How to track progress indoors beyond FTP
FTP is useful, but it doesn’t tell the whole story of your fitness. Indoors, where conditions are controlled, you can track additional signals that show whether you’re becoming more durable, more economical, and better able to hold watts for longer. Three metrics stand out: stamina, efficiency factor, and aerobic decoupling.
The metrics that matter indoors
Use these to complement FTP and training zones, not replace them. They work best on steady, aerobic efforts rather than short intervals.
| Metric | What it tells you | How to calculate | What improvement looks like |
|---|---|---|---|
| Stamina (power-curve based) | Your ability to sustain a high fraction of FTP for longer durations; overall aerobic durability. | From your platform’s power-duration model (often 0–100%) or by tracking time to exhaustion (TTE) at FTP. | Long-duration power rises; TTE at FTP extends by 5–15 min across a block. |
| Efficiency factor (EF) | Watts per heartbeat during steady aerobic work; a proxy for aerobic economy. | EF = Normalized Power / Average HR over a steady segment. | EF trending up 3–8% over several weeks at the same intensity and conditions. |
| Aerobic decoupling (Pa:Hr or Pw:Hr) | How much HR drifts up relative to power over time; a marker of endurance and fatigue resistance. | Compare EF (or power:HR) in first half vs second half of a steady effort. | <3% drift at upper endurance (Z2) for 40–60 min; downward trend week to week. |
Stamina: the long-game fitness
Stamina describes how well you hold power as durations lengthen. If your 20–60 min power improves even when FTP barely moves, you’re getting more durable. If your software provides a stamina score or TTE, watch it; otherwise, note how long you can ride near FTP before fading.
Efficiency factor (EF): watts per heartbeat
EF rises when you produce more watts at the same heart rate. Track EF only on steady aerobic rides, ideally around upper Z2 or low tempo. Compare apples to apples: same trainer, gearing, fans, temperature, caffeine timing, and similar cadence.
Aerobic decoupling: drift tells the truth
Low drift means your aerobic system matches the workload without excessive cardiovascular strain. As endurance improves, the power-to-heart-rate relationship stays tight for longer. Decoupling grows when you’re under-recovered, under-fueled, hot, or simply not conditioned for the duration.
EF = Normalized Power / Average HR (W per bpm)
Decoupling (%) ≈ ((EF_second_half - EF_first_half) / EF_first_half) × 100Simple indoor tests to measure them
Repeat these every 1–2 weeks to create clean trend lines.
1) Z2 aerobic benchmark (EF + decoupling)
- Warm up 10–15 minutes with a few cadence builds.
- Ride 40–60 minutes at upper endurance: 65–75% of FTP (Coggan Z2). Keep cadence steady (85–95 rpm).
- Record Normalized Power, average HR, EF, and decoupling for the full segment.
- Target: decoupling <3% at this intensity. If 3–5%, you’re close; >5–7% signals more endurance work is needed.
2) Stamina/TTE check
- After an easy day, ride a long steady effort near FTP (start ~95–100% of FTP). Hold as long as you can with stable power and cadence.
- Time to exhaustion (TTE) is the duration you can sustain. Stop when power fades >5% for several minutes despite best effort.
- Target: extend TTE toward 35–60 minutes across an 8–12 week block, depending on event demands.
3) Durability under fatigue
- Do 2×15–20 min at tempo or sweet spot (80–90% of FTP) with 5 min easy between.
- Immediately follow with 30–40 min at upper Z2.
- Measure decoupling only in the final Z2 segment. Improving drift here means your endurance holds up late, not just when fresh.
Consistency checklist
- Standardize environment: strong fans, similar room temperature, same bike fit and gearing.
- Fuel similarly: 30–60 g carbs per hour for Z2 tests; more for tempo; arrive hydrated.
- Run tests at similar times of day, after comparable recovery.
- Note confounders: sleep, caffeine, illness, heat. HR is sensitive to all of them.
Coach’s tip: Don’t chase single-day numbers. Look for 3–5 consecutive data points trending in the same direction before you adjust training zones or volume.
Turn trends into training choices
Use these simple rules to progress your work on the trainer.
If EF rises and decoupling stays low
- Progress endurance: add 10–15 minutes to your Z2 benchmark or bump power by 5–10 watts.
- Add a weekly tempo block (e.g., 2×20–30 min at 80–88% FTP) to push stamina without overtaxing recovery.
If decoupling is high at Z2
- First fix basics: more cooling, hydration, and carbs during the ride.
- Increase low-intensity volume. Start with adding 15–30 minutes to 2–3 rides per week.
- Keep intensity modest for a week and retest before raising FTP or tempo time.
If stamina/TTE stalls
- Swap some VO2 work for longer continuous efforts: 1×40–60 min at 88–92% FTP or 3×15–20 min at sweet spot.
- Practice steady fueling: 60–90 g carbs per hour on key stamina days.
- Every 3–4 weeks, schedule a recovery week and retest TTE fresh.
Red flags and pitfalls
- Comparing EF across different temperatures or fan setups is misleading; treat those as separate baselines.
- Short segments (<30 min) inflate EF and mute drift; stick to steady 40–60 minutes for endurance checks.
- Rapid EF drops with normal power often signal dehydration or inadequate fueling, not sudden fitness loss.
- Big decoupling spike after a race or hard block usually resolves with 2–3 easy days and proper recovery.
FTP sets your ceiling, but stamina, efficiency factor, and decoupling show how strong the walls are. Track them with simple indoor sessions, let the trends guide your training zones and progression, and you’ll arrive at spring with more durable watts—not just a bigger single number.
