Critical Speed

Critical Speed: training at the right intensity for optimal performance

One of the most important concepts in endurance training is Critical Speed (CS). It's the highest running speed you can sustain for an extended period without accumulating excessive fatigue. Training at the right intensity around your CS is essential for improving your endurance and performance.

What is Critical Speed?

Critical Speed is the threshold between sustainable and unsustainable effort. Below CS, your body can maintain a steady state where lactate production and removal are balanced. Above CS, lactate accumulates rapidly, leading to fatigue.

Think of CS as your metabolic boundary. It's not your maximum speed, but the speed you can maintain for 30-60 minutes depending on your fitness level. For most runners, CS corresponds closely to their tempo run pace or threshold pace.

Why is Critical Speed important?

• Training precision: Knowing your CS helps you train at the right intensity for specific adaptations
• Race pacing: CS is a reliable indicator for setting realistic race paces
• Performance tracking: Changes in CS reflect improvements in your aerobic capacity
• Injury prevention: Training within proper zones reduces overtraining risk

How to determine your Critical Speed

The most accurate way to determine CS is through testing:

Time trial method:

• Perform a 3 km time trial at maximum effort
• Rest 1-2 days
• Perform a 9 km time trial at maximum effort
• Calculate CS using the formula: CS = (9000m - 3000m) / (time for 9km - time for 3km)

Alternative: Use recent race times

You can also estimate CS from recent race performances at different distances, though this is less precise.

Training zones based on Critical Speed

Once you know your CS, you can establish training zones:

• Easy runs: 65-75% of CS - builds aerobic base
• Tempo runs: 85-95% of CS - improves lactate threshold
• Threshold runs: 95-105% of CS - pushes CS higher
• Interval training: 105-120% of CS - develops VO2max

Practical application

Let's say your CS is 4:00 min/km (15 km/h). Your training zones would be:

• Easy runs: 5:20-4:40 min/km
• Tempo runs: 4:25-4:12 min/km
• Threshold runs: 4:12-3:50 min/km
• Intervals: 3:50-3:20 min/km

Common mistakes

• Running too fast on easy days: Easy runs above 75% CS don't provide additional benefits and compromise recovery
• Running too slow on hard days: Threshold runs below 85% CS won't provide the desired training stimulus
• Ignoring progression: As your fitness improves, your CS increases and zones need adjustment
• Not testing regularly: Retest your CS every 8-12 weeks to track progress

The role of REN

REN continuously monitors your training data and adjusts your workouts to keep you training at the optimal intensity relative to your current CS. This ensures you're always training in the right zones for maximum adaptation without overtraining.

By tracking your performance across workouts, REN can detect when your CS has improved and automatically adjust your training zones, ensuring continued progress toward your goals.

Conclusion

Understanding and training around your Critical Speed is fundamental to structured training. It provides a scientific foundation for determining the right training intensity, helps prevent overtraining, and ensures you're getting the maximum benefit from every workout. Whether you're training for a 5K or a marathon, CS should be the cornerstone of your training plan.