⚡ Mechanical Output Science

Mechanical & Power Metrics

A rigorous guide to Functional Threshold Power (FTP), Normalized Power (NP), Variability Index (VI), and anaerobic W-Prime (W') modeling.

1. Core Mechanical Metrics & Stress Scores

FTP (Functional Threshold Power)

Your 60-minute quasi-steady physiological power ceiling in Watts. It anchors every training zone and represents the boundary above which lactate production outpaces clearance.

Normalized Power (NP)

An estimate of the true metabolic cost of surging efforts. Using a 30-second rolling average raised to the 4th power, it weights anaerobic spikes proportionally to glycogen consumption.

Intensity Factor (IF = NP / FTP)

Scales the difficulty of a workout relative to your personal threshold. 0.70 is conversational endurance; 1.00 equals riding at your absolute 60-minute limit.

TSS (Training Stress Score)

A composite number of total training dose: TSS = (seconds × NP × IF) / (FTP × 36) *Exactly 1 hour at 100% FTP equals exactly 100 TSS.

2. Average Power vs. Normalized Power: Why Averages Lie

Consider two riders with an FTP of 300W who both ride for 60 minutes with an Average Power of 220 Watts:

Scenario A: Steady Time Trial

Flat Road, Smooth Pacing

• Average Power: 220 Watts
• Normalized Power: 223 Watts
• Variability Index (VI = NP / Avg): 1.01
• Intensity Factor (IF): 0.74 (Zone 2 Endurance)
• Total Stress: 55 TSS
• Outcome: Aerobically gentle, legs feel fresh tomorrow.

Scenario B: Punchy Criterium Race

Repeated 600W Corner Surges + Coasting

• Average Power: 220 Watts (due to coasting)
• Normalized Power: 285 Watts
• Variability Index (VI = NP / Avg): 1.29
• Intensity Factor (IF): 0.95 (Threshold Cost!)
• Total Stress: 90 TSS
• Outcome: High muscle damage, lactic accumulation, heavy fatigue.

The Lesson: Average Power masks high-intensity surges. Normalized Power accurately captures the physiological penalty of punchy attacks, sprints, and hilly terrain.

3. Coggan 7-Zone Power Distribution Chart

Based on Dr. Andrew Coggan's power training levels, calibrated for a rider with an FTP of 250 Watts:

Zone	Name	% of FTP	Watts (250W FTP)	Primary Physiological Adaptations
Z1	Active Recovery	< 55%	< 137 W	Flushes metabolic waste; stimulates blood flow without fatigue.
Z2	Endurance	56% – 75%	140 – 187 W	Mitochondrial biogenesis, capillary proliferation, fat oxidation.
Z3	Tempo	76% – 90%	190 – 225 W	Muscle glycogen storage, fast-to-slow twitch fiber conversion.
Z4	Lactate Threshold	91% – 105%	227 – 262 W	Increases lactate buffering; raises maximum sustained speed.
Z5	VO2 Max	106% – 120%	265 – 300 W	Expands maximal cardiac output and aerobic consumption ceiling.
Z6	Anaerobic Capacity	121% – 150%	302 – 375 W	Increases glycolytic capacity and anaerobic tolerance.
Z7	Neuromuscular	> 150%	375+ W (Max)	Maximal motor unit recruitment, phosphagen (ATP-PC) power.

4. Anaerobic Work Capacity: W' (W-Prime)

W' (pronounced W-Prime) represents the total amount of energy (measured in kilojoules) you can generate above your Critical Power before reaching absolute exhaustion.

🔋 Real-Time Battery Analogy: Typical Capacity: 15 – 25 kJ