Energy zones
Swim Training Zones Explained: AEC, AEP, ANC and ANP
7 Min Read
Every set in a swim session trains something. Jan Olbrecht's energy-zone approach groups that work into four zones, so coaches can see exactly what a session, a week or a season is building. SwimWrite tags every set with its zone so you can see the balance at a glance.
AEC – Aerobic capacity
Low intensity with short rest. AEC work builds the aerobic base: capillaries, mitochondria and the ability to use fat as fuel. It should be long, steady and relaxed.
AEC Example
Easy to steady, conversational effort
AEP – Aerobic power
Threshold through VO₂max, with moderate rest. AEP sets raise the ceiling of aerobic output: strong, sustained and controlled. CSS pace is a practical target for threshold sets.
AEP Example
At or slightly faster than CSS pace
ANC – Anaerobic capacity
High intensity with incomplete rest. ANC work is lactate tolerance: holding race-specific speed while blood lactate is high. Hard, but repeatable.
ANC Example
At 200 race pace
ANP – Anaerobic power
Maximal efforts with full recovery. ANP is pure speed and power: short, all-out swims with enough rest that every one is fast. Use the HVO speeds tool to set split targets.
ANP Example
All-out, full recovery
Balancing the zones
- Most weekly volume usually sits in AEC.
- AEP work appears once or twice a week in most programmes.
- ANC and ANP are small doses, with more of them closer to competition.
- Heart-rate zones help: around Z2 for AEC, Z3 to Z4 for AEP, and Z4 to Z5 for ANC and ANP.
These are guides, not fixed rules: swimmers respond differently, and the right balance depends on the event, the phase of the season and the athlete.
Frequently Asked Questions
What do AEC, AEP, ANC and ANP stand for?
Aerobic capacity, aerobic power, anaerobic capacity and anaerobic power: the four energy-system zones in Jan Olbrecht’s training framework.
Which zone should most training be in?
For most swimmers the bulk of volume is aerobic capacity work, with smaller, targeted doses of the other zones.
How do I find my threshold pace?
A critical swim speed (CSS) test, using a 400 m and a 200 m time trial, gives a practical threshold pace per 100 m. The free SwimWrite CSS calculator works it out for you.