Glossary: Aerobic vs anaerobic

The two systems

• Aerobic metabolism: uses oxygen to convert fat and carbohydrate into ATP. Dominant at lower intensities and for longer durations. Higher yield per unit of fuel.
• Anaerobic metabolism: produces ATP without oxygen, via the phosphocreatine system and glycolysis. Dominant at high intensities and short durations. Faster but produces lactate and depletes phosphocreatine.

In real exercise both systems are always contributing; the question is the mix.

How intensity shifts the mix

Approximate aerobic contribution by duration of all-out effort:

• 6 seconds: about 10 percent aerobic, 90 percent anaerobic (phosphocreatine dominant).
• 30 seconds: about 25 percent aerobic.
• 2 minutes: about 50 percent aerobic.
• 5 minutes: about 80 percent aerobic.
• 10 minutes: about 90 percent aerobic.
• 30 minutes: about 97 percent aerobic.
• 2 hours: above 99 percent aerobic.

How this maps to training

• Sprint / neuromuscular work (under 30 seconds): trains phosphocreatine and anaerobic glycolysis. Yousuli Zone 7.
• Anaerobic capacity (30 sec to 3 min): trains anaerobic glycolysis. Yousuli Zone 6 and 7.
• VO2max work (3 to 8 min): trains aerobic ceiling. Yousuli Zone 6.
• Threshold work (8 to 60 min): trains aerobic sustainability. Yousuli Zone 4 and 5.
• Tempo / sweet spot (10 to 90 min): trains aerobic base with intensity. Yousuli Zone 3.
• Endurance (60+ min): trains pure aerobic base. Yousuli Zone 2.

What this means for race pacing

Long-course racing is almost entirely aerobic. The anaerobic system contributes briefly at the start, on short climbs, and at the finish. Pacing decisions that push you anaerobic for sustained periods produce a much worse race than holding aerobic ceiling.

See also: Glossary: zones, Glossary: VO2max, Glossary: lactate threshold vs FTP.