Training Stress Score (TSS): Complete Guide for Runners

What is Training Stress Score (TSS)?

Training Stress Score is a single number that represents the overall training load of a workout, accounting for both duration and intensity. Unlike simply tracking miles or time, TSS weights intensity appropriately: a 60-minute tempo run creates more training stress than a 60-minute easy run.

Key Concepts Behind TSS

• Objective Quantification: TSS provides a standardized metric to compare different workouts—intervals vs tempo vs long runs
• Intensity Weighting: Higher intensity workouts generate disproportionately more stress than duration alone would suggest
• Cumulative Tracking: TSS can be summed daily, weekly, and over training cycles to track total training load
• Individual Calibration: TSS is personalized to your threshold, making it specific to your fitness level
• Fatigue Management: TSS-derived metrics (CTL, ATL, TSB) predict fitness, fatigue, and optimal training load

TSS Benchmark Values

TSS integrates seamlessly with other training metrics covered in our running performance metrics guide and training load management resources.

rTSS: TSS for Running

While TSS was originally developed using cycling power data, rTSS (running Training Stress Score) adapts the concept for running using pace instead of power. The fundamental formula structure remains the same, but rTSS uses your Critical Running Speed (CRS) or threshold pace as the baseline.

Why CRS for rTSS?

CRS represents your sustainable aerobic-anaerobic transition speed—essentially your "functional threshold pace." Using CRS as the reference point ensures:

• Personalized intensity calculation specific to your physiology
• Accurate comparison across different runner ability levels
• Proper weighting of intensity relative to your actual capabilities
• Consistency with scientifically validated metrics

Alternative threshold markers include:

• 30-minute test pace: Functional threshold pace from 30-min time trial
• 10K race pace + 10-15 sec/mile: Estimated threshold from recent 10K
• Lactate threshold pace: From laboratory or field lactate testing

Learn how to test your CRS in our performance testing guide.

rTSS vs Heart Rate TSS (hrTSS)

For most runners, pace-based rTSS provides more consistent and actionable data, especially when training on measured courses or tracks. Heart rate-based TSS works better for trail running where pace is unreliable.

How to Calculate rTSS

The rTSS formula accounts for both workout duration and intensity relative to your threshold:

rTSS = (Duration in seconds × IF² × 100) / 3600

Where:
IF = Intensity Factor (workout pace / CRS pace)
Duration = Total workout time in seconds

Step-by-Step Calculation Example

Workout: 60-minute tempo run at 4:00/km pace
Your CRS: 4:20/km (270 m/min or 4.5 m/s)

Step 1: Convert paces to m/s

• Workout pace: 4:00/km = 4.17 m/s
• CRS pace: 4:20/km = 3.85 m/s

Step 2: Calculate Intensity Factor (IF)

IF = Workout pace / CRS pace
IF = 4.17 / 3.85 = 1.08

Step 3: Calculate rTSS

Duration = 60 minutes = 3,600 seconds
rTSS = (3,600 × 1.08² × 100) / 3,600
rTSS = (3,600 × 1.166 × 100) / 3,600
rTSS = 116.6 ≈ 117

This 60-minute tempo run at 108% of CRS generates 117 rTSS—slightly more stress than an hour at threshold would produce (which equals 100 TSS by definition).

Quick rTSS Calculator

Use our free rTSS calculator to quickly compute TSS for your workouts without manual calculation. Simply enter your CRS, workout pace, and duration.

rTSS for Mixed Workouts

For workouts with multiple segments at different intensities (intervals, progressive runs), calculate TSS for each segment and sum them:

Example: Interval Workout

• 15 min warm-up (60% CRS): IF = 0.60, TSS = 9
• 6 × 4 min at 110% CRS (24 min total): IF = 1.10, TSS = 48
• Recovery between intervals (12 min at 50% CRS): IF = 0.50, TSS = 2
• 10 min cool-down (60% CRS): IF = 0.60, TSS = 6

Total Workout rTSS: 9 + 48 + 2 + 6 = 65 rTSS

Notice that most of the training stress comes from the interval segments despite being less than half the workout duration—this demonstrates TSS's proper intensity weighting.

Understanding Intensity Factor (IF)

Intensity Factor is the ratio of your workout pace to your threshold pace. It represents how hard you're working relative to your maximum sustainable aerobic effort.

Intensity Factor Zones

TSS per hour = IF² × 100. For example, Zone 2 at IF 0.75 generates 56 TSS per hour.

Why IF is Squared

The quadratic relationship (IF²) reflects the exponential increase in physiological stress as intensity rises:

• IF 0.80 (easy run): Generates 64 TSS per hour
• IF 0.90 (tempo): Generates 81 TSS per hour (+27%)
• IF 1.00 (threshold): Generates 100 TSS per hour (+24%)
• IF 1.10 (VO2max): Generates 121 TSS per hour (+21%)

A 10% increase in pace (from 0.80 to 0.90 IF) increases training stress by 27%, not just 10%. This accurately reflects that running slightly faster requires disproportionately more energy and creates greater fatigue.

