Running Training Load: rTSS, CTL, ATL, and TSB Explained
How to quantify run stress, track fitness and fatigue, and time a taper without guessing
Quick Answer
Running training load combines rTSS for individual-session stress, CTL for long-term fitness, ATL for short-term fatigue, and TSB for freshness. It works best when your Critical Running Speed (CRS), training zones, and pace data are realistic.
- Best use: compare workouts, control weekly load, and time taper weeks
- Most important input: a believable CRS or threshold pace
- Biggest mistake: treating TSB as race truth when sleep, soreness, heat, and life stress say otherwise
- Update rhythm: retest CRS every 6-8 weeks or after a meaningful training block
Key Takeaways
- rTSS answers how much stress a single run created
- CTL shows the load your body has adapted to over weeks, not days
- ATL shows what is still in your legs from recent training
- TSB helps with taper timing and readiness, not ego management
- The cluster that matters:CRS, rTSS calculator, training zones, and periodization
If you want one system that turns running volume and intensity into decisions, use training load. The point is not to collect more numbers. The point is to know whether a run was easy enough, a block was productive enough, or a taper is actually making you fresh.
For runners, the practical questions are simple:
- How stressful was today's run?
- How much load have I actually adapted to?
- Am I carrying useful fatigue or too much fatigue?
- When should I push, when should I back off, and when should I race?
What Running Training Load Measures
Running training load is a practical framework for turning runs into interpretable stress signals. In this framework, rTSS measures session cost, CTL measures long-term fitness trend, ATL measures recent fatigue, and TSB shows the gap between the two.
| Metric | What it tells you | Best use | Common mistake |
|---|---|---|---|
| rTSS | The stress cost of one run | Compare workouts and total weekly load | Scoring runs against outdated CRS |
| CTL | The load you have adapted to over weeks | Track long-term fitness and capacity | Expecting it to move meaningfully in a few days |
| ATL | The fatigue still in your legs from recent work | Manage recovery between hard sessions | Ignoring impact damage, soreness, and life stress |
| TSB | Your freshness gap: fitness minus fatigue | Taper timing, testing windows, race readiness | Using one positive number as proof you are ready |
Useful Mental Model
Think of rTSS as today's cost, CTL as your fitness bank, ATL as your fatigue bill, and TSB as your freshness gap. The value of the system is in the trend, not one isolated number.
What Training Load Is Good For and Where It Fails
📊 Compare Workouts
rTSS lets you compare an easy long run, threshold session, and hill workout on one scale instead of guessing by feel alone.
🧭 Control Build Weeks
CTL and ATL help you decide whether a block is moving fitness forward or just stacking fatigue before it turns into poor sessions or injury risk.
🏁 Time the Taper
TSB becomes useful when you connect it to planned periodization, not when you stare at the chart without a race context.
⚠️ Important: Running's Higher Impact
The same TSS value represents more physical stress in running than cycling due to impact forces. A runner accumulating 400 TSS/week experiences similar training adaptation to a cyclist with 600-700 TSS/week. Running training load must account for both cardiovascular and musculoskeletal stress.
A positive TSB does not guarantee race-day form, and a negative TSB does not mean failure. Load models do not fully capture sleep debt, soreness, fueling errors, heat, or technique breakdown. Keep load numbers tied to efficiency metrics, subjective feel, and actual workout quality.
How Running Training Stress Score (rTSS) Is Calculated
The Formula for Running TSS
Where Intensity Factor (IF) is:
Normalized Graded Speed (NGS) accounts for pace variability and elevation:
📊 Pace-Based Simplified Method
For runners without power meters, use pace-based calculation or score the session with the rTSS calculator:
Example: If your CRS pace is 4:00/km and you run at 4:20/km:
IF = 4:00 / 4:20 = 240/260 = 0.923
Note: Slower pace = lower IF (inverted from speed calculation)
Worked Example: Tempo Run
Runner Profile:
- CRS Pace: 4:00/km (15 km/hr)
- CRS: Threshold pace for 1 hour
Workout Data:
- Total Distance: 10 km
- Moving Time: 43:20 (0.722 hours)
- Average Pace: 4:20/km
Step 1: Calculate Average Speed
Speed = 13.85 km/hr
Step 2: Calculate IF (Speed Method)
IF = 0.923
Step 3: Calculate IF (Pace Method)
IF = 0.923 ✓
Step 4: Calculate rTSS
rTSS = 0.852 × 0.722 × 100
rTSS = 61.5
Interpretation: This 10km tempo run at threshold-10% pace generated 61.5 TSS—a moderate training stimulus typical of quality aerobic sessions.
