GPXFIT

Efficiency Metrics

The Efficiency Metrics tool analyzes the efficiency and sustainability of your workouts by examining how power (or pace) relates to heart rate over time. This helps identify cardiovascular drift, fatigue patterns, and aerobic fitness.

What Does This Tool Analyze?

This tool provides insights into your aerobic fitness and pacing by measuring how efficiently your body converts effort (power or pace) into performance at a given heart rate.

  • For Cycling: Analyzes Power-to-Heart Rate (Pw:HR) ratio
  • For Running: Analyzes Pace-to-Heart Rate ratio and cadence relationships

Key Metrics

Aerobic Decoupling

The primary metric that compares the first half of your workout to the second half.

Formula: [(Ratio Second Half - Ratio First Half) / Ratio First Half] × 100

Interpretation:

Decoupling % Level Meaning
< 5% Excellent Great aerobic efficiency - well-paced for your fitness
5-10% Moderate Some cardiovascular drift - normal for harder efforts
> 10% Significant May indicate fatigue, dehydration, or intensity too high

Efficiency Over Time

A rolling chart showing how your efficiency changes throughout the workout.

For Cycling:

  • Y-axis: Watts per BPM (higher is better)
  • Declining trend = heart rate drifting up relative to power
  • Stable trend = good aerobic conditioning for that intensity

For Running:

  • Y-axis: Pace per BPM (lower is better)
  • Increasing trend = slowing down relative to heart rate
  • Stable trend = maintaining pace efficiency

Cadence vs Pace (Running Only)

If your file includes cadence data, this scatter plot shows the relationship between your cadence and pace.

What to look for:

  • Optimal cadence range: typically 170-190 steps per minute
  • Higher cadence with shorter stride = better efficiency
  • Helps identify if you're overstriding at certain paces

Data Requirements

Minimum Requirements:

  • All workouts: Heart rate data
  • Cycling: Power meter data + LTHR and FTP settings
  • Running: GPS data for pace + LTHR setting
  • Recommended: At least 100 data points (typically 10+ minute workout)

Setting Your Thresholds

LTHR (Lactate Threshold Heart Rate)

Required for all efficiency analysis.

How to find it:

  1. Do a 30-minute all-out time trial
  2. Record your average heart rate for the last 20 minutes
  3. That's your LTHR

FTP (Functional Threshold Power) - Cycling Only

Required for cycling efficiency analysis.

How to find it:

  1. Do a 20-minute all-out cycling effort
  2. Take your average power for those 20 minutes
  3. Multiply by 0.95

When to Use This Tool

Best for:

  • Steady-state aerobic workouts (long rides, tempo runs, endurance efforts)
  • Assessing if your training intensity matches your current fitness level
  • Tracking improvements in aerobic efficiency over time
  • Diagnosing pacing issues in endurance events

Not ideal for:

  • Interval workouts (will show high decoupling - this is normal)
  • Very short workouts (< 30 minutes)
  • Highly variable terrain or pacing

Understanding Your Results

Low Decoupling (< 5%)

Indicates excellent aerobic efficiency for this workout. Your cardiovascular system is coping well with the demands, suggesting good pacing and appropriate intensity for your fitness level.

Moderate Decoupling (5-10%)

Some cardiovascular drift is occurring. This is normal for:

  • Harder steady-state efforts
  • Hot weather conditions
  • Long duration workouts (> 2 hours)
  • Workouts at the upper end of your aerobic zone

High Decoupling (> 10%)

Significant drift suggests one or more of:

  • Intensity too high for current fitness level
  • Dehydration or heat stress
  • Accumulated fatigue from previous training
  • Poor pacing strategy (starting too fast)

Negative Decoupling

Getting more efficient during the workout. This can happen when:

  • You're properly warmed up in the second half
  • You started conservatively and negative-split the workout
  • Conditions improved (cooling temperature, tailwind, etc.)

Tips for Better Efficiency

  1. Train at appropriate intensities: Most aerobic work should show < 5% decoupling
  2. Start conservatively: The first 10-15 minutes should feel easy
  3. Stay hydrated: Dehydration accelerates heart rate drift
  4. Track trends over time: Compare similar workouts to measure fitness improvements
  5. Use for race pacing: Practice holding efforts that show minimal decoupling
  6. Consider cadence: For running, aim for 170-190 steps per minute

Comparing to TSS Calculator

While the TSS Calculator measures overall training stress, the Efficiency Metrics tool focuses on the sustainability of your effort:

  • TSS: Answers "How hard was this workout?"
  • Efficiency Metrics: Answers "How well-paced was this workout for my fitness?"

You can have a high TSS workout with excellent efficiency (well-paced hard effort) or a moderate TSS with poor efficiency (paced too hard for current fitness).

Limitations

  • Heart rate lag: HR responds slowly to effort changes, so first 10-15 minutes may not be representative
  • Environmental factors: Heat, humidity, and wind affect results
  • Interval workouts: Will always show high decoupling - this tool is designed for steady efforts
  • Equipment accuracy: Requires accurate HR and power/pace data

References