New Metric Proposal: Impact Load / Vertical Impact Load

Product suggestion for TrainingPeaks

Submitted by: Allostrain (ultra-trail and mountain endurance coaching methodology)
Date: August 2026

1. Executive Summary

TrainingPeaks robustly quantifies the metabolic/cardiovascular load of training (TSS, IF, rTSS, etc.), but has no native metric that quantifies the vertical impact mechanical load placed on the musculoskeletal system during running — particularly relevant in trail running, skyrunning, mountain running, and any discipline with significant elevation change (especially technical descents).

We propose that TrainingPeaks incorporate a complementary metric — which we will call the Impact Load Score (ILS) — focused on accumulated structural/mechanical load, independent of energy expenditure, in a way analogous to how TSS complements RPE.

2. The Problem: A Blind Spot in the Current Load Model

TSS and its derived metrics assume that fatigue is fundamentally metabolic. However, in vertical-impact sports there are two partially independent fatigue pathways:

• Metabolic/cardiovascular fatigue — well covered by TSS, HR-based TSS, power, etc.
• Structural/mechanical fatigue — muscle damage induced by repeated eccentric loading, especially on descents, which can be high even at moderate cardiovascular intensity.

Two sessions with the same TSS can carry completely different risk profiles if one takes place on flat terrain and the other includes several thousand meters of technical descent. The athlete may appear "recovered" according to TSS while the musculoskeletal system is severely damaged.

The scientific literature supports this distinction:

• Descents generate the greatest eccentric braking forces of a run, and are the primary driver of structural damage in trail and ultra-trail running, exceeding the cardiovascular demand of climbs themselves.
• In ultra-trail races with high negative elevation (>1000 m), markers of muscle damage (CK, LDH) rise very sharply — far beyond what energy expenditure or average heart rate alone would suggest.
• Peak vertical acceleration (measured via accelerometry, whether at the ankle, tibia, or even wrist/chest-worn devices) is a good surrogate for ground reaction force (GRF), the laboratory gold-standard variable for impact.
• Cumulative load models based on summing peak vertical acceleration per stride explain real-world load better than purely GPS-based parameters (pace, distance).
• Vertical instantaneous loading rate (VILR) and peak tibial acceleration have been retrospectively associated with overuse injuries such as tibial stress fractures.

3. Proposed Metric: Impact Load Score (ILS)

3.1 Concept

A composite index, expressed on an arbitrary scale (analogous to TSS), quantifying the accumulated vertical impact load of a session by integrating:

• Vertical acceleration / impact per stride (data already available on many watches and chest straps with an IMU: Garmin, COROS, Suunto, Polar).
• Negative gradient (%) and its duration, weighted non-linearly — very steep gradients generate disproportionately more structural damage than moderate ones, not in simple proportion.
• Cadence and stride length, since at equal gradient and speed, longer strides and lower cadence increase peak load per footstrike.
• Accumulated negative elevation (D-), as a coarse proxy when accelerometry data isn't available (fallback mode).

3.2 Differentiation from Existing TSS

| | TSS / IF | Impact Load Score (proposed) |
|---|---|---|
| What it measures | Metabolic/cardiovascular cost | Mechanical/structural cost |
| Sensitive to | Intensity, duration, power/HR | Negative gradient, slope, cadence, impact |
| Fatigue pathway | Systemic (cardio-metabolic) | Local (muscular, tendinous, articular) |
| Primary use | Dosing training load | Dosing injury risk and structural recovery needs |

3.3 Practical Applications for the End User

• Acute:chronic impact ratio, just as already exists for TSS, to flag risky spikes in structural load (e.g., after a race with heavy negative elevation).
• Structural recovery alerts independent of metabolic recovery: an athlete can be cardiovascularly "fresh" well before being structurally recovered.
• Session comparison for workouts that look equivalent in TSS but are nothing alike in injury risk (e.g., a mountain run with heavy technical descent vs. a track interval session).
• Particularly valuable for the trail running, ultra-trail, skyrunning, and ski mountaineering user segment — a growing part of TrainingPeaks' user base with monitoring needs distinct from flat-terrain cycling/triathlon, where the original TSS model was born.

4. Technical Feasibility

• Many sports watches already compatible with TrainingPeaks (Garmin, COROS, Suunto, Polar) already include an accelerometer/IMU, and in some cases already expose impact/"shock load" data within their own ecosystems (though in an isolated, non-standardized way).
• When accelerometry data isn't available, the metric could be calculated in a simplified mode using only altimetry (D-) and per-segment gradient, available in any GPX/FIT file with a barometer.
• This would allow a progressive rollout: a basic version (altimetry only) available to all users from day one, and an advanced version (with accelerometry) for compatible devices.

5. Summary of the Request

We ask TrainingPeaks to consider incorporating a vertical impact load metric complementary to TSS, specifically aimed at vertical-impact sports (trail running, ultra-trail, skyrunning, mountain running), with the goal of closing the current blind spot between metabolic load and structural load, and giving coaches and athletes a more complete tool for managing injury risk and real accumulated fatigue.

References Consulted

• Cumulative load estimation via accelerometry vs. GPS-based parameters in running — Frontiers in Sports and Active Living, 2020. https://www.frontiersin.org/journals/sports-and-active-living/articles/10.3389/fspor.2020.575596/full
• Tibial acceleration-based prediction of maximal vertical loading rate — Frontiers in Bioengineering and Biotechnology, 2020. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7010603/
• Peak impact accelerations and running-related injury risk — PMC, 2022. https://pmc.ncbi.nlm.nih.gov/articles/PMC8931222/
• Review of uphill and downhill running biomechanics and physiology — Frontiers in Bioengineering and Biotechnology, 2025. https://www.frontiersin.org/journals/bioengineering-and-biotechnology/articles/10.3389/fbioe.2025.1690023/full
• Exertional rhabdomyolysis and eccentric load in ultra-trail races — systematic review, PMC, 2024. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC12225290/
• Mechanical load, heat strain and dehydration in endurance trail runners — PMC. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8508486/