Use the rTSS calculator if you want the score without doing the math manually.
Running TSS Intensity Guidelines
| TSS Range | Intensity Level | Description | Example Workouts |
|---|---|---|---|
| < 50 | Easy Recovery | Light running, conversational pace, active rest | 30-45min easy run, recovery jog, strides |
| 50-100 | Moderate Training | Typical daily training volume, aerobic base | 60-90min steady run, long easy run |
| 100-200 | Hard Training | Quality sessions with tempo/threshold work | 90-120min with threshold intervals, progression runs |
| 200-300 | Very Hard | Long runs with quality, race simulation | 2-3hr long runs, half marathon pace work |
| > 300 | Extreme | Race day, ultra-distance events | Marathon racing, 50k+ ultras, Ironman runs |
📊 Weekly TSS Targets by Running Level
- Beginner Runners: 150-300 TSS/week (3-4 runs/week)
- Intermediate Runners: 300-500 TSS/week (4-6 runs/week)
- Advanced Runners: 500-700 TSS/week (6-8 sessions/week)
- Elite Runners: 700-900+ TSS/week (8-12 sessions/week)
These accumulate toward your Chronic Training Load (CTL), the fitness metric explained below. They should still be distributed through realistic training zones and a sensible periodization plan.
Understanding CTL, ATL, and TSB for Runners
The Performance Management Chart (PMC) visualizes three interconnected metrics that tell the complete story of your running training: fitness, fatigue, and form.
CTL - Chronic Training Load
42-day exponentially weighted average of daily TSS. Represents long-term aerobic fitness and training adaptation from consistent running.
ATL - Acute Training Load
7-day exponentially weighted average of daily TSS. Captures recent training stress and accumulated fatigue from the past week of running.
TSB - Training Stress Balance
Difference between yesterday's fitness and fatigue. Indicates readiness to perform or need to rest before your next quality run or race.
CTL: Your Running Fitness Metric
What CTL Represents for Runners
CTL quantifies the running training load your body has adapted to over the past 6 weeks. A higher CTL in running means:
- Greater aerobic capacity and running endurance
- Ability to handle more training volume and intensity
- Improved neuromuscular and metabolic adaptations
- Higher sustainable running performance
- Better tolerance to impact stress from ground contact forces
Time Constant: 42 Days
CTL has a half-life of ~14.7 days. After 42 days, approximately 36.8% (1/e) of a single workout's impact remains in your fitness calculation.
This slow decay means running fitness builds gradually but also fades slowly—protecting against detraining during short recovery breaks.
Typical CTL Values for Runners
Building base fitness, 3-4 runs/week
Consistent training, 4-6 runs/week
High volume, 6-8 sessions/week
Professional training load, 8-12+ sessions/week
- Beginners: +2-4 CTL per week
- Intermediate: +3-5 CTL per week
- Advanced: +5-7 CTL per week
Exceeding these rates significantly increases injury risk due to running's high impact stress. The "10% rule" (increase weekly volume by no more than 10%) aligns with these CTL ramp rates.
ATL: Your Running Fatigue Metric
ATL tracks short-term training stress—the fatigue accumulated in the past week of running. It rises quickly after hard training and drops quickly during rest, making it essential for managing recovery between quality sessions.
ATL Dynamics in Running Training
- Fast Response: 7-day time constant (half-life ~2.4 days)
- Spiky Pattern: Jumps after hard sessions, drops during recovery days
- Recovery Indicator: Falling ATL = dissipating fatigue between runs
- Overtraining Warning: Chronically elevated ATL suggests inadequate recovery
- Impact Stress: Running ATL reflects both cardiovascular and musculoskeletal fatigue
🔬 The Fitness-Fatigue Model for Running
Every training run produces two effects:
- Fitness stimulus (slow-building, long-lasting)
- Fatigue (fast-building, fast-dissipating)
Running Performance = Fitness - Fatigue. The PMC visualizes this model, enabling scientific periodization and optimal race timing for runners.
At Steady State
When running training load is consistent week-to-week, CTL and ATL converge:
Example: 400 TSS/week consistently
CTL approaches ~57
ATL approaches ~57
TSB approaches 0
Interpretation: Fitness and fatigue are balanced. No accumulating deficit or surplus—sustainable training maintenance.
During Build Phases
When increasing running volume or intensity:
ATL rises faster than CTL due to shorter time constant. TSB becomes negative (fatigue > fitness). This is normal and productive—you're applying overload to stimulate running adaptations.
During Taper for Races
When reducing training load before competition:
ATL drops faster than CTL. TSB becomes positive (fitness > fatigue). This is the goal—arrive at race day fresh with legs ready to perform while retaining fitness.
