Aerobic base : Endurance foundation

The low-intensity training phase that builds the capacity to sustain a long effort without burning out.

Aerobic base is built in Z2: long sessions at moderate intensity, conversational pace, stable HR. This phase increases mitochondrial density, improves fat oxidation (better energy efficiency over long distances), and raises the LT1 lactate threshold. In Ironman triathlon, a solid base (CTL 60-80) is essential before adding high-intensity volume, or injury and overtraining risk rises sharply.