Learn more about managing different intensity levels in our training zones guide.

CTL, ATL, and TSB Metrics

TSS for individual workouts is useful, but the real power comes from tracking cumulative metrics over time. Three derived metrics manage long-term training load:

Chronic Training Load (CTL): Fitness

CTL is your long-term training load, calculated as an exponentially weighted 42-day moving average of daily TSS. CTL represents your fitness—your capacity to absorb training stress.

CTL(today) = CTL(yesterday) + (TSS(today) - CTL(yesterday)) / 42

• Higher CTL: Greater fitness and work capacity
• Increasing CTL: Building fitness (but also accumulating fatigue)
• Stable CTL: Maintaining fitness level
• Declining CTL: Detraining (intentional taper or unplanned)

Acute Training Load (ATL): Fatigue

ATL is your short-term training load, calculated as an exponentially weighted 7-day moving average of daily TSS. ATL represents your fatigue—the impact of recent training.

ATL(today) = ATL(yesterday) + (TSS(today) - ATL(yesterday)) / 7

• Higher ATL: More accumulated fatigue
• ATL > CTL: Significant fatigue relative to fitness (overreaching or overtraining risk)
• ATL << CTL: Well-recovered, ready for hard training

Training Stress Balance (TSB): Form

TSB is the difference between fitness and fatigue. It predicts your form—readiness to perform or train hard.

TSB = CTL - ATL

• TSB = -30 to -10: High fatigue, productive overload phase
• TSB = -10 to +5: Optimal training range, good balance
• TSB = +5 to +15: Fresh, tapered, race-ready
• TSB = +15 to +25: Very fresh, peak form for key race
• TSB > +25: Detraining, lost fitness

CTL, ATL, TSB Relationship Visual

This example shows typical CTL/ATL/TSB patterns through a training block and taper. Notice how TSB becomes strongly negative during hard training weeks (fatigue accumulation) then swings positive during taper (fatigue dissipates faster than fitness).

TSS Guidelines by Training Phase

Appropriate TSS targets vary by training phase, experience level, and goals. These guidelines help structure training load progression throughout your season.

Weekly TSS by Experience Level

Weekly TSS ranges account for individual differences in work capacity, training history, and recovery ability.

CTL Targets by Race Distance

Higher CTL provides greater race-specific endurance and resilience. However, rapid CTL increases risk injury—limit CTL growth to 5-8 points per week maximum.

TSS Periodization Through Training Cycle

For a 16-week marathon build (intermediate runner example):

Learn more about structuring training phases in our marathon periodization guide.

Weekly TSS Targets

How you distribute TSS across the week matters as much as total weekly TSS. Proper distribution balances training stimulus with adequate recovery.

TSS Distribution Patterns

Pattern 1: Traditional Weekday/Weekend

Pattern 2: Double Key Workout Week

TSS Ramping Guidelines

Progressive overload requires steady TSS increases, but too rapid growth risks injury and overtraining:

• Maximum weekly increase: 5-10% increase in weekly TSS
• Maximum CTL increase: 5-8 CTL points per week
• Recovery weeks: Every 3-4 weeks, reduce TSS by 30-50%
• Injury/illness: After layoff, return at 50% previous TSS, add 10% per week

Recovery Week TSS

Scheduled recovery weeks prevent overtraining and allow adaptation:

• Frequency: Every 3-4 weeks
• TSS reduction: 30-50% of previous week
• Intensity maintenance: Keep some quality work but reduce volume significantly
• CTL impact: Minimal CTL decrease (1-3 points) but significant ATL decrease (10-20 points)
• TSB improvement: TSB should rise to +5 to +15 during recovery week

TSS Distribution & Intensity

Not all TSS is created equal. The intensity distribution of your TSS accumulation significantly impacts training adaptations and outcomes.

80/20 TSS Distribution

The 80/20 training rule suggests 80% of training should be easy (Zone 1-2), 20% moderate to hard (Zone 3-5). TSS distribution should reflect this:

Calculating Intensity Distribution

Track TSS by zone to ensure proper distribution:

Example Week (Target: 500 weekly TSS, 80/20 split):

• Monday: Rest (0 TSS)
• Tuesday: 10 miles easy, IF 0.75 → 85 TSS (Zone 2)
• Wednesday: 8 miles with 6 × 4min at VO2max → 95 TSS total
  • WU/CD: 55 TSS (Zone 2)
  • Intervals: 40 TSS (Zone 5)
• Thursday: 6 miles easy, IF 0.75 → 55 TSS (Zone 2)
• Friday: 8 miles tempo, IF 0.92 → 110 TSS total
  • WU/CD: 40 TSS (Zone 2)
  • Tempo: 70 TSS (Zone 3-4)
• Saturday: Rest (0 TSS)
• Sunday: 16 miles long run, IF 0.78 → 155 TSS (Zone 2)

Weekly Totals:

• Total TSS: 500
• Zone 1-2 TSS: 390 (78%)
• Zone 3-5 TSS: 110 (22%)
• Distribution: 78/22 ✓ Within 80/20 guideline

Polarized Training TSS Distribution

Polarized training takes 80/20 further: ~85% easy, minimal Zone 3, ~15% very hard (Zone 5):

Polarized distribution often produces better results than traditional pyramidal or threshold-heavy approaches, particularly for endurance events.

