The three energy systems and when each one takes over
Muscle contraction runs on adenosine triphosphate, and because muscle stores only a tiny amount of it, three linked pathways rebuild it while you play. The phosphagen system breaks down phosphocreatine already stored in the muscle, and it delivers energy fastest but holds the least. The glycolytic system breaks down muscle glycogen without oxygen, and it also produces energy at a high rate but is limited by fuel depletion and the rising acidity of hard work. The oxidative system burns carbohydrate and fat with oxygen, and its capacity is enormous but its rate is capped by how fast your heart, lungs and blood can deliver oxygen to working muscle. None of them switches off. All three contribute to every effort, and only the balance shifts with how hard you are working and for how long.
| All-out effort lasting | Anaerobic share | Aerobic share |
|---|---|---|
| 5 seconds | 95% | 5% |
| 10 seconds | 91% | 9% |
| 30 seconds | 75% | 25% |
| 60 seconds | 58% | 42% |
| Around 80 seconds | Even | Even |
| 2 minutes | 38% | 62% |
| 5 minutes | 19% | 81% |
Those splits come from a pooled analysis of just over a hundred measurement studies of adults performing single maximal efforts, and they are modelled averages across many protocols, so treat them as a map rather than a lab report on you. The shape is what matters for rugby: nothing you do on a pitch runs on one system, the anaerobic pathways dominate anything under about seventy-five seconds, and once an effort stretches past that, oxygen is paying most of the bill.
How a rugby match spends the systems
Rugby is an intermittent high-intensity sport: maximal strength and power efforts, static work, collisions and high-speed running, spread between low-intensity movement and rest. A sprint to the breakdown, a tackle or a lineout jump is over in a few seconds, which puts it at the phosphagen-heavy end of the table above. A defensive set or a phase sequence that keeps you working towards the minute mark pulls the glycolytic system into the lead, and anything longer hands over to oxygen. And every walk back to the mark is the oxidative system doing repayment work, because rebuilding what a sprint spends takes oxygen and time.
Mostly anaerobic
One explosive effort
In pooled adult measurements a ten-second maximal effort drew about nine tenths of its energy from the anaerobic systems, with the phosphagen system leading.
Around 80 seconds
Where the balance tips
At about eighty seconds of maximal work the aerobic and anaerobic systems supply equal shares, and everything longer is mostly aerobic.
Phosphocreatine
The sprint tank
One thirty-second all-out sprint dropped muscle phosphocreatine to about a fifth of its resting level in the eight men who gave muscle samples.
Oxygen
The refill
Oxygen taken up early in recovery rebuilds spent phosphocreatine, but ninety seconds after that sprint the store was only about two thirds restored.
Why the fourth sprint feels nothing like the first
Fourteen men did thirty-second all-out cycle sprints, and muscle samples taken through recovery in eight of them explain the feeling. Phosphocreatine finished the sprint at about a fifth of its resting level, climbed back to about two thirds after ninety seconds, and was still only around 85 per cent restored after six full minutes. The refill's half-time was just under a minute. Sprint power came back in step with the phosphocreatine, not with the muscle's acidity, and the two tracked each other closely through recovery. So when the gaps between efforts are short, you start each one part-empty, and no amount of willpower changes the chemistry.
What your position asks for
Nobody has measured energy-system contributions position by position during real matches, so any table claiming your exact anaerobic and aerobic split by shirt number is made up. What has been measured is what each position does: across fifty-one GPS studies of rugby union, backs cover more total, relative and high-speed distance while forwards bank more collisions, impacts and static work. Map that onto the duration table and you get something honest enough to train from. Everyone needs the phosphagen system for the explosive seconds and the oxidative system for the recovery between them, a forward's match is built from short heavy efforts with little running between them, so the refill between efforts is the part most worth training, and an outside back lives on fewer but faster sprints with more running in between, so top-end speed and the engine that restores it matter most out wide.
How to train each system
Three levers set which system a session trains: how long each effort lasts, how hard it is, and how long you rest. The work durations follow from the measured contribution splits, and the rests follow from the refill chemistry.
| Target | Work | Rest | Why it works |
|---|---|---|---|
| Phosphagen power | Maximal efforts of a few seconds | Several minutes, close to full recovery | Phosphocreatine was only about two thirds back ninety seconds after an all-out sprint, so short rests mean you stop training top-end output |
| Repeat-effort capacity | Short maximal efforts | Deliberately short, well under a minute | Each effort starts part-empty on purpose, which is the state a match puts you in around the breakdown |
| Glycolytic tolerance | Hard efforts of thirty to seventy-five seconds | A few minutes between efforts | Maximal work in this range still draws most of its energy anaerobically, and this is where the glycolytic system carries its biggest load |
| Aerobic base | Steady and tempo running in longer blocks | Short or none | Everything past about eighty seconds is mostly aerobic, and oxygen does the phosphocreatine refill between the efforts of every other session too |