TrainingPeaks: How Fitness, Fatigue, and Form Are Calculated

How is fitness calculated in TrainingPeaks?

In TrainingPeaks, fitness is primarily calculated and represented by Chronic Training Load (CTL), a metric derived from an exponentially weighted moving average of your daily Training Stress Score (TSS), reflecting your long-term training consistency and physiological adaptation.

Introduction to TrainingPeaks Metrics

For athletes and coaches utilizing TrainingPeaks, understanding the core metrics of fitness, fatigue, and form is paramount for optimizing performance and preventing overtraining. These metrics, rooted in the work of Dr. Andrew Coggan and Dr. Allen Lim, provide an objective framework for quantifying training stress and its physiological impact. The calculation of "fitness" in TrainingPeaks is a multi-layered process, built upon the foundation of daily training stress.

The Foundation: Training Stress Score (TSS)

Before we can understand fitness, we must first grasp the concept of Training Stress Score (TSS). TSS is a proprietary metric designed to quantify the overall physiological stress of a training session. It considers both the intensity and duration of an activity, providing a single number that reflects the workout's impact.

TSS Calculation Principles:

• Intensity Factor (IF): This is the ratio of your Normalized Power (NP) for cycling, Normalized Graded Pace (NGP) for running, or average heart rate (for other activities) to your functional threshold (FTP for cycling, FTHR for heart rate, or Threshold Pace for running). An IF of 1.0 means the workout was performed at your threshold intensity.
• Duration: The total time of the activity in hours.
• Functional Threshold Power (FTP), Functional Threshold Heart Rate (FTHR), or Threshold Pace: These are critical inputs that define your individual physiological ceilings for a given duration. Accurate threshold settings are vital for meaningful TSS calculations.

TSS Formulas by Discipline (Conceptual):

• Cycling TSS: Based on power data, it uses Normalized Power (NP), Intensity Factor (IF), and duration. NP is a power average that accounts for the physiological cost of fluctuating power outputs, providing a more accurate measure of intensity than simple average power.
  • Formula roughly: TSS = (Duration in seconds * Normalized Power * Intensity Factor) / (FTP * 3600) (simplified for conceptual understanding)
• Running TSS (rTSS): Calculated using pace data, it leverages Normalized Graded Pace (NGP) and Intensity Factor (IF). NGP adjusts for changes in terrain (uphills/downhills) to reflect the true physiological effort. If pace data isn't available, heart rate can be used (HRTSS).
• Swimming TSS (sTSS): Typically calculated using heart rate data or a manual entry based on perceived exertion and duration, as power meters are less common for swimming.
• Strength Training TSS (STRSS): Often manually entered or calculated based on session duration and perceived intensity, as objective metrics like power are not standard for most strength exercises.

Interpretation of TSS:

• A TSS of 100 for an hour is roughly equivalent to an all-out, threshold effort for that hour.
• A higher TSS indicates a more stressful workout.

The Core Metric: Chronic Training Load (CTL) – Your Fitness

Chronic Training Load (CTL), often referred to as "fitness" in TrainingPeaks, is the primary metric used to quantify an athlete's long-term training consistency and accumulated physiological adaptations. It represents your body's ability to handle training stress over an extended period.

How CTL is Calculated: CTL is an exponentially weighted moving average (EWMA) of your daily TSS. This means that:

• It considers all your past training, not just recent workouts.
• More recent training sessions have a greater impact on the CTL value than older sessions.
• The time constant for CTL is typically 42 days (6 weeks). This means that approximately 63% of your current CTL value is influenced by training from the last 42 days, with the impact diminishing exponentially for older workouts.

Interpretation of CTL:

• Rising CTL: Generally indicates an increase in fitness, as you are consistently accumulating training stress.
• Plateaued CTL: Suggests a maintenance phase or a period where training stress is not significantly increasing.
• Falling CTL: Implies a decrease in fitness, often due to reduced training volume or intensity (e.g., during an off-season or forced break).
• A higher CTL value generally correlates with a higher potential for endurance performance, assuming appropriate recovery and peaking.

