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Effects of shoe longitudinal bending stiffness and runner level on Achilles tendon loading in experienced and novice male runners: a musculoskeletal modelling study

This musculoskeletal modelling study reveals that both runner experience level and increased shoe longitudinal bending stiffness independently elevate Achilles tendon loading, with novice runners adapting to stiffer shoes through proximal compensations while experienced runners do not, supporting an individualized approach to optimal shoe stiffness.

Original authors: Shixin Lin, Hao Yuan, Jiaying Shao, Dongwei Liu, Mianfang Ruan, Ye Ma

Published 2026-07-13

📖 4 min read☕ Coffee break read

CC BY 4.0

Original authors: Shixin Lin, Hao Yuan, Jiaying Shao, Dongwei Liu, Mianfang Ruan, Ye Ma

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ⚕️ This is an AI-generated explanation of a preprint that has not been peer-reviewed. It is not medical advice. Do not make health decisions based on this content. Read full disclaimer

Imagine your legs are a high-tech suspension system on a race car, and your Achilles tendon is the super-strong bungee cord connecting your calf muscles to your heel. This cord is the engine's secret weapon: it stretches to store energy and snaps back to launch you forward. But stretch it too hard, too often, and it can snap or get sore.

A team of scientists decided to see how two things change the tension on this bungee cord: who is driving (an experienced racer vs. a new driver) and how stiff the car's chassis is (the shoe). They didn't just watch runners; they built a super-detailed computer simulation of 31 male runners to peek inside their muscles and tendons without needing surgery.

The Two Types of Drivers

First, the study found that experience changes the driving style.

The Veterans (Experienced Runners): These runners are like pro racers who know exactly how to use the engine. They land with their feet flatter and push off harder. Because they are so good at using their calf muscles, their Achilles tendon takes a bigger hit. In the simulations, the veterans' tendons felt forces up to 9.23 times their body weight.
The Newbies (Novice Runners): These runners are more like learners. They land with their hips slightly turned out and don't push off as aggressively. Their tendons felt less stress, peaking around 8.51 times their body weight.

The "Stiff Shoe" Experiment

Next, the researchers put everyone in three different pairs of shoes, all identical except for how hard it was to bend them lengthwise (like bending a ruler).

Shoe 1 (T1): Soft and floppy (10.9 Nm/rad).
Shoe 2 (T2): Medium stiffness (17.5 Nm/rad).
Shoe 3 (T3): Super stiff, like a carbon-fiber plank (33.0 Nm/rad).

The Big Surprise: The stiffer the shoe, the more the Achilles tendon had to work.
When the runners wore the stiffest shoes (T3), the tendon force went up. Why? Think of the shoe's stiff plate like a rigid lever. It stops your toes from bending as much. So, instead of your foot doing the bending, your ankle has to bend more to compensate. This extra ankle bending pulls harder on the calf muscles, which yanks harder on the Achilles tendon. The simulations showed that as the shoes got stiffer, the tendon force and the total "load" (impulse) went up significantly.

The "Cheat Code" vs. The "Real Deal"

Here is where it gets really interesting. The newbies and the veterans reacted to the stiff shoes in totally different ways.

The Veterans: They were like pros who could handle the stiff car. They kept their running style mostly the same, just pushing a bit harder with their ankles. They didn't need to change their whole body to make the stiff shoe work.
The Newbies: They were like learners trying to drive a stiff car. Because they couldn't handle the stiff shoe with just their ankles, they started "cheating" by moving their hips and knees weirdly.
- They twisted their hips more.
- They bent their knees harder.
- They rotated their legs in ways the veterans didn't.

The paper suggests that while the stiff shoe might help the pros, it forces the newbies to use their knees and hips to do the work. This might actually be risky for their knees, even if it saves their Achilles tendon a little bit.

What This Means for You

The study argues against the idea that "stiffer is always better." It suggests that the perfect shoe isn't the same for everyone.

If you are a veteran, a moderately stiff shoe might help you run fast, but if it gets too stiff, it might overload your Achilles tendon (which is already working hard).
If you are a newbie, a super-stiff shoe might force your knees and hips to do too much work, potentially causing other injuries.

The researchers conclude that there is likely a "sweet spot" for shoe stiffness that looks like a U-shape: too soft is bad, too stiff is bad, and the middle is best—but that "middle" depends entirely on how experienced you are.

Important Note: These results come from a computer simulation based on real data, not from measuring the actual tendons inside the runners' legs (which would be very painful and difficult). So, while the trends are strong and the math is solid, the exact numbers are estimates from the model. Also, this study only looked at men running at a specific speed of 4.17 m/s, so we can't be sure if the same rules apply to women or slower joggers.

