CTL, ATL, and TSB Explained: Understanding Your Training Load

The Fitness-Fatigue Model

Before diving into the metrics themselves, it helps to understand the theory behind them. Every workout you do produces two simultaneous effects on your body: a fitness gain and a fatigue cost. The fitness effect is positive — your cardiovascular system adapts, your muscles grow stronger, your running economy improves. The fatigue effect is negative — your legs are sore, your glycogen stores are depleted, your nervous system is taxed.

Here's the critical insight: these two effects operate on different timescales. Fitness builds slowly and decays slowly. It takes weeks of consistent training to meaningfully raise your aerobic capacity, and it takes weeks of inactivity to lose it. Fatigue, on the other hand, builds quickly and dissipates quickly. A brutal tempo run on Tuesday leaves you wrecked on Wednesday, but by Friday you feel mostly recovered.

This asymmetry is the reason tapering works. When you reduce your training volume before a race, you stop piling on new fatigue. But because fitness decays so much more slowly than fatigue, your fitness level barely drops during a one- or two-week taper. The result: fatigue falls away while fitness stays high, and you arrive at the start line feeling both fit and fresh.

This concept was formalized by Dr. Eric Banister in 1975 as the impulse-response model (sometimes called the fitness-fatigue model). Banister showed that you could model an athlete's performance at any point in time by subtracting the accumulated fatigue effect from the accumulated fitness effect. CTL, ATL, and TSB are the practical, day-to-day implementation of Banister's model — and they've become standard tools in endurance sports from cycling to running to triathlon.

Chronic Training Load (CTL) — Your Fitness

CTL stands for Chronic Training Load, and it represents your long-term training fitness. Technically, it's an exponentially weighted moving average (EWMA) of your daily TSS values calculated over approximately 42 days. In plain English: CTL is a rolling measure of how much training stress your body has absorbed and adapted to over the past month and a half.

Because CTL uses a 42-day window, it's inherently slow-moving. A single hard workout barely nudges your CTL. A week off doesn't crater it. This matches the biological reality of aerobic fitness — it's the product of sustained, cumulative work, not any one session. Think of CTL as the answer to the question: "How fit am I right now, based on what I've consistently been doing?"

How CTL Behaves in Practice

If you average 50 TSS per day for six weeks, your CTL will stabilize around 50. If you suddenly stop training entirely, your CTL doesn't drop to zero overnight — it decays at roughly 3-5% per week. This is why you don't lose all your fitness during a recovery week or even a short injury layoff. Conversely, it's why building a high CTL takes patience: you're essentially stacking day after day of consistent work until the moving average climbs.

A runner with a CTL of 60 is objectively fitter (in terms of recent training load) than when their CTL was 40. But a crucial caveat: CTL is relative to you. A CTL of 80 for an elite marathoner running 120 km per week means something entirely different than a CTL of 80 for a recreational runner. The number only has meaning in the context of your own training history.

PeakRunner calculates your CTL automatically from your synced activity data, so you can track your fitness trend on your dashboard without doing any math yourself. Watching your CTL gradually rise across a training block is one of the most satisfying things in endurance sport — it's tangible proof that the work is adding up.

Acute Training Load (ATL) — Your Fatigue

ATL stands for Acute Training Load, and it represents your short-term fatigue. Like CTL, it's an exponentially weighted moving average of your daily TSS — but over a much shorter window of approximately 7 days. ATL answers the question: "How much have I been training in the past week?"

Because the window is short, ATL is highly responsive to changes in your training. Do a monster long run on Saturday and your ATL will jump noticeably by Sunday. Take three days off and it drops fast. This makes ATL a useful real-time indicator of how much stress you've recently loaded onto your body.

ATL in Different Training Phases

Consider this scenario: you've been resting for a week (ATL near zero), and then you run hard sessions of 100 TSS each for three consecutive days. Your ATL will spike rapidly — perhaps to 70 or 80 — while your CTL barely moves because it's averaging over 42 days. This extreme divergence between ATL and CTL is a red flag: you've loaded a lot of acute stress onto a body that isn't conditioned for it.

During a heavy training block, your ATL will typically be higher than your CTL. This is normal and expected — you're deliberately accumulating fatigue to force adaptation. During a taper phase, the opposite happens: you reduce training volume so ATL drops below CTL. Fatigue dissipates while fitness remains. This is the physiological setup for peak performance on race day.

High ATL isn't inherently bad — it simply means you've been working hard recently and your body needs adequate recovery. The danger comes when ATL stays elevated for too long without recovery, which we'll explore in the TSB section below.

Training Stress Balance (TSB) — Your Form

TSB is where everything comes together. The formula is simple:

TSB tells you how ready you are to perform right now. When your fitness exceeds your recent fatigue (positive TSB), you're fresh and primed. When recent fatigue exceeds your fitness (negative TSB), you're in a fatigued state. Neither is inherently good or bad — it depends on where you are in your training cycle.