TSB: Your Fitness-Fatigue Balance and Race Readiness
TSB (Training Stress Balance) is the difference between yesterday's fitness (CTL) and yesterday's fatigue (ATL). For runners, it indicates whether you're fresh or fatigued, ready to race or in need of recovery days.
TSB Interpretation Guide for Runners
| TSB Range | Status | Interpretation | Recommended Action |
|---|---|---|---|
| < -30 | Overtraining Risk | Extreme fatigue. High injury risk. | Immediate recovery needed. Reduce volume 50%+. Consider rest days. |
| -20 to -30 | Optimal Training Block | Productive overload. Building fitness. | Continue plan. Monitor for excessive soreness or fatigue signs. |
| -10 to -20 | Moderate Training Load | Standard training accumulation. | Normal training. Can handle quality interval or tempo sessions. |
| -10 to +15 | Transition/Maintenance | Balanced state. Light fatigue or freshness. | Good for B/C races, testing, or recovery weeks. |
| +15 to +25 | Peak Race Form | Fresh and fit. Optimal performance window. | A-priority races. Expected peak running performance. |
| +25 to +35 | Very Fresh | Highly rested. Good for short races. | 5K-10K races, time trials, track events. |
| > +35 | Detraining | Losing fitness from inactivity. | Resume training. Fitness declining from extended rest. |
🎯 Target TSB by Running Race Distance
- 5K/10K Races: TSB +20 to +30 (7-10 day taper for freshness)
- Half Marathon: TSB +15 to +25 (10-14 day taper)
- Marathon: TSB +10 to +20 (14-21 day taper with volume reduction)
- Ultra Marathon (50K+): TSB +5 to +15 (7-14 day taper, maintain volume but reduce intensity)
Shorter races need higher TSB for speed/power. Longer races need moderate TSB to retain endurance.
PMC Example: Running Training Block → Taper → Race
12-Week Marathon Training Cycle
Weeks 1-3: Base Building Phase
- Weekly TSS: 300 → 350 → 400
- CTL: Gradually rises from 40 → 48
- ATL: Tracks weekly load, fluctuates 45-60
- TSB: Slight negative (-5 to -15), manageable training stress
- Focus: Aerobic base development, easy miles
Weeks 4-7: Build Phase 1
- Weekly TSS: 450 → 500 → 500 → 450 (recovery week)
- CTL: Continues rising 48 → 60
- ATL: Higher fluctuations 55-75
- TSB: More negative (-15 to -25), productive overload
- Focus: Adding tempo runs, threshold intervals
Weeks 8-11: Peak Phase
- Weekly TSS: 500 → 550 → 550 → 500
- CTL: Peaks at ~65
- ATL: Highest values 65-80
- TSB: Most negative (-20 to -30), maximum fitness stimulus
- Focus: Long runs with marathon pace segments, quality sessions
Week 12: Recovery Week
- Weekly TSS: 300 (45% reduction)
- CTL: Slight dip to ~63 (fitness retained)
- ATL: Drops rapidly to ~50
- TSB: Rises to +5 to +10 (partial freshness)
- Focus: Recovery runs, absorbing training adaptations
Week 13-14: Taper + Race Week
- Week 13 TSS: 250, Week 14 TSS: 120 + race (~250 total)
- CTL: Gentle decline to ~60 (minimal fitness loss)
- ATL: Rapid drop to ~35 (fatigue eliminated)
- TSB: Peaks at +20 to +25 on race day
- Result: Fresh legs, retained fitness, ready for peak marathon performance
✅ Why Taper Works for Runners
The different time constants (42 days for CTL, 7 days for ATL) create the taper effect that optimizes running performance:
- ATL responds quickly → Running fatigue (both cardiovascular and musculoskeletal) disappears within 7-10 days
- CTL responds slowly → Aerobic fitness and running adaptations persist for weeks
- Result: Fitness remains while fatigue and soreness vanish = fresh legs for peak race performance
Practical Running Training Load Management Guidelines
1️⃣ Track Daily rTSS Consistently
Log every run's TSS to build accurate CTL/ATL/TSB trends. Missing data creates gaps in the fitness curve. Use GPS watch data or pace-based calculations for all runs.
2️⃣ Monitor CTL Ramp Rate Carefully
Increase CTL gradually. A 3-5 point weekly increase is sustainable for most runners. Jumping 10+ points invites overtraining and injury due to running's impact stress.
3️⃣ Schedule Recovery Weeks
Every 3-4 weeks, reduce running volume 30-40% for one week. Let TSB rise to -5 to +10. This consolidates fitness adaptations and prevents cumulative fatigue.
