Ae/An TISS: Why Identical TSS Is Not Identical Stress

TSS (Training Stress Score) is a powerful metric, but it has a blind spot: it treats all stress as equal. A kilojoule of work below threshold and a kilojoule above threshold get the same weighting in the TSS formula. AeTISS (Aerobic Training Impact Scoring System) and AnTISS (Anaerobic Training Impact Scoring System) solve this by splitting the total training stress into two distinct buckets. AeTISS captures the load placed on your aerobic system -- slow-twitch muscle fibers, mitochondria, cardiovascular capacity. AnTISS captures the load on your anaerobic system -- fast-twitch fibers, phosphocreatine stores, glycolytic pathways. Each system fatigues differently, recovers on a different timeline, and produces different adaptations. Tracking them separately gives you a far more honest picture of what your training is actually doing to your body.

Why TSS Is Blind

The TSS formula, developed by Andrew Coggan, normalizes every workout to a single number based on intensity and duration relative to your FTP. It is elegant and useful, but it makes one critical assumption: that a minute of Zone 2 riding and a minute of Zone 6 sprinting, when normalized to FTP, produce equivalent physiological stress. They do not.

A Zone 2 session primarily stresses slow-twitch muscle fibers, the cardiovascular system, and aerobic metabolic pathways. Recovery takes 24-36 hours. The adaptations produced -- mitochondrial density, capillarization, fat oxidation -- accumulate gradually and require high volume.

A VO2max interval session recruits fast-twitch fibers, depletes phosphocreatine and glycogen stores, generates significant neuromuscular fatigue, and elevates stress hormones. Recovery takes 48-72 hours. The adaptations -- lactate buffering, VO2max expansion, neuromuscular power -- are potent but require adequate recovery between doses.

When you mash both into a single TSS number, you lose the ability to distinguish between these fundamentally different types of stress. Two athletes can have identical weekly TSS, but if one achieved it through steady volume and the other through repeated high-intensity sessions, their recovery needs and adaptation trajectories are worlds apart.

How AeTISS and AnTISS Work

The split is based on the Critical Power (CP) boundary. Any work performed below CP contributes primarily to AeTISS. Work above CP contributes primarily to AnTISS. The key difference is in how each is weighted:

AeTISS uses a roughly linear weighting for sub-CP intensities. Riding at 90% of CP produces proportionally more aerobic stress than riding at 60% of CP. The relationship is straightforward: more aerobic work equals more aerobic stress.

AnTISS uses a quadratic (or exponential) weighting for supra-CP intensities. This means that riding at 120% of CP does not produce 20% more anaerobic stress than riding at CP -- it produces substantially more, because the metabolic cost of supra-CP work escalates nonlinearly. A 3-minute effort at 130% of CP generates far more AnTISS per second than a 3-minute effort at 105% of CP, even though the power difference is only 25%.

This weighting reflects physiology. The fatigue produced by supra-threshold work is disproportionate to the power difference because it involves fundamentally different and more costly metabolic processes.

Recovery Timelines

This is where the split becomes practical. AeTISS fatigue recovers within 24-48 hours for most athletes. You can stack Zone 2 sessions on consecutive days without meaningful performance degradation. Your legs might feel heavy, but the aerobic system is not compromised.

AnTISS fatigue takes 48-72 hours to clear. Stack two VO2max sessions on consecutive days and you will see it immediately: the second session's power numbers are down 5-10%, heart rate is elevated, and the subjective effort feels far worse. You are not weaker. You are spending from an anaerobic account that has not been replenished.

Phase-Appropriate Ratios

The balance between AeTISS and AnTISS should shift across your training season:

• Base Phase: AeTISS 85-90%, AnTISS 10-15%. You are building the aerobic engine. High-intensity work is minimal, just enough to maintain neuromuscular recruitment patterns.
• Build Phase: AeTISS 72-78%, AnTISS 22-28%. Intensity increases as you introduce threshold and VO2max work, but the aerobic volume remains the foundation.
• Peak Phase: AeTISS 58-65%, AnTISS 35-42%. Intensity is at its highest proportion, sharpening race-specific fitness. Total volume often drops to accommodate the higher recovery demands of AnTISS.
• Taper: AeTISS 70-80%, AnTISS 20-30%. Volume drops significantly, but short, sharp intensity sessions maintain neuromuscular sharpness while allowing deep recovery.

