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🧠 Subjective Exertion Science

RPE & Perceived Fatigue (sRPE)

Bridging subjective mental strain with objective sensor outputs to uncover hidden fatigue, overtraining, and under-recovery.

1. Carl Foster's Session-RPE (sRPE) Model

Power meters and heart rate straps measure external work and cardiovascular output. But they cannot capture psychological burnout, sleep deficits, muscular soreness, or environmental distress (extreme heat/cold). Session RPE (sRPE) captures the total integrated systemic strain:

sRPE Training Load Formula:
sRPE Load = Borg CR-10 Rating (0–10) × Duration (minutes)
Example: 60 minutes @ RPE 7 (Very Hard) = 60 × 7 = 420 sRPE Load Units.

2. The Borg CR-10 Category-Ratio Scale

Unlike linear 1–10 scales, the Borg CR-10 scale reflects exponential sensory biology: the leap from 8 to 9 feels vastly harder than the step from 2 to 3:

Score Descriptor Verbal & Respiratory Sensory Cue Equivalent Intensity Domain
1 Very Light Effortless spinning. Conversation is completely normal. Active Recovery (Z1)
2 Light / Easy Pleasant aerobic pace. Full sentences without pauses. Aerobic Endurance (Z2)
3 Moderate Steady rhythmic breathing. Talking requires a breath every sentence. Aerobic Tempo (Z3)
4 Somewhat Hard Concentration required. Beginning to feel mild muscular burning. Sweet Spot
5 Hard Lactate threshold. Breathing heavily; talking limited to short phrases. FTP Threshold (Z4)
6 Vigorous Above threshold. Strong leg burn. Complete mental focus needed. Supra-Threshold
7 Very Hard VO2 Max intervals (3–5 mins). Gasping for air; speaking is impossible. VO2 Max (Z5)
8 Extremely Hard Severe anaerobic lactic acid buildup. Severe discomfort. Anaerobic Repeats (Z6)
9 Near Maximal Full-gas sprint or final hill climb finish. Muscular failure imminent. Max Sprint / Finish
10 Maximum Effort Absolute physical limit. Collapse / cannot pedal one second longer. Absolute Exhaustion

3. The Fatigue Gap: Uncovering Systemic Under-Recovery

Ability.ICU automatically normalizes your subjective sRPE Load against your mechanical TSS. The delta is your Fatigue Gap:

Low RPE + High Power
Diagnosis: Supercompensation. Your nervous system is primed and fatigue has completely cleared. High power outputs feel effortless. Ideal race form!
Expected RPE = Expected Power
Diagnosis: Normal Equilibrium. Sensations match power output. Training is progressing predictably with balanced adaptation.
High RPE + Low Power (Wide Gap)
Diagnosis: High Systemic Fatigue. Severe red flag! You feel exhausted at very low wattages. Caused by sleep deprivation, dehydration, glycogen depletion, or viral illness.