# Consequences of muscular deconditioning on the force transmitted to the Achilles enthesis: a preliminary study using musculoskeletal modeling

Manon Jamet, Claire Camy, Sandrine Roffino, Martine Pithioux, Benjamin Goislard de Monsabert

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Manon Jamet, Claire Camy, Sandrine Roffino, Martine Pithioux, Benjamin Goislard de Monsabert. Consequences of muscular deconditioning on the force transmitted to the Achilles enthesis: a preliminary study using musculoskeletal modeling. Multidisciplinary Biomechanics Journal, 2025, 50th congress of the.., 10.46298/mbj.16138. hal-05191690v2

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# Consequences of muscular deconditioning on the force transmitted to the Achilles entheses: a preliminary study using musculoskeletal modeling

Manon Jamet*, Claire Camy, Sandrine Roffino, Martine Pithioux, Benjamin Goislard de Monsabert

Institut des Sciences du Mouvement, Marseille, France

* Corresponding author: manon.jamet@univ-amu.fr

Received date: 06/04/2025

Accepted date: 27/06/2025

Publication date: 27/10/2025

**Keywords:** deconditioning, tendon force, muscular synergies, musculoskeletal modeling

© 2025 The Authors

Licence CC-BY 4.0

Published by Société de Biomécanique

## 1. Introduction

An optimal loading is necessary to maintain the integrity of the biological tissues (Pizzolato et al., 2017).

Reduced body weight leads to musculoskeletal deconditioning, affecting all tissues involved in movement production. In microgravity, studies have shown that deconditioning impacts astronauts throughout their space-flight, resulting in atrophied muscles (Lang et al., 2017).

Our research team has shown the effect of deconditioning on the Achilles entheses: the insertion of the Achilles tendon on the calcaneus. This structure is crucial to transfer the force to the bone. In the mouse model, cellular and mechanical changes in this tissue have been experimentally quantified in the deconditioning context (Camy et al., 2022).

Nevertheless, little is known regarding the link between the tissue adaptations and motor strategy changes. In particular, reduced body weight leads to muscle atrophy which has been observed in humans (Lang et al., 2017), but also in mice (Hanson et al., 2013).

Atrophy is characterized by muscle mass loss and hence muscle force capacity loss (Pizzolato et al., 2017). The impact of muscular atrophy on the force transmitted to the calcaneus during locomotion has never been quantified. Indeed, motor strategies, such as levels of co-contraction, depend on the force-generating capacities of the involved muscles.

The relationship between locomotion parameters, atrophy and the force withstood by the Achilles tendon

can be studied using musculoskeletal modeling. The mechanical functioning of body is simulated using non-invasive measurements of segmental kinematics and forces. Validated models are available within the scientific community including the mouse model (Charles et al., 2018).

The aim of this study is to test the effect of modifying muscle capacities of a musculoskeletal model on Achilles tendon's mechanical loading, as well as on ankle co-contractions, using muscle atrophy data to simulate the impact of deconditioning on entheses tissue.

Understanding these impacts takes root in a long-term challenge: guide rehabilitation protocols after a period of hypoactivity like space exploration.

## 2. Methods

### 2.1 Musculoskeletal modeling and simulated movement

The musculoskeletal model used to perform the simulation is Charles *et al.*'s model (2018) corresponding to the lower limbs of a mouse. Muscle parameters tie in the Hill model. The muscle component is considered active as it is physiologically contractile and its force capacity is defined by a maximal isometric force. The tendon is represented by a passive force-strain relationship, resistant to stretching, and positioned in series with the muscle. Locomotion in mouse at natural speed is studied using data from Charles et al. (2018). The modeling software used is OpenSim® 4.5.
## 2.2 Simulation of the atrophy impact

The data used to simulate atrophy were taken from (Hanson et al., 2013). Muscle morphology was studied during adaptation to underloading at different times: Day 0; D+1; D+3; D+7; D+14 for thirty-five mice, equally distributed in five groups. To simulate microgravity, the mice were suspended by the tail. As proof of concept, only one scenario is tested using data at D+14. The simulated force of Gastrocnemius and Soleus muscles are diminished by 10% and 25% respectively. Comparison is made with data from the control scenario at D0 (Tab.1).

