Definition of Critical Speed (CS): Critical Speed is the boundary between metabolically sustainable and unsustainable speeds, distinguishing the threshold between high-intensity aerobic and anaerobic efforts.

Critical Speed as a Training and Racing Tool: Running at speeds just above or below critical speed, even by a few percentage points, results in significantly different metabolic conditions within the muscles.

Unlike lactate threshold training, improving critical speed does not mean training exactly at CS. Instead, combine interval training at CS+ (3% faster than CS) with fast continuous runs or intervals at CS- (3% slower than CS).

Training with Critical Speed: When training with critical speed, runners should not train exactly at CS but use modified paces: CS- and CS+.

• CS-: For high-intensity aerobic training, use CS-, which is slightly slower than CS, set at 97% of CS.
• CS+: To enhance anaerobic capacity, use CS+, which is slightly faster than CS, set at 103% of CS.

Training at CS-: Training at CS- slows the onset of fatigue, allowing for substantial training volume while maintaining stable biochemical conditions in the muscles.

How to Implement CS- and CS+ in Training: Training induces physiological adaptations by applying stress to the body.

For endurance sports, the following physiological characteristics are key: mitochondrial density, muscle fiber composition, and capillary density. Enhancing these aspects improves endurance.

1. Training to Increase Mitochondrial Density: Easy runs and long easy runs increase mitochondrial density and gradually alter muscle fiber type distribution. Training volume positively correlates with mitochondrial density, while intensity does not. Control your pace during these sessions.
   Key Training:
   • Easy Runs: 80% of marathon pace (MP), for over 30 minutes per session.
   • Long Easy Runs: 80-85% of MP, with single sessions not exceeding 30% of weekly mileage or 150 minutes, whichever is lower.
2. Training to Stimulate Capillary Growth: Long fast runs stimulate capillary growth.
   Key Training:
   • Long Fast Runs: Example: 12-16 km at 80% of 5 km pace.
3. Training to Increase Mitochondrial Energy Output: Training at 80-90% of VO2 max enhances mitochondrial energy output. Most runners’ CS falls within this range. Research indicates that CS+ training, CS- training, and continuous high-intensity aerobic runs are crucial for enhancing mitochondrial energy output.
   Key Training:
   • Daniels-style Tempo Runs and Cruise Intervals: Example:
     • 6 x 4 minutes at 92% of 5k pace, with 1-minute slow jogs.
     • Continuous 8 km at 90% of 5k pace.
   • CS- Training (below critical speed): Examples:
     • 8 x 3 minutes at CS- (or 95% of 5k pace), with 1 minute 30 seconds slow jogging.
     • 5 x 1 km at CS- (or the same as above), with 2 minutes slow jogging.
     • 3 x 2 km at CS- (or the same as above), with 3 minutes slow jogging.
     • Continuous 5 km at CS- (or the same as above).
   • CS+ Training (above critical speed): Examples:
     • 8 x 600 meters at CS+ (or 100-102% of 5k pace), with 2 minutes slow jogging.
     • 10 x 2 minutes at CS+ (or the same as above), with 1 minute 30 seconds slow jogging.

Critical speed is a valuable tool for understanding training and racing but should be part of a broader training strategy. Effective training follows a pyramid model, balancing different intensities. Endurance training is a long-term process that requires gradual progression to enhance capabilities and minimize injury risk.