CTL, ATL & TSB: Your Fitness Dashboard

CTL (Chronic Training Load) represents your fitness -- a rolling exponential average of your daily training stress over approximately 42 days. ATL (Acute Training Load) represents your fatigue -- a shorter exponential average over approximately 7 days. TSB (Training Stress Balance) is simply CTL minus ATL, and it tells you how fresh or fatigued you are right now. When TSB is positive, you are rested and likely to perform well. When it is deeply negative, you are carrying heavy fatigue. The art of training is growing CTL steadily while managing TSB so that it is positive on race day.

Banister's Fitness-Fatigue Model

In 1991, exercise scientist Eric Banister formalized an idea that coaches had understood intuitively for decades: every training session produces two competing effects. The first is a fitness gain -- your body gets stronger, more efficient, and more resilient. The second is fatigue -- you are tired, your muscles are damaged, and your hormonal system is stressed.

The crucial insight is that these two effects decay at different rates. Fatigue dissipates relatively quickly (half-life of about 7 days), while fitness persists much longer (half-life of about 42 days). This means that after a hard training block, if you rest for a week or two, the fatigue melts away but the fitness remains. That is the entire basis of tapering for a race.

Banister expressed this mathematically as:

Performance = Fitness - Fatigue

The PMC translates this into three trackable metrics.

CTL: Chronic Training Load (Your Fitness)

CTL is an exponentially weighted moving average of your daily training stress, with a time constant of 42 days. Each day, the formula gives slightly more weight to recent training and slightly less to older sessions. In practical terms:

• A session from yesterday has a strong influence on today's CTL.
• A session from 3 weeks ago has a moderate influence.
• A session from 2 months ago has almost no influence.

CTL rises when you train consistently and falls when you rest. It is slow to change in either direction, which is why building fitness requires patience and why a single bad week does not destroy your fitness.

A useful way to think about CTL: if your CTL is 80, it means your body is adapted to handling a daily average training stress of 80 TSS. If you suddenly do a 150 TSS day, that is a significant spike. If you do a 60 TSS day, that is an easy day for you.

ATL: Acute Training Load (Your Fatigue)

ATL uses the same exponential formula but with a time constant of 7 days. It reacts quickly to recent training. A massive weekend of training will spike your ATL within days. Three easy days in a row will drop it noticeably.

ATL represents how tired you are right now. High ATL after a big training week is expected and normal. The problem comes when ATL stays elevated for weeks without relief -- that is the path to overreaching and eventually overtraining.

TSB: Training Stress Balance (Your Freshness)

TSB is the simplest metric of the three:

TSB = CTL - ATL

When CTL is greater than ATL (you have trained consistently but your recent load is moderate), TSB is positive. You feel fresh, your legs have snap, and you are likely to perform well. When ATL exceeds CTL (you have been training harder than your body is used to), TSB is negative. You feel heavy, sluggish, and your performance suffers.

Interpreting TSB values:

• TSB above +25: Very rested, possibly losing fitness if maintained too long. This is common during an extended taper or a vacation from training.
• TSB +10 to +25: Fresh and race-ready. This is the target range for race day.
• TSB 0 to +10: Slightly fresh. Good for testing sessions or tune-up races.
• TSB -10 to 0: Mildly fatigued. Normal training state for a moderate week.
• TSB -10 to -30: Functionally overreaching. This is where adaptation happens during hard training blocks, but you should not stay here for more than 2-3 weeks without a recovery period.
• TSB below -30: Deep fatigue. Risk of illness, injury, and non-functional overreaching increases significantly. If you find yourself here, back off immediately.

How to Read the PMC Chart

On a Performance Management Chart, you will typically see three lines:

• CTL (blue): A gradually rising or falling line showing your long-term fitness trend. Ideally, this climbs steadily during a training block.
• ATL (pink/red): A jagged, more volatile line showing recent fatigue. It spikes during hard weeks and drops during recovery.
• TSB (yellow/gold): An oscillating line that mirrors the relationship between the other two. It dips below zero during training blocks and rises above zero during recovery weeks and tapers.

The healthy pattern looks like a sawtooth wave: TSB drops during 2-3 weeks of building load, then pops back up during a recovery week. Over time, CTL rises while the TSB oscillations become a familiar rhythm.