TSB Ranges and What They Mean

• TSB +5 to +25 (race-ready zone): You're fresh and fit. This is the sweet spot for racing. A well-executed taper will land you here on race day.
• TSB 0 to -10 (productive training): Slight fatigue from recent work, but nothing concerning. You can train normally.
• TSB -10 to -30 (heavy training): You're accumulating fatigue in a build phase. This is expected and productive, but you'll need a recovery period soon to absorb the gains.
• TSB below -30 (danger zone): You're carrying a significant fatigue debt. Risk of overreaching, breakdown, or injury increases sharply. Back off and recover.
• TSB above +30 (detraining): You've been resting for too long. Fitness is beginning to decay. Time to resume structured training before you lose the gains you've built.

The common mistake is thinking that the highest possible TSB is the goal. It's not. A TSB of +40 means you've been doing almost nothing for weeks — you might feel fresh, but you're also losing fitness. The ideal race-day TSB is typically +5 to +15: enough rest to shed fatigue, not so much rest that fitness erodes.

A Real-World Example

Let's walk through a four-week training block to see how CTL, ATL, and TSB interact in practice. Imagine a runner starting with a CTL of approximately 42 from their previous training cycle.

Notice what happened across this cycle: the runner started with a CTL of 42 and finished at 50 — a meaningful fitness gain of 8 points. And thanks to the recovery week, they ended with a positive TSB of +21, meaning they're both fitter AND fresher than when they started. This is periodization in action: strategically alternating stress and recovery to build fitness while managing fatigue.

Without these metrics, the runner would be relying entirely on feel — which is important but unreliable. Metrics like CTL and TSB give you objective confirmation that your training structure is working.

How PeakRunner Uses These Metrics

PeakRunner calculates CTL, ATL, and TSB automatically from your Strava data, updating in real time as new activities sync. Your PeakRunner dashboard displays a clear visualization of your fitness, fatigue, and form trends so you always know exactly where you stand — no spreadsheets or manual calculations needed.

But PeakRunner doesn't just display these numbers passively. The AI coaching engine actively uses TSB to calibrate your upcoming workouts. When your TSB is negative (indicating accumulated fatigue), PeakRunner prescribes easier sessions — recovery runs, lighter intervals, or rest days — to prevent you from digging too deep. When your TSB turns positive (indicating freshness), the AI recognizes that you're ready to handle more intensity and programs appropriately challenging sessions.

There are also automated safety guardrails built in. When your TSB drops below -30, PeakRunner automatically reduces your upcoming workload to pull you back from the overtraining danger zone. Similarly, your CTL growth rate is continuously monitored: if it rises faster than 5-7 points per week, the system flags elevated injury risk and adjusts your plan accordingly. Rapid CTL increases are one of the most reliable predictors of injury in endurance athletes, and catching them early is far better than dealing with a stress fracture.

Common Misconceptions

These metrics are powerful, but they're often misunderstood. Here are the most common mistakes runners make when interpreting CTL, ATL, and TSB:

"Higher CTL Is Always Better"

This is perhaps the most dangerous misconception. It's tempting to chase an ever-higher CTL as proof that you're getting fitter. But CTL needs to be sustainable for YOUR body, your recovery capacity, and your life circumstances. A CTL of 90 is meaningless if maintaining it requires you to skip sleep, neglect recovery, and run yourself into the ground. The goal is the right CTL for your goals — high enough to support your target race performance, low enough to be sustainable across a full training cycle. For most recreational runners training for a marathon, a CTL in the 50-70 range represents serious, committed training.

"Negative TSB Means I'm Doing Something Wrong"

Absolutely not. Training requires controlled fatigue. If your TSB were always positive, you'd never be training hard enough to improve. A TSB of -10 to -20 during a build phase is completely normal and indicates productive training. The key is that negative TSB phases are followed by recovery periods that bring TSB back toward zero or slightly positive. It's the pattern of stress and recovery that matters, not any single day's TSB value.

"I Should Race When TSB Is Highest"

Not exactly. The optimal race-day TSB for most runners is +5 to +15 — enough freshness to perform without having rested so long that fitness begins to erode. A TSB of +30 or higher often correlates with flat, uninspired race performances because the extended rest has started to undo some of the fitness adaptations you worked so hard to build. The art of tapering is finding the TSB sweet spot where fatigue has dissipated but fitness remains sharp.

"CTL, ATL, and TSB Replace Listening to My Body"

These metrics are tools that complement subjective feedback, not replace it. Some days your TSB says you should be fresh, but your legs feel dead anyway — maybe you didn't sleep well, or you're fighting off a cold. Other days your TSB looks terrible but you feel surprisingly good. Always use metrics as one input among many. The best approach is to cross-reference the numbers with how you actually feel: when both the data and your body agree, you can be highly confident in your training decisions.