Running Data Analysis: Understanding Garmin and COROS Training Load Metrics

Introduction

More and more runners in Taiwan are wearing Garmin Forerunner or COROS PACE series watches, devices that offer training analytics far beyond a simple heart rate monitor. Yet the warning “Your Training Load is too high, be careful” is often all people notice — few understand how that number is actually calculated, let alone how to turn these metrics into real training decisions.

Breaking Down the Core Metrics

Training Load / TRIMP

Training Load quantifies the total physiological impact of a workout by combining duration and intensity. The most fundamental calculation is TRIMP (Training Impulse):

TRIMP = Training Duration × Average Heart Rate Weighting Factor

The heart rate weighting factor increases exponentially with intensity, so high-intensity sessions contribute far more load than low-intensity sessions of the same duration. Garmin uses an Internal Load approach based on Heart Rate Deviation, while COROS offers both heart-rate-based and pace-based versions.

Acute Training Load vs. Chronic Training Load (ATL / CTL)

The logic behind Garmin’s “Performance Condition” traces back to Banister’s Impulse-Response Model:

• Negative TSB: Fatigue exceeds adaptation — you’re still recovering (normal during a heavy pre-race build phase).
• Positive TSB: Good physical readiness, suitable for high-intensity training or racing (the ideal state after a taper).
• Excessively high positive TSB (>30): Undertraining — fitness may start to decline.

Training Status

Garmin assigns a training status label based on the trend in your VO2max:

• Peaking: VO2max rising, training load moderate — the best racing condition.
• Productive: Moderate overload — fitness is improving.
• Maintaining: Load is just enough to sustain your current fitness.
• Recovery: Load is relatively low — the body is resting.
• Overreaching: Load is too high — recovery needs attention.
• Detraining: Training volume is insufficient — VO2max is declining.

Body Battery (Garmin)

Body Battery combines HRV status, sleep quality, activity level, and other dimensions into a single 5–100 score representing your available energy reserve. As a general guide:

• >75 points: Suitable for high-intensity or long-distance training.
• 50–75 points: Moderate-intensity training is safe; avoid maximal effort testing.
• <25 points: Stick to easy activity or full rest.

COROS EvoLab’s Distinctive Metrics

COROS’s EvoLab system provides:

• Base Fitness: Similar to CTL, reflecting long-term training accumulation.
• Training Load: Similar to ATL, reflecting recent training stimulus.
• Recovery: Predicts the minimum safe time before your next high-intensity session.
• Adaptation Pace: Based on recent training trends, suggests the marathon pace best suited to your current fitness.

How to Apply These Metrics in Real Training

1. Establish a personal baseline: Train at moderate intensity for at least 6 weeks initially so CTL (Chronic Training Load) can establish a reliable baseline.
2. TSB target for race-week taper: Most research recommends a TSB of +10 to +20 three days before a race, avoiding tapering too early (TSB too high) or not enough (TSB still negative).
3. Avoid an excessive ATL/CTL ratio: If the ratio (ATL ÷ CTL, also called the Monotony Index) stays above 1.5, the risk of overtraining rises significantly.
4. Cancel high-intensity workouts when Body Battery is below 30: Even if intervals are scheduled, prioritize your body’s signals and switch to an easy run or rest instead.

Limitations and Caveats

• Assumptions behind VO2max estimation: Optical heart rate sensor accuracy is limited during exercise, and users with darker skin tones or extensive tattoos may see larger errors — a chest strap heart rate monitor is recommended for better precision.
• Numbers aren’t comparable across brands: Garmin’s Training Load figures and COROS’s calculation formulas differ, so you can only meaningfully compare trends within the same system over time.
• Individual variation: The algorithms are based on population averages, and recovery speed for beginners or veteran athletes often diverges from what the system predicts.

Conclusion

Garmin and COROS training metrics are powerful tools, but their real value lies in reading trends, not over-interpreting any single data point. Treat your watch data as a mirror of your body’s state — combine it with how you actually feel and how you actually perform, and technology can genuinely serve your progress as a runner.

Related Reading

• The Complete Guide to GPS Running Watches: Garmin and COROS Features and Training Applications
• High Training Load Warnings: Using Data to Prevent Overtraining
• The Complete Guide to Reading Garmin Connect Data: Beyond Mileage Tracking
• How to Choose and Use a GPS Running Watch: Comparing Garmin, COROS, and Suunto