Fatigue Indicator Details
Why Two Fatigue Indicators?#
During a marathon, two distinct systems fatigue: the cardiovascular system and the neuromuscular system. These progress independently, and when either reaches its limit, a sudden pace drop occurs.
A key finding from Prigent et al. (2022):
“Neuromuscular indicators change even with mild fatigue, while heart rate only changes with moderate or greater fatigue.”
In other words:
- NM = Early warning system: Changes in early stages of fatigue
- CV = Definitive fatigue indicator: Changes as fatigue progresses
Monitoring both provides a multi-faceted view of fatigue progression.
🫀 CV (CardioVascular Fatigue)
What It Measures#
Changes in the ratio of heart rate to speed (HR/Speed Ratio).
Principle#
During running, heart rate and speed normally maintain a constant ratio. However, as fatigue progresses, a higher heart rate is needed to maintain the same speed. This “decoupling” phenomenon indicates decreased cardiovascular efficiency.
Main causes:
- Blood volume reduction due to dehydration
- Glycogen depletion
- Rising body temperature (Cardiac Drift)
Formula#
DECOUP% = ((Current HR/Speed ratio) / (Baseline HR/Speed ratio) - 1) × 100
CV Display Value = (DECOUP% / 20%) × 100
※ 20% = Typical decoupling rate at marathon finish
Threshold Basis#
Smyth et al. (2022) analyzed 82,303 London Marathon finishers, statistically classifying the relationship between HR/Speed ratio deterioration (decoupling) and finish times.
| DECOUP% | Status | CV Value |
|---|---|---|
| < 10% | Normal | < 50 |
| 10-20% | Caution | 50-100 |
| ≥ 20% | Danger | > 100 |
🦵 NM (NeuroMuscular Fatigue)
What It Measures#
Changes in stride length.
Principle#
As muscle fatigue progresses, force production capacity decreases. This results in reduced stride length (weakened push-off).
This appears as “form breakdown,” which runners often don’t notice themselves, but can be clearly measured objectively. Stride length reduction is a precursor to significant pace drops (25%+) in the latter half.
Stride Length Estimation#
Due to Connect IQ API technical limitations in obtaining data from Running Dynamics sensors like HRM-Pro, stride length is estimated from speed and cadence using physics:
Stride Length (mm) = Speed × 60 / Cadence × 1000
Formula#
STRIDE% = ((Current stride length) / (Baseline stride length) - 1) × 100
NM Display Value = -(STRIDE% / 10%) × 100
※ 10% = Typical stride length reduction at marathon finish
Examples:
- STRIDE% = -5% → NM = 50 (yellow)
- STRIDE% = -10% → NM = 100 (red boundary)
Threshold Basis#
According to Lin et al. (2025) and Chan-Roper et al. (2012), the typical stride length change at marathon finish is approximately -10%.
| Stride Change | Status | NM Value |
|---|---|---|
| > -5% | Normal | < 50 |
| -5% to -10% | Caution | 50-100 |
| < -10% | Danger | > 100 |