Worked Example: 12-Week Ironman Build

Here is how CTL, ATL, and TSB might evolve during a 12-week Ironman build for an athlete starting at a CTL of 70.

Key observations from this build:

CTL grew from 70 to 88 over the peak training weeks. That is a 26% increase in fitness, achieved through patient, progressive loading. The athlete is significantly fitter than when they started.

TSB during training blocks ranged from -11 to -20. This is the functional overreaching zone -- hard enough to drive adaptation but not so deep as to risk breakdown. The recovery weeks in Weeks 4 and 8 brought TSB back to positive territory (+14 and +16), allowing the body to consolidate gains.

Race-day TSB of +39 in Week 12 might seem too high. In practice, for an Ironman (a long, sub-threshold event), some coaches prefer a TSB of +15 to +25. A TSB of +39 suggests the taper might have been slightly too aggressive. For shorter, more explosive events like an Olympic triathlon or a criterium, a TSB of +5 to +15 is often more appropriate because you want snap in the legs without being "flat" from too much rest.

The CTL decline during taper (88 to 85) is minimal. This is the beauty of the exponential decay -- fitness erodes slowly. Two weeks of reduced training costs only 3 CTL points while shedding enormous fatigue. This is why tapers work.

Practical Rules

1. Grow CTL at no more than 5-7 points per week. Faster CTL growth almost always means unsustainable training spikes. A 12-week block that raises CTL by 20-30 points is a highly productive training period.

2. Avoid sustained TSB below -30. Brief dips into this range (during a training camp, for example) are acceptable, but spending more than 1-2 weeks there increases the risk of illness and injury significantly.

3. Plan recovery weeks to bring TSB back to positive. A recovery week is not wasted time. It is where your body converts training stress into actual fitness. Aim for TSB of +5 to +20 during recovery weeks.

4. Target race-day TSB based on event duration. For short, explosive events (5K, sprint triathlon, criterium): TSB of 0 to +15. For medium events (half-marathon, Olympic triathlon): TSB of +10 to +20. For long events (marathon, Ironman): TSB of +15 to +30.

5. Do not chase CTL for its own sake. A higher CTL is not always better. What matters is that your CTL reflects productive, well-distributed training. Junk volume inflates CTL without improving performance.

6. Use TSB trends, not single-day values. TSB can fluctuate day to day based on one hard or easy session. Look at the weekly trend line to gauge your readiness.

7. Remember that CTL does not distinguish between disciplines. For triathletes, a CTL of 100 built mostly from easy cycling is very different from a CTL of 100 built from a balanced swim-bike-run program. Consider tracking discipline-specific CTL if your platform supports it.

Evidence Base

The fitness-fatigue model originated with Banister's 1991 chapter "Modeling Elite Athletic Performance," where he proposed that performance at any point in time could be predicted by the difference between accumulated fitness and accumulated fatigue, each decaying exponentially. The model was originally validated using swimming and weightlifting data, showing strong correlations between predicted and actual performance.

Busso (2003) refined the model by introducing variable dose-response relationships, acknowledging that the same training load can produce different effects depending on an athlete's current state. An athlete who is fresh will gain more fitness from a hard session than one who is already deeply fatigued. This non-linearity is one reason why the PMC is a useful guide rather than a precise predictor.

Jobson et al. (2009) brought the model into practical cycling training, demonstrating how CTL, ATL, and TSB tracking could inform taper strategies for competitive cyclists. Their analysis showed that athletes who achieved appropriate TSB values on race day (positive, but not excessively so) tended to produce better performances than those who were either under-tapered (negative TSB) or over-tapered (TSB above +30).

It is worth noting that CTL, ATL, and TSB are models, not measurements. They estimate fitness and fatigue based on training load inputs, but they cannot account for sleep quality, nutrition, psychological stress, or the specific physiological demands of different sessions. A 100-TSS tempo ride and a 100-TSS VO2max interval session produce the same TSS but very different physiological responses. The PMC is most useful when combined with subjective monitoring (how do you feel?) and objective markers (resting heart rate, HRV, sleep data). Treat it as one valuable lens among several, not as an oracle.