Critical speed is the dividing line between paces your body can hold in a steady state and paces where fatigue keeps building until you have to stop. Run 3% below it and you can keep going for a long time. Run 3% above it and the clock is ticking. Running right at critical speed, most people reach exhaustion somewhere around 20 to 40 minutes, so for trained runners it usually sits a little slower than 5K pace and a little faster than 10K pace (Jones and Vanhatalo, 2017).
That makes it a useful anchor for workouts, and unlike a lab lactate test, you can estimate it from two race times.
How to estimate your critical speed
You need two all-out efforts of different lengths, ideally between about 3 and 20 minutes. Recent race results work, or a 1.6 km and a 5 km time trial on separate days.
Then divide the difference in distance by the difference in time:
Critical speed = (longer distance – shorter distance) / (longer time – shorter time)
Example: 1.6 km in 6:00 (360 seconds) and 5 km in 21:00 (1,260 seconds). That is 3,400 m in 900 seconds, or 3.78 m per second. Convert to pace and you get about 4:25 per km.
The two efforts need to be genuinely hard, on similar ground, in similar weather. A 5 km run at 7am in KL humidity and a 1.6 km at 6pm after rain will give you a muddled number.
Train around critical speed, not exactly at it
The coaching approach set out by John Davis at Running Writings uses two paces either side of critical speed rather than CS itself:
- CS minus (about 97% of CS): slightly slower. You stay in a stable state, so you can accumulate a lot of quality work.
- CS plus (about 103% of CS): slightly faster. Harder, shorter reps that push the top end.
Example workouts
At CS minus (roughly 95% of 5K pace)
- 8 x 3 min, 90 sec jog
- 5 x 1 km, 2 min jog
- 3 x 2 km, 3 min jog
- 5 km continuous
At CS plus (roughly 100 to 102% of 5K pace)
- 8 x 600 m, 2 min jog
- 10 x 2 min, 90 sec jog
Tempo and cruise intervals
- 6 x 4 min at about 92% of 5K pace, 1 min jog
- 8 km continuous at about 90% of 5K pace
Where this fits in a week
Critical speed sessions are the sharp end of training, not the whole of it. Most of your running should still be easy, with long easy runs building the aerobic base these sessions sit on. One or two CS-based sessions a week is plenty for most runners training for a half or full marathon.
In Malaysian heat, your critical speed on a hot afternoon is slower than on a cool morning. Run these sessions early, or run them by effort and accept slower splits. For how to set up the reps and recoveries themselves, see The Interval Training Mistakes Most Runners Make. For why the burn in these sessions isn’t what you think, read Is Lactic Acid Why Your Legs Burn?
References
- Jones AM, Vanhatalo A (2017). The ‘critical power’ concept: applications to sports performance with a focus on intermittent high-intensity exercise. Sports Medicine, 47(Suppl 1), 65-78.
- Davis J (2024). Critical speed guide for runners. Running Writings. runningwritings.com
