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Speedendurance Index Calculator

Our performance calculator computes speed–endurance index instantly. Get accurate stats with historical comparisons and benchmarks.

Reviewed by Sher, Sports Science & Nutrition Specialist

Reviewed by Sher, Sports Science & Nutrition Specialist

Formula

Speed-Endurance Index = (Average Speed / Max Speed) x 100

The Speed-Endurance Index expresses average speed as a percentage of maximum speed, providing a direct measure of speed sustainability. Speed Maintenance Ratio compares second-half speed to first-half speed. Deceleration Index measures the percentage speed drop between halves. These metrics together provide a comprehensive picture of pacing effectiveness and endurance capacity at high speeds.

Worked Examples

Example 1: 400m Sprint Analysis

Problem:A sprinter with max speed 10.5 m/s completes 400m in 50.2 seconds. First 200m in 23.1s, second 200m in 27.1s. Average speed is 7.97 m/s. Calculate their speed-endurance profile.

Solution:Speed-Endurance Index = (7.97 / 10.5) x 100 = 75.9%\nFirst half speed = 200 / 23.1 = 8.66 m/s\nSecond half speed = 200 / 27.1 = 7.38 m/s\nSpeed Maintenance Ratio = (7.38 / 8.66) x 100 = 85.2%\nDeceleration Index = (8.66 - 7.38) / 8.66 x 100 = 14.8%\nSplit differential = 27.1 - 23.1 = 4.0 seconds (17.3%)\nEven split time = 50.2 / 2 = 25.1s per 200m

Result:SEI: 75.9% (Average) | Speed Maintenance: 85.2% | Split Diff: 4.0s | Decel: 14.8%

Example 2: 800m Runner Pacing Analysis

Problem:An 800m runner with max speed 8.5 m/s finishes in 118 seconds. First 400m in 56s, second 400m in 62s. Average speed is 6.78 m/s.

Solution:Speed-Endurance Index = (6.78 / 8.5) x 100 = 79.8%\nFirst half speed = 400 / 56 = 7.14 m/s\nSecond half speed = 400 / 62 = 6.45 m/s\nSpeed Maintenance Ratio = (6.45 / 7.14) x 100 = 90.3%\nDeceleration Index = (7.14 - 6.45) / 7.14 x 100 = 9.7%\nSplit differential = 62 - 56 = 6.0 seconds (10.7%)\nEven split = 118 / 2 = 59s per 400m

Result:SEI: 79.8% (Good) | Speed Maintenance: 90.3% | Split Diff: 6.0s | Decel: 9.7%

Frequently Asked Questions

What is the Speed-Endurance Index and what does it measure?

The Speed-Endurance Index (SEI) quantifies an athlete ability to maintain their maximum speed over a given distance or duration, expressed as a percentage. It is calculated as average speed divided by maximum speed, multiplied by 100. An SEI of 85% means the athlete maintained 85% of their peak speed throughout the effort. This metric bridges the gap between pure sprint testing and endurance testing by specifically measuring the capacity to resist speed decline during sustained high-intensity effort. The SEI is particularly valuable in sports like 200m and 400m sprinting, 100m and 200m swimming, and track cycling where athletes must maintain near-maximal speeds for durations that exceed the capacity of the phosphocreatine energy system. Higher SEI values indicate superior anaerobic endurance and lactate tolerance.

How does the Speed-Endurance Index differ from the Fatigue Index?

While both metrics assess an athlete ability to resist performance decline, they differ in measurement approach and application context. The Fatigue Index (typically from Wingate testing) uses power output as the primary variable and is measured during a standardized all-out test protocol. The Speed-Endurance Index uses speed as the primary variable and can be applied to any distance or duration. The SEI compares average to maximum speed, while the Fatigue Index compares peak to minimum power. This means the SEI captures the overall sustainability of performance, while the Fatigue Index captures the worst-case decline. An athlete could have a moderate Fatigue Index but a high SEI if their peak power is very high but their minimum power drops late. Practically, the SEI is more relevant for predicting race performance while the Fatigue Index is more relevant for assessing physiological capacity.

What training methods improve the speed-endurance index?

Improving the SEI requires targeting the physiological systems that limit speed maintenance during sustained high-intensity effort. Speed-endurance training involves repetitions at 90-100% of maximum speed over distances that induce significant fatigue (typically 150-600m for sprinters), with long recovery periods of 8-15 minutes between reps. Special endurance training uses longer distances at 85-95% intensity to develop lactate tolerance and clearance capacity. Tempo runs at 75-85% speed with moderate recovery develop the aerobic support for anaerobic recovery. Glycolytic interval training with work intervals of 30-60 seconds at near-maximal intensity targets the same metabolic pathways used during sustained sprinting. Additionally, resistance training focusing on muscular endurance (higher reps at moderate loads) and plyometric training improve the fatigue resistance of fast-twitch muscle fibers. A periodized approach that addresses all these components across a training cycle produces the greatest SEI improvements.

How does race distance affect the speed-endurance index?

Race distance dramatically influences both the expected SEI value and the physiological determinants of that value. In the 100m sprint, SEI values are very high (95-98%) because the race is short enough that speed decline is minimal. At 200m, SEI drops to 90-95% as the bend exit and final 30m typically show speed reduction. At 400m, SEI ranges from 78-90% because the race extends well beyond phosphocreatine system capacity, requiring substantial anaerobic glycolysis. At 800m and beyond, SEI values paradoxically improve (82-92%) because athletes pace more conservatively, never reaching their absolute maximum speed. The energy system contributions also shift dramatically: the 100m is approximately 90% anaerobic, the 400m is roughly 50-50 aerobic-anaerobic, and the 800m is approximately 60-70% aerobic. Understanding these distance-specific relationships helps athletes set appropriate SEI targets and select training methods specific to their event.

References

Reviewed by Sher, Sports Science & Nutrition Specialist · Editorial policy