATP along with phosphocreatine, which can be used to regenerate ATP extremely quickly. This is known as the ATP-PC system.

Think about a heavy deadlift, an explosive jump or the first few seconds of an all-out sprint. These activities require a huge amount of energy almost immediately, making the ATP-PC system particularly important.

The downside is capacity. It can generate energy rapidly, but its readily available stores are limited. As an intense effort continues, other energy pathways must contribute more.

This also helps explain why adequate rest is important during heavy strength training. If the goal is to produce high levels of force on each set, rushing your recovery can mean starting again before you’re ready to reproduce the same quality of effort.

The Anaerobic Glycolytic System

As a hard effort continues, anaerobic glycolysis becomes increasingly important.

This system breaks down glucose, including glucose obtained from glycogen stored in your muscles, to regenerate ATP relatively quickly. It can’t match the immediate rate of the ATP-PC system, but it has greater capacity and can support intense activity for longer.

You experience this during activities such as repeated sprints, hard intervals, higher-repetition resistance training or a tough conditioning session.

As exercise intensity rises, so does the challenge of maintaining your output. Your breathing becomes harder, your muscles start to burn, and eventually you have to slow down or stop.

Lactate Isn’t The Enemy

This is one area where fitness advice has changed considerably.

For years, lactate was blamed for the burning sensation during intense exercise, muscular fatigue and even the soreness experienced the following day. You’ll still hear people talking about needing to “flush lactic acid” from their muscles.

That’s an oversimplification.

During rapid glycolysis, lactate production increases, but lactate itself isn’t simply a toxic waste product. It can be transported around the body and used as an energy source.

Fatigue during high-intensity exercise is much more complicated. Changes in muscle acidity, the availability of phosphocreatine, accumulation of other metabolites, changes in ion concentrations, nervous system fatigue and the sheer demand being placed on the working muscles can all contribute.

And the soreness you experience a day or two after a difficult workout isn’t trapped lactic acid either. That’s delayed onset muscle soreness, or DOMS, and is associated with the stress placed on muscle tissue during unfamiliar or demanding exercise.

Anaerobic Training Isn’t Just Sprinting

When you mention anaerobic exercise, most people immediately think of sprinting or HIIT.

They’re certainly examples, but anaerobic metabolism is also central to resistance training.

A challenging set of squats, deadlifts, presses, rows or loaded carries can place a substantial demand on your anaerobic energy systems.

Other examples include:

short, hard sprints
jumping and other explosive movements
heavy resistance training
repeated high-intensity intervals
hill sprints
rowing or cycling intervals
circuit-style conditioning

The exact contribution from each energy system depends on the intensity, duration, rest periods and your individual fitness.

Anaerobic Power and Anaerobic Capacity

Although the terms are sometimes used interchangeably, anaerobic power and anaerobic capacity aren’t quite the same thing.

Anaerobic power is essentially about how quickly you can produce energy and, as a result, how much power you can generate. Think about jumping as high as possible, accelerating rapidly or producing a very short maximum effort.

Anaerobic capacity relates more closely to how much high-intensity work you can sustain before fatigue causes your performance to fall significantly.

Both can be trained.

A rugby player might need explosive power to accelerate into a tackle but also enough anaerobic capacity to repeat hard efforts throughout the game. You might not intend to play rugby. Still, the same principles matter when you’re lifting weights, tackling a conditioning session or simply trying to become fitter and more capable.

Rest Is Part Of Anaerobic Training

More work isn’t automatically better work.

After a demanding effort, your body needs time to replenish phosphocreatine and restore its ability to produce another high-quality effort. How much rest you need depends on what you’re trying to achieve.

If you’re lifting heavily and trying to maximise strength, longer rest periods generally allow you to maintain force and technique between sets.

If you’re deliberately training your ability to tolerate repeated high-intensity efforts, rest periods may be shorter. That creates a very different training stimulus.

Neither approach is inherently superior. The important thing is that your work-to-rest ratio matches the purpose of the session.

This is one of the problems with assuming every workout needs to leave you exhausted. Sometimes the best thing you can do between sets is absolutely nothing.

Anaerobic Training and Strength

Strength training is one of the clearest examples of why anaerobic energy matters beyond competitive sport.

Lifting a challenging weight requires your muscles to generate force quickly. As sets become longer, the balance between the ATP-PC and glycolytic systems changes, but anaerobic metabolism continues to play an important part.

Over time, structured resistance training can help you build muscle, increase strength and improve your ability to produce force.

Those adaptations matter outside the gym too. Being stronger makes everyday physical tasks easier and helps you build a body that’s more capable of dealing with whatever life throws at it.

That’s why Foundry places such a strong emphasis on learning fundamental movements such as squatting, hinging, pushing and pulling rather than simply trying to make every session as tiring as possible.

Anaerobic and Aerobic Fitness Work Together

It’s tempting to put training into two boxes: aerobic or anaerobic.

Real life isn’t that tidy.

Imagine completing several hard intervals. Your anaerobic systems help you produce the high power required during each work period, but your aerobic system plays an important part in recovery between efforts.

A better-developed aerobic system can therefore help you recover more effectively and repeat high-intensity work.

The reverse is also useful. Anaerobic training can improve your ability to produce the short bursts of higher power that occur during otherwise predominantly aerobic activities.

You don’t need to choose one or the other. A good training programme develops the qualities you actually need.

Starting Anaerobic Training

If you’re relatively new to exercise, there’s no reason to start with maximal sprints, brutal circuits or highly technical lifts.

First, build the foundations.

Learn good movement patterns, become comfortable with resistance training and gradually increase the amount and intensity of work you can handle. Foundry’s approach to beginner fitness is based on developing those foundations rather than throwing people into workouts they’re not ready for.

As your strength, movement quality, and general fitness improve, higher-intensity training can be introduced and progressed appropriately.

You don’t need to train like an elite athlete to benefit from your anaerobic energy systems.

Recovery is Important

Hard training creates the stimulus. Recovery allows your body to adapt to it.

That means looking beyond the workout itself.

Sleep, nutrition, hydration and sensible training frequency all influence your ability to recover and perform again. Consistently cutting your sleep short while increasing training intensity isn’t a particularly clever combination, which is why getting enough quality sleep should be part of your training, not something separate from it.

Nutrition doesn’t need to become unnecessarily complicated either. Eating enough protein, including plenty of vegetables, limiting highly processed foods and matching your overall energy intake to your goals provides a strong starting point.

Pain Doesn’t Mean More Progress

One of the biggest mistakes with anaerobic training is assuming that the harder a workout feels, the more effective it must be.

It doesn’t work like that.

There are times when training should be hard. You may breathe heavily, your muscles may burn, and you may have to work through considerable discomfort. But exhaustion isn’t the goal of every session.

If fatigue causes your movement quality to collapse, drastically reduces your output or leaves you unable to recover for subsequent sessions, piling on more work may achieve very little.

Good programming manages intensity rather than worshipping it.

Getting More From Your Anaerobic System

Improving anaerobic fitness doesn’t require one magical exercise or training method. It requires progressively exposing your body to the kind of demands you want it to handle.

That could mean getting stronger through progressive resistance training, becoming more powerful through explosive movements or improving your ability to repeat high-intensity efforts through appropriately programmed intervals.

The key word is appropriately.

At Foundry, training isn’t about throwing random exercises together until you’re exhausted. We want you to move well, become stronger, improve your fitness and gradually build the capacity to handle more.

Our personal training sessions are delivered in small groups of up to six, with the training adapted to your ability, experience and goals. You still do the work, but you have a coach making sure that work is actually taking you somewhere.

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