# Quantifying intervertebral efforts during slow-dynamic trunk flexion/extension via a hybrid approach: a validation study

Simon Hinnekens, Philippe Mahaudens, Christine Detrembleur, Paul Fisette

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Simon Hinnekens, Philippe Mahaudens, Christine Detrembleur, Paul Fisette. Quantifying intervertebral efforts during slow-dynamic trunk flexion/extension via a hybrid approach: a validation study. Multidisciplinary Biomechanics Journal, 2025, 49th congress of the Société de Biomécanique, 10.46298/mbj.14544. hal-04738031v2

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Submitted on 31 Jan 2025

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# Quantifying intervertebral efforts during slow-dynamic trunk flexion/extension via a hybrid approach: a validation study

Simon Hinnekens<sup>a*</sup>, Philippe Mahaudens<sup>b</sup>, Christine Detrembleur<sup>b</sup>, Paul Fisette<sup>a</sup>

<sup>a</sup> UCLouvain/iMMC/MEED, Louvain-la-Neuve, Belgium

<sup>b</sup> UCLouvain/IREC/NMSK, Bruxelles, Belgium

* Corresponding author: simon.hinnekens@uclouvain.be

Received date: 04/04/2024

Accepted date: 28/06/2024

Publication date: 31/01/2025

**Keywords:** spine, intervertebral efforts, hybrid approach

© 2025 The Authors

Licence CC-BY 4.0

Published by Société de Biomécanique

## 1. Introduction

In-vivo measurements are required to validate models and methods of computing muscle forces and joint efforts. These measurements are gathered in open-access databases to allow a larger accessibility. Among the latter, Orthoload (www.orthoload.com) gathers efforts of several joints measured in-vivo thanks to implanted prostheses. In this study, the device of interest is the vertebral body replacement, which consists in replacing a spinal functional unit, i.e. a vertebra with its two adjacent intervertebral discs. Up to now, six patients were implanted (Dreischarf et al. 2015) to replace a vertebra. Once implanted, the patients were proposed to perform various daily tasks during which the in-vivo efforts inside the replaced vertebra were recorded in real-time.

Our objective was to confront our hybrid approach, i.e. an EMG-driven model, developed for static configurations to a dynamic case. Our approach relies on making the most of electromyography (EMG) as input of a musculoskeletal model of the trunk to compute muscle forces and resulting intervertebral efforts in the lumbar spine. These intervertebral efforts could be compared to the ones reported in the Orthoload database.

## 2. Methods

### 2.1 Experiment

Two subjects participated voluntarily in this study. They were selected to match their anthropometric features with these from the subjects recovered in the Orthoload database – two for the selected motion. These features

are summarised in Table 1 for all the subjects. Our subjects were with a good self-reported health, were recreationally active and never had back pathology.

<table><caption>**Table 1. Anthropometric features**</caption><thead><tr><th></th><th>This study</th><th>Orthoload</th></tr></thead><tbody><tr><td>Subject</td><td>S1</td><td>S2</td></tr><tr><td>Height (m)</td><td>1.77</td><td>1.82</td></tr><tr><td>Mass (kg)</td><td>68</td><td>63</td></tr><tr><td>Age (years)</td><td>60</td><td>59</td></tr><tr><td>Sex</td><td>M</td><td>M</td></tr><tr><td>Fractured vertebra</td><td>/</td><td>L1</td></tr></tbody></table>

Data for the Orthoload subjects are from Dreischarf et al. (2015)

Credit: Hinnekens

The motion selected for the study was a trunk flexion/ extension with a straight back when standing upright. The motion was performed three times: two as slow as possible and one at a self-selected speed.

EMG signals of back — lumbar (LP) and thoracic paravertebral (TP) muscles, quadratus lumborum (QL), latissimus dorsi — and abdominal — rectus abdominis, externus obliquus — muscles were recorded during the experiment. Maximal voluntary contractions were recorded prior to the exercises for EMG normalisation.

### 2.2 Model

A multibody model representing the studied motion was developed on the basis of a fully articulated
thoracolumbar spine and rib cage model (Bruno et al. 2015). After analysis of the recorded EMG signals and several sensitivity analyses, only LP, TP and QL were involved into the model.

