Your energy systems: what does it actually mean for your training?
You don't need a degree in exercise physiology to understand energy systems.
But understanding the basics can help you make better decisions about how hard to train, how long to recover between efforts, what to eat before and after training, and how to structure your week.
How your body creates and uses energy
ATP-PC system – stored energy for explosive effortsYour muscles keep small amounts of ATP and phosphocreatine (PC) stored and ready to use. This system can regenerate ATP very quickly, giving you energy for short, powerful efforts such as a heavy lift, jump or sprint. The downside is that these stores are limited, so you need recovery time before you can produce the same level of power again.
Anaerobic glycolysis – quickly turning carbohydrate into energyWhen you need energy faster than your aerobic system can provide it, your body can break down stored carbohydrate (glycogen) to rapidly make more ATP without relying directly on oxygen. This helps fuel harder efforts lasting from several seconds into a few minutes, such as hard intervals, hills or repeated efforts. It produces energy quickly, but you can't sustain this level of effort for long.
Aerobic system – producing energy for longerYour aerobic system uses oxygen to break down carbohydrate and fat and produce ATP. This happens mainly inside your muscle cells' mitochondria. It produces energy more slowly than the other systems, but it has a much greater capacity, making it the main source of energy for longer, steadier activities such as running, cycling or walking.
The important bit: these systems don't work one at a time. They are all contributing to energy production, but the balance between them changes depending on how hard and how long you're exercising.
So what does that actually mean when you walk into the gym or head out for a run?
Strength and power: quality matters
Think heavy squats, deadlifts, plyometrics and short sprints.
These activities require you to produce force quickly, so you need to be relatively fresh to get the most from them. The ATP-PC system makes a particularly important contribution during very short, high-intensity efforts.
Rest between sets:
For heavy strength work, around 2–4 minutes is a useful starting point.
Power work also needs enough recovery to maintain speed and quality.
Don't automatically start your next set because the clock says you've rested for two minutes.
Ask yourself:
Can I produce the same quality of movement?
Has my breathing settled?
Do I feel ready to produce force rather than just get the set done?
Can I maintain good technique and speed?
If not, take a little longer.
For power work, this is particularly important. If your box jump has become slow and you're landing heavily because you're exhausted, you're no longer getting the same power stimulus.
For runners, cyclists or endurance athletes, power is worth training — but it also needs to be respected.
What should you eat?
For a short strength or power session, you don't necessarily need a special pre-workout meal if you've eaten normally during the day.
But if you're training first thing, haven't eaten for several hours or you're doing a long or demanding session, some carbohydrate beforehand can be useful.
Think simple and familiar:
Banana + yoghurt
Toast + honey
Porridge + fruit
Cereal + milk
Bagel + a little jam
Afterwards, aim for a normal meal containing protein, carbohydrate and fluids. Protein supports muscle repair and adaptation, while carbohydrate helps replenish glycogen, particularly when you have another demanding session coming up.
Zone 2: the work you should be able to recover from
Easy running, cycling, rowing and other steady-state work rely predominantly on the aerobic system.
This is generally the intensity where you can maintain a conversation and control your breathing.
Because the intensity is lower, you don't need recovery periods during a continuous Zone 2 session. But don't make the mistake of thinking: “It's Zone 2, so it doesn't matter.”
A two-hour easy run can still create a significant training load, particularly for your feet, calves, tendons and joints.
For shorter sessions, normal meals and hydration will often be sufficient. As sessions get longer, carbohydrate and fluid intake become increasingly important.
A useful practical guide is:
Up to ~60 minutes: normal meals and hydration may be enough.
60–90 minutes: consider carbohydrate before and/or during the session depending on intensity and your overall training.
90+ minutes: have a deliberate fuelling and hydration strategy rather than relying entirely on what you ate several hours earlier.
Zone 3–4: your quality sessions
Tempo running, threshold work, hard hills and intervals are a different proposition.
As intensity increases, your body needs to produce energy more quickly. Carbohydrate becomes increasingly important and anaerobic pathways contribute more alongside the aerobic system.