Technical Summary: Effects of Shoe Longitudinal Bending Stiffness and Runner Level on Achilles Tendon Loading

Problem Statement
Achilles tendon (AT) injuries are a prevalent overuse issue among runners, with lifetime risks reaching 52% in habitual runners. While AT loading is influenced by lower-limb biomechanics and footwear, the specific interaction between runner experience level and shoe longitudinal bending stiffness (LBS) remains unclear. Traditional methods of estimating AT force (dividing net ankle moment by a moment arm) oversimplify ankle anatomy by ignoring synergistic muscle contributions and antagonist co-contraction. Furthermore, it is unknown whether experienced and novice runners adapt differently to carbon-fiber plate shoes with varying stiffness, and how these adaptations affect AT loading and injury risk profiles.

Methodology
This study employed a musculoskeletal modeling approach to investigate these interactions in 31 healthy male rearfoot strikers, divided into two groups: 18 experienced runners (monthly volume >100 km) and 13 novice runners (weekly volume <10 km).

Experimental Conditions: Participants ran at a controlled speed of 4.17 m/s on an overground runway while wearing three prototype shoes with identical construction but varying LBS levels: Low (T1: 10.9 Nm/rad), Medium (T2: 17.5 Nm/rad), and High (T3: 33.0 Nm/rad). Shoe mass was matched across conditions using counterweights.
Data Collection: Kinematic data were captured via a 12-camera motion capture system (200 Hz), and ground reaction forces were recorded via a force plate (1000 Hz).
Modeling and Analysis: A subject-specific, full-body musculoskeletal model (based on Rajagopal et al., 2016, and optimized by Lai et al.) was scaled to each participant. Muscle forces were estimated using static optimization. AT force was calculated as the sum of the forces from the soleus, medial gastrocnemius, and lateral gastrocnemius.
Statistical Analysis: Discrete variables (peak angles, moments, forces) were analyzed using two-way mixed ANOVA. Continuous time-series data were analyzed using one-dimensional Statistical Parametric Mapping (SPM1D) to identify significant differences across the stance phase.

Key Results

Runner Level Effects: Experienced runners generated significantly higher peak AT forces than novices ( $p=0.038$ ). Biomechanically, experienced runners adopted a "distally-loaded" strategy characterized by smaller initial ankle dorsiflexion, greater hip extension, and significantly higher ankle plantarflexion moments during mid-to-late stance. This strategy optimized propulsion but increased per-stride AT tensile load.
Shoe Stiffness (LBS) Effects: Increasing LBS independently elevated both peak AT force ( $p=0.004$ ) and AT impulse ( $p=0.006$ ). This was driven primarily by a significant increase in peak ankle dorsiflexion angle ( $p<0.001$ ) as stiffness increased. The study suggests that stiffening the forefoot redirects sagittal plane excursion proximally to the ankle, thereby increasing the mechanical demand on the triceps surae complex.
Interaction Effects: Significant interactions were found between runner level and LBS for peak knee extension moment, peak hip external rotation angle, and transverse-plane hip range of motion.
- Novice Runners: Adapted to stiffer shoes by increasing proximal compensations, specifically showing greater knee extension moments and larger transverse-plane hip motions.
- Experienced Runners: Did not exhibit these significant proximal compensations, maintaining stable kinematics across stiffness levels.

Key Contributions

Refined AT Force Estimation: The study utilized a subject-specific musculoskeletal model to estimate AT force by summing specific triceps surae muscle forces, offering a more anatomically accurate assessment than the traditional net-moment approach.
Mechanistic Insight into LBS: The research demonstrates that the mechanism by which LBS increases AT loading is the redirection of sagittal excursion from the forefoot to the ankle, rather than a direct linear increase in ankle moment generation.
Differential Adaptation Profiles: The study identifies distinct biomechanical responses to footwear stiffness based on training level. Novice runners rely on proximal joint adjustments (knee and hip) to accommodate stiff shoes, whereas experienced runners rely on distal mechanisms.

Significance and Claims
The paper argues that runner level is the principal determinant of baseline AT loading, while shoe LBS acts as an independent factor that further elevates this load. The findings challenge the "stiffer is better" paradigm for carbon-fiber footwear prescription. Instead, the authors support an individualized, U-shaped view of optimal LBS:

For Experienced Runners: They already carry high baseline AT loads due to their propulsion-oriented biomechanics; increasing LBS further amplifies this load, potentially contributing to the high incidence of AT tendinopathy in this group.
For Novice Runners: They lack the distal capacity to absorb the redistributed kinetic demand from stiff shoes, leading to unstable proximal compensations (increased knee and hip torque) that may elevate the risk of acute knee and hip injuries rather than tendon injury.

The authors conclude that footwear stiffness should be matched to the runner's training level and neuromuscular capacity, suggesting that novice runners may benefit from lower stiffness until their proximal control and triceps surae strength are sufficiently developed. The study acknowledges limitations, including the focus on male runners at a single speed and the inherent uncertainties in model-based force estimation, but asserts that the relative comparisons provide a robust basis for individualized injury prevention strategies.

The Two Types of Drivers

The "Stiff Shoe" Experiment

The "Cheat Code" vs. The "Real Deal"

What This Means for You

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