Muscle forces were quantified via an EMG-based deterministic distribution of a global equivalent force among muscle fascicles (Hinnekens et al. 2021). Muscle forces and resulting intervertebral efforts were computed for each time step by inverse dynamics, but considering the motion as a succession of quasi-equilibrium configurations for which velocities and accelerations were assumed to be equal to 0.

It was assumed that the whole trunk flexion/extension happened at the S1/L5 level. Intervertebral efforts were compared at the L2/L1 level which corresponds to the fractured level in the Orthoload subjects (Table 1). In parallel with the validation of the database, linear regressions were performed between compressive forces computed by the model and flexion angles recorded during the experiment for comparison with the literature.

## 3. Results and discussion

Starting with the Orthoload comparison, compressive and shear forces computed by equivalent models of healthy subjects and those measured in Orthoload patients were in a similar range of magnitudes (Figure 1). Furthermore, they looked similar in their curve shapes, except for S1's shear forces. Additional comparisons with normalised forces were performed but are not presented here.

For compressive forces, the similarity was relatively obvious with a bell-shaped curve, in particular for healthy subjects. There was a plateau for each curve of Orthoload subjects, as observed for the flexion angles (not showed). The plateau was actually explained by the motion performed by the Orthoload subjects, i.e. they did not keep their back exactly straight. Indeed, the flexion could be split in two, with a sagittal rotation of the pelvis first followed by a slight rotation at the level of another spine segment. Globally the back was straight but the real strategy produced a plateau on compressive forces. Dreischarf et al. (2015) corroborated this explanation in their study.

Shear forces in Orthoload subjects were opposite to these obtained with the model and the hybrid approach. These differences might be explained by the implant itself. Indeed, forces measured by the implant were for a vertebral functional unit while the forces computed

with the hybrid approach were associated with one disc level. Also, the Orthoload subjects could have developed an adaptive strategy following their injury, operation and rehabilitation which could result in a different distribution of intervertebral efforts.

Finally, there was a linear correlation between compressive forces and the flexion angle (not showed). In particular, compressive forces at the L5/L4 level fitted well the linear regression proposed by Takahashi et al. (2006). It meant that even with a relatively simple deterministic distribution of a global equivalent force into all the muscle fascicles, the method was accurate enough to compute representative compressive forces. It would suggest that considering the back muscles as pure force transmitters, i.e. no muscle-length or muscle-velocity relationships, could be sufficient for dynamic configurations.

<figure><img src="image_2.png" /><figcaption>**Figure 1.** Compressive and shear forces for all subjects and all trials – Comparison between this study (S1, S2) and Orthoload (wp1, wp2).</figcaption></figure>

Credit: Hinnekens
4. Conclusions

This study highlighted the potential of using a hybrid
approach developed for static configurations to dynamic
cases. Intervertebral efforts, especially compressive
forces, were similar with in-vivo measurements from
an open-access database. Furthermore, compressive
forces computed with our method matched the linear
regressions reported in the literature. Future work will
focus on considering the muscle dynamics for a more
detailed comparison.

Conflict of Interest Statement

The authors state that they do not have any conflicts
of interest.

References

Bruno, A. G., Bouxsein, M. L., Anderson, D. E. (2015). Development and Validation of a Musculoskeletal Model of the Fully Articulated Thoracolumbar Spine and Rib Cage. *J Biomech Eng, 137*(8), 081003. doi: 10.1115/1.4030408

Dreischarf, M., Albiol, L., Zander, T., Arshad, R., Graichen, F., Bergmann, G., Schmidt, H., & Rolhmann, A. (2015). In vivo implant forces acting on a vertebral body replacement during upper body flexion. *J Biomech, 48*(4), 560-565. doi: 10.1016/j.jbiomech.2015.01.007

Hinnekens, S., Mahaudens, P., Detrembleur, C., & Fisette, P. (2021). Quantifying back muscle forces in the Sorensen test: influence of individual strategies on intervertebral efforts. *Comput Methods in Biomech Biomed Eng, 24*(S1), S163-S164. doi: 10.1080/10255842.2021.1978758

Takahashi, I., Kikuchi, S., Sato, K., & Sato N. (2006). Mechanical load of the lumbar spine during forward bending motion of the trunk—a biomechanical study. *Spine, 31*(1), 18–23.