These sessions are supposed to be hard. They're also supposed to be high quality.
How long should you recover between intervals?
It depends on the session and what you're trying to achieve.
For example:
6 × 3 minutes around threshold:Perhaps 1–2 minutes of easy running recovery.
6 × 800m at a hard 5K-type effort:Perhaps around 2 minutes easy recovery.
Short, very hard intervals or sprints:You may need 3–5 minutes or more if the objective is to reproduce high-quality efforts.
The important question isn't: “Have I rested for exactly two minutes?”
It's: “Am I sufficiently recovered to perform the next repetition at the intended intensity?”
Research on interval training supports the idea that recovery duration changes what you can produce in the next repetition; longer recovery can allow higher running speeds, while shorter recovery can increase fatigue and alter the nature of the stimulus.
Use heart rate as another recovery guide
This is something I find particularly useful during interval sessions.
I don't necessarily want my heart rate to return to resting levels before starting the next interval. That's unnecessary for most sessions.
Instead, I look at whether it has dropped sufficiently for me to be able to produce the next effort properly.
For example:
Hard interval → HR rises
Easy recovery → HR falls
Next interval → HR rises again
If your HR is still very high, your breathing is ragged and your legs feel heavy, you may need a little longer.
But don't become obsessed with hitting an arbitrary HR number. Heart rate is affected by temperature, hydration, caffeine, stress, sleep and fatigue, and it doesn't tell you everything about muscular or neuromuscular recovery. Research has also found limitations in using HR alone to manage recovery during interval training.
I use HR + pace + perceived effort.
Can I hit the intended pace?
Does the effort feel appropriate?
Has my HR recovered reasonably?
If all three are telling you you're ready, go.
What should you eat before a quality session?
This is where I wouldn't deliberately turn up under-fuelled.
If you've got a hard session planned, have some carbohydrate available.
2–3 hours before:A normal carbohydrate-based meal such as porridge and fruit, toast and eggs, rice with chicken, or a bagel with yoghurt and fruit.
30–60 minutes before:Something smaller and easy to digest — perhaps a banana, toast with honey or jam, or a small cereal bar.
The closer you get to training, the smaller and simpler the meal generally needs to be.
And practise what works for you. Race day is not the time to discover that your pre-run breakfast doesn't agree with you.
Recovery afterwards
After a hard session, think:
Refuel + repair.
A meal or snack containing carbohydrate and protein is a good starting point, particularly if you have another demanding session within the next 24 hours.
That could be:
Greek yoghurt, fruit and granola
Eggs on toast plus fruit
Chicken, rice and vegetables
Tuna pasta
Milk-based smoothie with fruit and yoghurt
You don't need a complicated recovery shake if you can eat a proper meal.
The bigger picture is making sure you're eating enough across the day to support the training you're doing.
Think about the cost of the session
This is probably the most useful takeaway.
20 minutes of plyometricsShort session, but high force and power demand → needs quality and recovery.
60 minutes of Zone 2Lower intensity → generally more manageable, although the physical load still matters.
45 minutes of intervalsHigh cardiovascular and metabolic demand → requires preparation and recovery.
2–3 hours of easy enduranceLower intensity but potentially significant total training load → fuelling, hydration and recovery become increasingly important.
So when planning your week, don't simply count how many hours you've trained.
Think about what each session is asking from you.
And this matters even more as a masters athlete
Getting older doesn't mean you can't train hard. It does mean that how you organise the hard training matters.
You want enough stimulus to improve strength, power, aerobic capacity and performance — but enough recovery for your body to actually adapt.
That means looking at the whole week rather than treating every session independently.
Hard sessions need preparation. Easy sessions need to stay easy. Long sessions need fuelling and recovery. Power sessions need freshness.
Your watch gives you useful data. Your pace gives you feedback. Your body gives you context. Use all three.
The goal isn't to maximise fatigue. It's to get the right training stimulus, recover from it and be ready to do it again.
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