Endurance training physiology

Sustainable increases in cycling power require raising both maximum aerobic capacity ($VO_2max$) and the percentage of that capacity that can be sustained without fatigue Verified Answer #1. This sustainable capacity is known as Functional Threshold Power (FTP) or the second lactate threshold ($LT_2$) Verified Answer #1. Zone 2 and interval training are complementary methods that target distinct biological signaling pathways to improve these metrics Verified Answer #1.

Zone 2 Physiology

Zone 2 is defined as low-intensity, steady-state endurance training that stays strictly below the first lactate threshold ($LT_1$) Verified Answer #1. This intensity typically corresponds to 56–75% of FTP or 60–70% of maximum heart rate Verified Answer #1.

Metabolic Signaling and Biogenesis

• Prolonged, low-intensity muscle contractions cause sustained, low-level intracellular calcium ($Ca^{2+}$) fluxes Verified Answer #1.
• These fluxes activate Calcium/calmodulin-dependent protein kinase (CaMK) Verified Answer #1.
• CaMK triggers the transcription of PGC-1$\alpha$, which is the master regulator of mitochondrial biogenesis Verified Answer #1.
• This pathway increases mitochondrial density specifically within Type I (slow-twitch) muscle fibers Verified Answer #1.
• Zone 2 training also stimulates vascular endothelial growth factor (VEGF), which increases capillary density around muscle fibers to improve oxygen delivery and waste removal Verified Answer #1.

Lactate Clearance

Zone 2 training upregulates monocarboxylate transporter 1 (MCT1) Verified Answer #1. Under the "Lactate Shuttle" theory, MCT1 is responsible for importing lactate from the bloodstream into Type I muscle fibers and cardiac tissue Verified Answer #1. Once imported, lactate is converted back to pyruvate and oxidized as fuel Verified Answer #1. This mechanism improves baseline efficiency, allowing an athlete to ride at higher wattages before producing excess lactate Verified Answer #1.