<table><caption>**Table 1.** Atrophy Data (Hanson et al., 2013) and values of maximal isometric force used for simulating on OpenSim® 4.5. Software.</caption><thead><tr><th></th><th>Normalized Muscle Mass (mg) mean ± SEM</th><th>Maximum Isometric Force (N)</th><th>Loss percentage of Muscle Force (%)</th></tr></thead><tbody><tr><td>MG & LG</td><td>5,62 ± 0,19</td><td>1,75 & 3,784</td><td>0</td></tr><tr><td>SOL</td><td>0,32 ± 0,03</td><td>0,233704722</td><td>0</td></tr><tr><td></td><td></td><td>Day +14</td><td></td></tr><tr><td>MG & LG</td><td>5,04 ± 0,14</td><td>1,575 & 3,406</td><td>10</td></tr><tr><td>SOL</td><td>0,24 ± 0,01</td><td>0,1752785475</td><td>25</td></tr></tbody></table>

MG & LG: Medial & Lateral Gastrocnemius; SOL: Soleus.

The atrophy was simulated by applying the percentage reduction of muscle mass to the maximal isometric force in the OpenSim model, while the segment kinematics and ground reaction force patterns remained the same. The study will focus on the forces in the Achilles tendon and the tibialis anterior, as antagonistic muscle.

## 3. Results & Discussion

The Achilles tendon force was approximately 10% lower in the case of atrophy of the triceps surae (maximum peak force: 4.98 N), compared to control individual (5.57 N). The force pattern was preserved (Fig.1).

Reducing the maximal isometric force of the triceps surae muscles affected the mechanical loading of the Achilles tendon. Indeed, with a reduced force capacity, the optimization process attributed a smaller part of mechanical loading to this muscle group. Moreover, tibialis anterior was not affected by this change (data not shown), suggesting a variation in ankle co-contraction and joint stability. Nevertheless, this simulation did not take into account the effect of microgravity on other leg muscles or on the joint angle and ground reaction force pattern. A reorganization of muscular synergies between the different muscles has to be considered.

<figure><img src="image_3.png" /><figcaption>**Figure 1.** Comparison of control and atrophied triceps surae tendon force during trotting locomotion.</figcaption></figure>

## 4. Conclusion

This study provided initial insights about the links between muscle atrophy, motor strategy and mechanical loading in the Achilles enthesis. The co-contraction, illustrating the motor strategies, seems to vary and reflect a functional adaptation to the microgravity. Future work should include atrophy of all leg muscles and the leg kinematic and kinetic patterns of atrophic individuals to clarify the effect of deconditioning. Then, to further understanding, a larger-scale study about structural-functional adaptations will be carried out to overcome these limits.

## Conflict of interest statement

The authors declare that they have no competing interests.

## References

Camy, C., Brioche, T., Senni, K., Bertaud, A., Genovesio, C., Lamy, E., Fovet, T., Chopard, A., Pithioux, M., & Roffino, S. (2022). Effects of hindlimb unload- ing and subsequent reloading on the structure and mechanical properties of Achilles tendon-to-bone attachment. *The FASEB Journal*, 36(10). https://doi. org/10.1096/fj.202200713R

Charles, J. P., Cappellari, O., & Hutchinson, J. R. (2018). A dynamic simulation of musculoskeletal function in the mouse hindlimb during trotting locomotion. *Frontiers in Bioengineering and Biotechnology*, 6, 61. https:// doi.org/10.3389/fbioe.2018.00061

Hanson, A. M., Harrison, B. C., Young, M. H., Stodieck, L. S., & Ferguson, V. L. (2013). Longitudinal characterization of functional, morphologic, and
biochemical adaptations in mouse skeletal muscle
with hindlimb suspension. *Muscle & Nerve*, 48(3),
393-402. https://doi.org/10.1002/mus.23753

Lang, T., Van Loon, J. J. W. A., Bloomfield, S., Vico, L., Chopard, A., Rittweger, J., Kyparos, A., Blottner, D., Vuori, I., Gerzer, R., & Cavanagh, P. R. (2017). Towards human exploration of space: The THESEUS review series on muscle and bone research priorities. *Npj Microgravity, 3*(1), 8. https://doi.org/10.1038/s41526-017-0013-0

Pizzolato, C., Lloyd, D. G., Barrett, R. S., Cook, J. L., Zheng, M. H., Besier, T. F., & Saxby, D. J. (2017). Bioinspired Technologies to Connect Musculoskeletal Mechanobiology to the Person for Training and Rehabilitation. *Frontiers in Computational Neuroscience, 11*, 96. https://doi.org/10.3389/fncom.2017.00096