Using TSS for Training Management

TSS transforms from interesting data into actionable guidance when applied to training decisions.

Planning Weekly TSS

1. Establish baseline: Track 2-3 weeks of current training to determine sustainable weekly TSS
2. Set target CTL: Determine goal CTL for your target race (see tables above)
3. Calculate required TSS: Work backward from goal CTL to determine weekly TSS needed
4. Plan ramp rate: Increase weekly TSS by 5-10% maximum per week
5. Schedule recovery: Every 3-4 weeks, reduce TSS by 30-50%

Example: Building from CTL 50 to CTL 85 for marathon

• Current CTL: 50 (current weekly TSS ~350)
• Goal CTL: 85 (requires weekly TSS ~595)
• Weeks available: 16 weeks
• Required CTL increase: 35 points
• Sustainable rate: ~2.2 CTL points per week
• Weekly TSS progression: 350 → 385 → 420 → 455 → 490 → 525 → 560 → 595
• With recovery weeks factored: Achieve goal CTL with buffer for taper

Using TSB for Workout Timing

TSB predicts readiness for hard training or racing:

Practical applications:

• Schedule key workouts when TSB is -5 to +5
• If TSB drops below -20, consider extra rest day
• Target TSB of +15 to +25 for goal races
• If TSB exceeds +25 mid-season, increase training load

Taper Management with TSS

TSS metrics optimize taper: maintain fitness (CTL) while dissipating fatigue (ATL) to peak form (TSB).

2-Week Marathon Taper Example:

CTL drops only 7 points (8%) while ATL plummets 58 points (66%), creating optimal race day freshness.

Learn more about taper strategies in our periodization guide.

Common TSS Mistakes

1. Chasing TSS Numbers

Problem: Treating TSS as a score to maximize rather than a management tool.

Why it's wrong: More TSS is not always better. Excessive TSS without adequate recovery leads to overtraining, injury, and performance decline.

Solution: Use TSS to ensure adequate stimulus while respecting recovery needs. Quality matters more than quantity.

2. Ignoring Intensity Distribution

Problem: Hitting weekly TSS targets with too much Zone 3 "gray zone" training.

Why it's wrong: 500 weekly TSS from mostly Zone 3 work produces worse results than 500 TSS with proper 80/20 distribution. You accumulate fatigue without sufficient hard or easy training stimulus.

Solution: Track TSS by zone. Ensure 75-85% comes from easy running, with hard work truly hard (Zone 4-5).

3. Rapid CTL Ramping

Problem: Increasing CTL by 10+ points per week to "catch up" on fitness.

Why it's wrong: Connective tissue adaptation lags cardiovascular adaptation. Rapid TSS increases cause injury even if you feel aerobically ready for the load.

Solution: Limit CTL increases to 5-8 points per week maximum. Include recovery weeks every 3-4 weeks.

4. Neglecting Individual Response

Problem: Following generic TSS guidelines without adjusting for individual recovery capacity.

Why it's wrong: Recovery varies by age, training history, sleep quality, stress, and genetics. A 500 TSS week may be perfect for one runner and overwhelming for another of the same ability level.

Solution: Use TSS guidelines as starting points. Monitor performance, sleep, motivation, and resting heart rate. Adjust TSS targets based on individual response.

5. Overvaluing External TSS Estimates

Problem: Relying on smartwatch TSS estimates without understanding calculation methods.

Why it's wrong: Many watches use proprietary algorithms that don't align with standard TSS methodology. Some overestimate, others underestimate. Inconsistent calculations make trend tracking unreliable.

Solution: Use a consistent calculation method based on CRS or threshold pace. Our rTSS calculator uses validated methodology for accurate, consistent results.

6. Comparing TSS Between Runners

Problem: Assuming equal TSS means equal training stimulus for different runners.

Why it's wrong: TSS is relative to individual threshold. A 100 TSS workout for an elite runner (e.g., 60 minutes at 5:30/mile threshold pace) differs dramatically from 100 TSS for a recreational runner (60 minutes at 9:00/mile threshold pace).

Solution: Use TSS only for self-comparison over time. Never compare absolute TSS values between athletes.

7. Neglecting Non-Running Stress

Problem: Managing TSS perfectly but ignoring life stress, work demands, and non-running factors.

Why it's wrong: Total stress (not just training stress) determines recovery needs and performance. High life stress + high TSS = overtraining.

Solution: Reduce TSS targets during high-stress life periods. Monitor HRV, sleep quality, and subjective fatigue. Adjust training based on total stress load, not just TSS.

Frequently Asked Questions

Scientific References

The Training Stress Score methodology and concepts presented in this guide are based on peer-reviewed research in exercise physiology and sports science: