What is D' (D-prime)?

D' represents a finite anaerobic reserve capacity measured in meters. Whenever you run faster than Critical Speed, you draw down this battery until exhaustion occurs at D' = 0.

Updated: August 2026 • Verified v1.2

Running Critical Speed & Anaerobic Capacity (D') Calculator

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CS = (Distance_2 - Distance_1) / (Time_2 - Time_1); D' = Distance_1 - CS * Time_1

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Understanding Your Results

Displays Critical Speed in m/s, km/h, and min/km threshold pace, D' anaerobic reserve in meters, training intensity domains, and predicted race times.

Practical Guidance & Next Steps

Critical Speed represents the boundary between the heavy and severe exercise domains; running above CS depletes your finite D' battery.

What is this?

An exercise physiology calculator that applies the two-parameter hyperbolic model to determine a runner's aerobic threshold and anaerobic reserve capacity.

How does it work?

1. Enter distance and time for a shorter time trial (e.g. 1500m or 3km). 2. Enter distance and time for a longer time trial (e.g. 5km or 10km). 3. Review Critical Speed, D' reserve battery, training pace zones, and race predictions.

Why use it?

Provides a more objective, lab-grade threshold pace benchmark than standard arbitrary heart rate zone approximations.

Technical Scope & Calculation Limitations

Requires two maximal-effort time trials on flat terrain under similar environmental conditions.

Frequently Asked Questions

Critical Speed is the highest running speed that can theoretically be sustained without a continuous depletion of anaerobic energy reserves and progressive rise in blood lactate.

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