Training Load & Fitness

Training load metrics help you understand the balance between training stress and recovery, so you can train smarter and avoid injury or burnout.

What is Training Load?

Every workout creates stress on your body. Your body adapts to this stress over time, building fitness. But if you accumulate too much stress without enough recovery, you risk overtraining, injury, or illness.

Training load metrics quantify this balance using three key numbers: CTL, ATL, and TSB.

The Three Key Metrics

CTL (Chronic Training Load) - Your "Fitness"

CTL represents your long-term training load - essentially, your current fitness level built up over time.

• How it's calculated: A 42-day exponentially-weighted average of your daily training stress
• What it means: Higher CTL = more training capacity = better fitness
• How it changes: Slowly - it takes consistent training over weeks to significantly raise CTL

Think of CTL as your "training bank account" - it grows slowly with consistent deposits (training) and shrinks slowly with inactivity.

ATL (Acute Training Load) - Your "Fatigue"

ATL represents your short-term training load - how tired you are from recent training.

• How it's calculated: A 7-day exponentially-weighted average of your daily training stress
• What it means: Higher ATL = more accumulated fatigue from recent workouts
• How it changes: Quickly - a hard week raises it fast, a rest week drops it fast

Think of ATL as your "fatigue meter" - it spikes after hard training blocks and drops during recovery.

TSB (Training Stress Balance) - Your "Form"

TSB is the difference between your fitness and fatigue (CTL minus ATL). It tells you how ready you are to perform.

• How it's calculated: TSB = CTL - ATL
• Positive TSB: Your fitness exceeds your fatigue - you're fresh and ready to perform
• Negative TSB: Your fatigue exceeds your fitness - you're tired but building fitness

TSB is the key metric for race readiness. Tapers work by reducing ATL (rest) while CTL stays high, creating positive TSB.

TSB Ranges and What They Mean

TSB Range	Status	What's Happening	Recommendation
Above +15	Very Fresh	Fitness well above fatigue - ideal race readiness	Race day! Extended freshness outside a taper can mean detraining
+5 to +15	Fresh	Well-recovered, good energy	Great for hard workouts or B-races
-10 to +5	Normal	Balanced training/recovery	Normal training - sustainable long-term
-25 to -10	Productive	Productive overload - the normal state mid-block	Building fitness - keep easy days easy, protect sleep
-35 to -25	Overreached	Heavy fatigue - fine briefly at the peak of a hard block	Prioritize recovery before your next hard session
Below -35	Severely Fatigued	Very deep fatigue - risk zone	Back off! Rest before illness/injury

How Training Stress is Calculated

Each workout contributes a Training Stress Score (TSS) based on:

• Duration - Longer workouts = more stress
• Intensity - Harder efforts = more stress per minute
• Your fitness - The same workout is less stressful as you get fitter

TSS is calculated using heart rate data relative to your threshold. A 1-hour workout at threshold pace = 100 TSS.

Reading Your Fitness (CTL) Number and Trend

Your CTL number is personal - it's your own average daily training load over roughly the last six weeks, so there is no universal "good" value. A dedicated recreational runner might sit at 40 while an ultra athlete sits at 100; what matters is your direction of travel. The Analytics page summarizes that direction as a trend:

Trend	What's Happening	What It Means
Building	Fitness trending up week-over-week	Training is working - keep the progression sustainable
Holding	Fitness stable	You're maintaining, not building - fine unless you're working toward something
Fading	Fitness slipping week-over-week	Recent training is below what maintains your base - expected in recovery weeks or time off

Practical Applications

Building Fitness Safely

• Increase CTL by 3-7 points per week for sustainable gains
• Allow CTL to drop during recovery weeks (every 3-4 weeks)
• Don't chase CTL at the expense of recovery

Tapering for Races

• Reduce training volume 1-3 weeks before race
• Goal: Positive TSB (+10 to +25) on race day
• Maintain some intensity to keep CTL from dropping too much

Avoiding Overtraining

• Watch for a sustained High Fatigue form label — the bands scale with your fitness, so the label (not a fixed TSB number) is the signal
• Include easy weeks every 3-4 weeks
• If TSB stays very negative, schedule recovery time