LEAI

Low-Energy Absorption Index (LEAI):

Quantifying Harness Performance in Small and Moderate Vertical Impacts

Abstract

Current paragliding harness certification standards focus primarily on high-energy impacts associated with severe crashes. However, the vast majority of real-world pilot loads arise from low-energy vertical impacts, such as hard landings, micro-drops, flare misjudgments, and repeated “culates”.
To address this gap, the Low-Energy Absorption Index (LEAI) has been introduced as a complementary biomechanical metric designed to quantify how effectively a harness attenuates small and moderate impact energy before critical acceleration thresholds are reached.

What is the LEAI (Low-Energy Absorption Index)

The LEAI (Low-Energy Absorption Index) is a biomechanical severity indicator used to describe how violently impact energy is applied to the human body during an impact event.

LEAI does not measure how much energy is absorbed.
Instead, it measures how aggressively the remaining energy is delivered over time.

In simple terms:

LEAI tells you how “hard” the impact feels to the body, taking into account both force and how quickly that force is applied.

Simple explanation for the user

Two impacts can involve the same energy and the same peak acceleration, but still feel very different to the human body.

A smooth, progressive deceleration is generally well tolerated.
A short, sharp deceleration is much more dangerous.

LEAI captures this difference.

It increases when:

acceleration is high,
force is applied very quickly,
the impact duration is short.

It decreases when:

energy is dissipated over a longer time,
deceleration is progressive.

Difference between LEAI and classic impact metrics

Peak acceleration (G)

Peak G only describes the single worst instant of the impact.
It ignores how long the load lasts.

Absorbed energy (J)

Absorbed energy tells you how much energy was dissipated, but not how violently it was applied.

LEAI

LEAI combines:

force magnitude,
time exposure,
dynamic loading behavior.

In short:

Peak G measures intensity,
absorbed energy measures quantity,
LEAI measures violence of delivery.

What LEAI measures conceptually

LEAI represents the time-weighted severity of force application during an impact.

It penalizes:

high forces,
rapid force onset,
short dissipation times.

It rewards:

long deceleration strokes,
smooth force buildup.

LEAI is therefore strongly correlated with tissue stress and discomfort, not just structural damage.

1. Motivation and Rationale

Traditional indices such as DRI_EN, DRI_BIOM, and FAR spinal load are optimized for injury prevention under high-severity events. While essential, these metrics are relatively insensitive to:

Initial impact sharpness
Early jerk behavior
Progressive vs impulsive energy absorption
Pilot comfort and cumulative spinal loading

In practice, pilots experience far more low-amplitude, short-duration impacts than catastrophic ones. A harness that performs well at certification g-levels may still transmit excessive load during everyday operations.

LEAI was designed to capture this missing dimension.

2. Conceptual Definition of LEAI

The Low-Energy Absorption Index evaluates how much mechanical “violence” is transmitted to the pilot before a given acceleration threshold is exceeded.

Unlike peak-based metrics, LEAI focuses on the early phase of the impact, where the harness should ideally:

Delay acceleration buildup
Reduce jerk
Spread energy over time

A low LEAI value indicates gradual, progressive absorption.
A high LEAI value indicates rigid or impulsive load transfer.

3. Why Multiple LEAI Thresholds?

Human tolerance to acceleration is strongly time-dependent. Short exposures to moderate g-levels are usually harmless, while sharp transitions cause discomfort and cumulative spinal fatigue.

For this reason, LEAI is evaluated at multiple acceleration ceilings:

Adopted LEAI Levels

Index - Threshold - Physical Meaning

LEAI-5 - 5 g - Micro-impacts, flare errors, minor culates
LEAI-10 - 10 g - Typical hard landings
LEAI-15 - 15 g - Severe but non-catastrophic vertical impacts
LEAI-20 - 20 g - Upper bound before injury-oriented metrics dominate

4. Calculation Principle (Conceptual)

For each LEAI threshold GlimG_{lim}Glim:

Identify the time interval from impact onset to the first crossing of GlimG_{lim}Glim
Analyze the acceleration-time history within this window
Penalize:
- High early jerk
- Rapid acceleration buildup
- Concentrated impulse

In simplified biomechanical terms, LEAI reflects:

How fast acceleration rises
How abruptly energy is transmitted
How early the pilot experiences significant load

This makes LEAI particularly sensitive to differences between:

Airbags vs foam protectors
Progressive vs stiff harness architectures

5. Relationship to Existing Indices

LEAI does not replace established safety metrics. Instead, it complements them:

Index - Primary Focus

DRI_EN - Certification compliance
DRI_BIOM - Injury probability
FAR spinal load - Structural spinal tolerance
LEAI - Early-phase energy absorption & comfort

A harness may pass certification limits while still scoring poorly in LEAI, revealing deficiencies invisible to traditional tests.

6. Interpretation of LEAI Values

LEAI values are relative indicators, not absolute injury predictors.

General interpretation:

Low LEAI → Progressive damping, comfort-oriented design
Moderate LEAI → Acceptable but noticeable impact transmission
High LEAI → Rigid response, poor micro-impact absorption

This makes LEAI especially useful for:

Comparative harness testing
Design optimization
Informing pilots about comfort vs protection trade-offs

7. Why LEAI Matters in Practice

From a pilot’s perspective, LEAI addresses questions that certification does not answer:

Why does one harness “feel softer” on landing?
Why do repeated small impacts cause fatigue or back pain?
Why does an airbag feel better even when peak g is similar?

LEAI translates these subjective impressions into a quantifiable engineering metric.

8. Conclusions

The Low-Energy Absorption Index fills a long-standing gap between certification safety and real-world usability.
By focusing on early-phase dynamics and moderate acceleration thresholds, LEAI provides:

A clearer picture of harness damping behavior
Better discrimination between modern protection systems
A new design target for future harness development

LEAI does not redefine safety limits — it refines our understanding of impact quality.

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