Acute Training Load (ATL) – Your Fatigue

While CTL represents fitness, Acute Training Load (ATL) quantifies your short-term training stress, or "fatigue." It reflects the immediate impact of your recent workouts on your body.

How ATL is Calculated: Similar to CTL, ATL is an exponentially weighted moving average of your daily TSS, but with a much shorter time constant.

• The time constant for ATL is typically 7 days (1 week). This means ATL is heavily influenced by your training from the past week.

Interpretation of ATL:

• Rising ATL: Indicates increasing fatigue due to recent hard training.
• Falling ATL: Suggests decreasing fatigue, often during a recovery or taper period.
• A high ATL without adequate recovery can lead to overtraining or injury.

Training Stress Balance (TSB) – Your Form/Readiness

Training Stress Balance (TSB), often called "form" or "readiness," is the critical metric that integrates both fitness (CTL) and fatigue (ATL) to provide an indication of your current performance potential.

How TSB is Calculated: TSB is simply the difference between your Chronic Training Load (CTL) and your Acute Training Load (ATL) for a given day:

• TSB = CTL - ATL

Interpretation of TSB:

• Positive TSB (e.g., +5 to +25): Generally indicates that fatigue has dissipated relative to your fitness, suggesting you are fresh and ready for peak performance. This is the desired range for key races or events.
• Neutral TSB (e.g., -5 to +5): You are likely maintaining fitness and not overly fatigued, potentially suitable for consistent training or less critical events.
• Negative TSB (e.g., -10 to -30 or lower): Indicates that your current fatigue (ATL) outweighs your accumulated fitness (CTL). This typically occurs during periods of heavy training and suggests you are not fresh enough for peak performance. While necessary for building fitness, prolonged negative TSB without recovery can lead to burnout.

Practical Application and Limitations

TrainingPeaks' fitness metrics (TSS, CTL, ATL, TSB) provide a powerful framework for intelligent training.

Benefits:

• Objective Quantification: Moves beyond subjective feelings to provide data-driven insights.
• Load Management: Helps coaches and athletes manage training volume and intensity to optimize adaptation and prevent overtraining.
• Performance Prediction: TSB can be a strong indicator of readiness for competition.
• Long-Term Tracking: CTL allows for monitoring fitness trends over months and years.

Limitations and Considerations:

• Garbage In, Garbage Out: The accuracy of these metrics heavily relies on accurate threshold settings (FTP, FTHR, Threshold Pace) and consistent, high-quality data (e.g., power meter calibration).
• Individual Variability: While the formulas are standard, how an individual responds to a given TSS can vary greatly based on genetics, lifestyle stress, sleep, nutrition, and experience level.
• Non-Quantifiable Stress: These metrics do not account for external life stressors (work, family, sleep deprivation, illness) that significantly impact recovery and performance.
• Single-Discipline Focus: While TrainingPeaks can aggregate TSS from different activities, the underlying calculations for each discipline are distinct, making direct cross-sport comparisons of intensity or stress sometimes challenging without careful calibration.
• Not a Replacement for Coaching Intuition: These tools are aids, not substitutes, for a skilled coach's judgment and an athlete's self-awareness.

Conclusion

In TrainingPeaks, "fitness" is a dynamic and data-driven concept embodied by Chronic Training Load (CTL). By understanding how CTL is built upon daily Training Stress Score (TSS) and balanced against Acute Training Load (ATL) to determine Training Stress Balance (TSB), athletes and coaches gain invaluable insights into an individual's physiological state. This scientific approach empowers more precise training prescription, intelligent recovery planning, and ultimately, optimized athletic performance. While powerful, remember to integrate these metrics with your subjective feelings, the nuances of your body's responses, and your overall life context for truly holistic training management.