Worked Example: Same TSS, Different Worlds

Week A -- Base Phase Focus: - Monday: Rest - Tuesday: 90 min Z2 bike (TSS 75, AeTISS 70, AnTISS 5) - Wednesday: 60 min Z2 run (TSS 55, AeTISS 50, AnTISS 5) - Thursday: 90 min Z2 bike (TSS 75, AeTISS 70, AnTISS 5) - Friday: 45 min Z1 swim (TSS 35, AeTISS 33, AnTISS 2) - Saturday: 2.5 hr Z2 bike (TSS 120, AeTISS 110, AnTISS 10) - Sunday: 75 min Z2 run (TSS 70, AeTISS 65, AnTISS 5)

Week A totals: TSS 430, AeTISS 398 (93%), AnTISS 32 (7%)

Week B -- Same TSS, Intensity-Heavy: - Monday: Rest - Tuesday: 60 min with 5x4min Z5 intervals (TSS 85, AeTISS 45, AnTISS 40) - Wednesday: 45 min Z2 recovery spin (TSS 35, AeTISS 33, AnTISS 2) - Thursday: 50 min with 8x2min Z6 run intervals (TSS 80, AeTISS 35, AnTISS 45) - Friday: 40 min with 10x100m sprint swim (TSS 45, AeTISS 22, AnTISS 23) - Saturday: 90 min with 3x10min Z4 bike (TSS 110, AeTISS 65, AnTISS 45) - Sunday: 60 min Z2 run (TSS 55, AeTISS 50, AnTISS 5)

Week B totals: TSS 410, AeTISS 250 (61%), AnTISS 160 (39%)

Both weeks sit around TSS 420. A traditional PMC would treat them almost identically. But Week A places 93% of its stress on the aerobic system and requires minimal recovery between sessions. Week B hammers the anaerobic system with 39% AnTISS -- more than double the recommended proportion even for a peak phase. An athlete who trains like Week B consistently will accumulate deep neuromuscular and hormonal fatigue that a standard TSS-based recovery model will miss entirely.

The athlete finishing Week A feels pleasantly tired and ready for another similar week. The athlete finishing Week B needs a genuine recovery week -- and if they do not get one, performance will decline within 2-3 weeks.

Practical Rules

1. Keep AnTISS below 15% of total TISS during base phase. If your weekly AnTISS is creeping above 15% and you are supposed to be building aerobic fitness, you are doing too much intensity. Dial back the interval sessions and add easy volume instead.

2. Allow 48 hours between sessions with high AnTISS contribution. If Tuesday's interval session generated significant AnTISS, Wednesday should be Zone 1-2 only. Stacking high-AnTISS days is the fastest path to overtraining and stagnation.

3. Zone 2 sessions after hard intervals are fine and encouraged. A 60-minute easy ride the day after VO2max intervals adds AeTISS without meaningfully delaying anaerobic recovery. Active recovery promotes blood flow and accelerates clearance of metabolic byproducts.

4. Monitor weekly AnTISS trends, not just weekly TSS. If your weekly AnTISS has been climbing for 3 or more consecutive weeks without a drop, schedule a recovery week regardless of what your TSS trend looks like. The anaerobic system does not signal overload as clearly as the aerobic system until it is too late.

5. Increase AnTISS proportion gradually as you move from base to build to peak. A jump from 10% AnTISS to 35% AnTISS in a single week is a shock your body is not prepared for. Add 3-5 percentage points per week over a 4-6 week build.

6. During peak phase, accept that higher AnTISS means lower total TSS. Your total weekly TSS will likely drop 15-25% during peak and taper phases even as the proportion of AnTISS rises. This is correct and intentional. You are trading volume for specificity and sharpness.

7. Use the AeTISS/AnTISS split to design your taper. In the final 10-14 days before a race, cut AeTISS by 40-60% (reduce volume) while maintaining or slightly reducing AnTISS (keep short, sharp sessions). This preserves neuromuscular readiness while shedding accumulated aerobic fatigue. The result is legs that feel fresh and responsive on race morning.

Evidence Base

The conceptual foundation for splitting training stress into aerobic and anaerobic components traces back to Skiba's work on the Critical Power model and the BikeScore system. Recognizing that aggregate training load metrics like TSS (Coggan & Allen, 2010) could not distinguish between fundamentally different physiological demands, Skiba and colleagues developed frameworks that weighted sub-CP and supra-CP work separately.

The mathematical approach works as follows. For any given workout, each second of data is categorized relative to the athlete's CP. Seconds spent below CP contribute to AeTISS using a function that scales roughly linearly with intensity -- more power means proportionally more aerobic stress. Seconds spent above CP contribute to AnTISS using a quadratic or exponential function, reflecting the disproportionate metabolic cost of supra-threshold work. The two scores are summed independently, giving you two numbers instead of one.

Coggan and Allen's original TSS framework (2010) remains valuable as a total-load metric. It normalizes training stress relative to FTP, making it possible to compare sessions across different durations and intensities. However, as the authors themselves acknowledged, TSS does not capture the qualitative differences between training types. Two sessions with identical TSS can produce vastly different physiological responses and require vastly different recovery periods.

The practical value of the Ae/An split has been validated in coaching practice, where athletes who monitor both metrics show better periodization compliance and fewer episodes of unexplained performance decline. By tracking which energy system is being stressed and ensuring the balance matches the current training phase, coaches can make more precise adjustments to weekly programming than TSS alone allows.