4️⃣ Time Your Race Taper
Target TSB +15 to +25 on race day for most distances. Begin taper 10-21 days out depending on race distance and current TSB level.
5️⃣ Accept Negative TSB During Build
TSB of -20 to -25 during build phases is normal and productive for runners. It means you're applying training stimulus for adaptation without excessive injury risk.
6️⃣ Rebuild CTL Gradually After Breaks
After injury or off-season breaks, don't try to resume at previous CTL immediately. Rebuild gradually at 3-5 CTL points per week to avoid re-injury.
7️⃣ Listen to Your Body
TSS metrics are guides, not absolute rules. Persistent soreness, poor sleep, or declining performance despite good TSB numbers indicate need for additional recovery.
8️⃣ Account for Non-Running Stress
Life stress, poor sleep, and cross-training affect recovery. Consider reducing TSS targets during high-stress periods even if running-specific metrics look good.
Frequently Asked Questions: Training Stress Score for Running
How do I calculate TSS for running without a power meter?
Use the pace-based method: Calculate your Intensity Factor (IF) by dividing your threshold pace by your average pace for the run. Then apply the formula: rTSS = (IF²) × Duration (hours) × 100. For example, if your threshold pace is 4:00/km and you run 10km at 4:30/km pace in 45 minutes (0.75 hours): IF = 4:00/4:30 = 0.889, so rTSS = (0.889²) × 0.75 × 100 = 59.2 TSS.
If you want the shortest workflow, use the rTSS calculator after you set a realistic CRS.
What's the difference between running TSS and cycling TSS?
Both use the same IF² formula, but running TSS represents more total physical stress due to impact forces from ground contact. A runner with 400 TSS/week experiences similar training adaptation to a cyclist with 600-700 TSS/week. Running also uses pace/speed as input instead of power watts, and requires more recovery time for the same TSS value.
What's a good weekly TSS for marathon training?
Weekly running TSS for marathon training varies by experience: Beginners building to first marathon: 250-400 TSS/week; Intermediate marathoners: 400-550 TSS/week; Advanced/competitive marathoners: 550-700+ TSS/week. Peak weeks may reach 600-800 TSS for advanced runners, with taper weeks dropping to 200-300 TSS.
What matters more than one number is how that load sits inside your marathon build and periodization.
What TSB should I target for a 10K race?
For a 10K race, target TSB of +20 to +30 on race day. Begin a 7-10 day taper to achieve this. The higher TSB (more freshness) is beneficial for shorter races where speed and power matter more than endurance. Start reducing volume 10 days out while maintaining some intensity to preserve neuromuscular sharpness.
Can I use TSS if I don't know my Critical Running Speed?
Knowing your Critical Running Speed (CRS) or threshold pace is essential for accurate TSS calculation since IF is calculated relative to this value. If you don't know your CRS, perform a threshold test: either a 30-minute all-out time trial (average pace = CRS) or an 8km race pace. Alternatively, estimate using recent race times: 10K pace + 10-15 seconds/km approximates CRS.
Once CRS is set, convert it into training zones so your easy, threshold, and race-pace work all feed the same load model.
How fast should I increase my CTL?
Increase running CTL by 2-4 points per week for beginners, 3-5 points for intermediate runners, and 5-7 points maximum for advanced runners. This aligns with the "10% rule" for weekly mileage increases and minimizes injury risk. Exceeding these rates significantly increases overtraining and injury risk due to running's high impact stress on muscles, tendons, and bones.
What does negative TSB mean during training?
Negative TSB means fatigue (ATL) exceeds fitness (CTL)—you're currently fatigued from recent training. This is normal and productive during build phases. TSB of -15 to -25 indicates productive training stress. TSB below -30 suggests overtraining risk and need for recovery. Negative TSB during builds creates the stimulus for fitness adaptations while positive TSB during tapers allows performance.
Use Training Load as Your Running Metrics Hub
Training load becomes genuinely useful when it is connected to threshold pace, session scoring, training zones, and race planning. On its own, the chart is just a chart.
Build the cluster around this page
If training load is your hub, the next pages to use are:
- Critical Running Speed (CRS) to anchor threshold pace
- rTSS Calculator to score individual runs
- Training Zones to convert CRS into session intent
- Training Load and Periodization to plan build, recovery, and taper blocks
Running Training Load: rTSS, CTL, ATL, and TSB Explained
Running training load uses rTSS to score sessions, CTL to track long-term fitness, ATL to track short-term fatigue, and TSB to gauge freshness for training and taper decisions.
- 2026-04-02
- running training load · rTSS · CTL ATL TSB running · running fatigue and fitness · running taper timing
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