Critical speed (CS / CSS)
Critical speed (CS) is the running speed on the boundary between hard and very hard effort, estimated from your performances over several distances or durations. The swimming variant is called Critical Swim Speed (CSS); in cycling the same quantity, expressed in power, is called Critical Power (CP).
What is critical speed?
The model behind it is elegant: plot your best performances over different durations against each other, and together they point to a limit value — the pace your duration curve creeps towards as the duration grows longer. Below that limit the effort is stable and long to sustain; above it you draw on a finite reservoir (called D′ in running, W′ in cycling) and exhaustion is only a matter of time.
So critical speed isn’t a pace you literally hold forever — it is a model boundary that serves as an extra anchor alongside threshold pace or FTP, and it predicts well how long you can sustain efforts above threshold.
How do you find CS and CSS?
For both the recipe is the same: several maximal efforts over clearly different durations or distances — for example a test of a few minutes and one of just over ten minutes — from which the model estimates the boundary and the reservoir. Recent race results over different distances work too.
For swimmers the classic CSS test is a maximal 400 metres and 200 metres in the pool: the time difference over the extra 200 metres sets the estimate. Pool sessions are measured accurately for this; GPS distances in open water are too noisy from current and measurement gaps underwater, and so don’t belong in a CSS calibration.
How Trimetra personalises this
Trimetra continually estimates your CS, CP and CSS from your existing training and races — a separate anchor per sport — and keeps the duration curves current without a dedicated test being required. If your level shifts consistently, a suggestion follows to update the anchor; swim data only counts in the calibration when the distance is measured accurately.
Frequently asked questions
What is the difference between critical speed and threshold pace?
Both mark the boundary between hard and very hard, but they come about differently. Threshold pace is estimated from a single effort or race; critical speed follows from a model over your performances across multiple distances or durations. They serve side by side as anchors for your training.
Can you sustain your critical speed indefinitely?
No. Critical speed is a model boundary, not a pace you literally hold forever. Above it you drain a finite reservoir (D′ for running, W′ for cycling) and exhaustion is a matter of time; below it the effort is sustainable for a long while.
How do you determine your critical speed?
From several maximal efforts over clearly different durations — for example one test of a few minutes and one of well over ten minutes. Recent race results over different distances work too; the model estimates the boundary and your reservoir from them.
Every term at a glance in the full glossary.