Energy systems
Your muscles all run on the same fuel, but your body makes it in three ways: lightning fast and very briefly, quickly and in limited supply, or slowly and virtually without limit. Which route dominates depends on how hard and how long you go — and that determines what a session actually trains.
The three systems
The first system delivers immediate power from a small store of energy-rich substance inside the muscle itself. It gives everything you have, but is empty after roughly ten seconds: this is the sprint, the jump, the explosive attack on a hill. The second system burns carbohydrate without oxygen and takes over from there. It delivers a lot of power for about half a minute to a few minutes, producing the by-products you know as burning, acidic legs.
The third is the aerobic system: with oxygen you extract energy from carbohydrate and fat. It supplies less power per second but keeps going for hours, which makes it the engine under almost all endurance sport. Important: the three never stand apart. They always run at the same time, and the aerobic system also does the recovery work between intervals — which is why a strong base even improves your repeat sprints.
How do you spread your training?
In practice people often work with three training areas: easy endurance work that serves your aerobic base and recovery, tempo and threshold work that stretches your durability around threshold, and short hard stimuli above threshold that address your maximal oxygen uptake and anaerobic capacity. Each area has its own price in recovery: the third costs by far the most.
The common lesson from research and practice is that the bulk of your hours should be genuinely easy, with a smaller, deliberately dosed share of hard work. The classic mistake is the reverse: everything at medium pace, so your endurance base gets too little stimulus and your hard sessions are never hard enough. Which mix suits you depends on your goal, your experience and the time you have — a sprinter and a marathon runner do not need the same split.
How Trimetra personalises this
Trimetra sorts both your completed and your planned load into three energy-system groups: low aerobic, high aerobic and anaerobic. For a finished session it does that on your actual heart-rate zones wherever possible; if heart-rate coverage is too thin, the app falls back on the intensity and the session type. Per experience level and training phase there is a range for each group, and when a group drops below its lower bound the planner picks the next quality stimulus there — rotating within the same physiological need, so you do not get exactly the same session back every time. On the card of a finished session you can also see how strongly that training drew on your anaerobic reserve.
Every term at a glance in the full glossary.