Author(s): HAUTIER, C., GASSIER, R., HENRY, L., SOULIE, T., STAUFFER, E., HINTZY, F. , Institution: UNIVERSITÉ CLAUDE BERNARD LYON 1, Country: FRANCE, Abstract-ID: 2326

INTRODUCTION:
Running generates impacts when the foot contacts the ground, transmitted to the body as transient vibrations. While exposure to shocks and vibrations can have beneficial effects, their multimodal characteristics induce resonance in soft tissues vibration (STV) at their natural frequencies, leading to amplification and an increased risk of deleterious effects. The muscle tuning paradigm (MTP) proposes that muscles adapt to reduce vibrations by actively adjusting their mechanical properties (1). Although MTP is well established for continuous vibrations, further research is needed to confirm its applicability to transient vibrations. This study aimed to objectify the occurrence and the evolution of MTP under fatigue, using an instrumented ergometer capable of reproducing the shocks encountered when running, while eliminating the interference of voluntary contractions during running (2).
METHODS:
Sixteen active volunteers, running at least once per week, underwent nine hundred impacts corresponding to running at 14 to 15 km/h. Ground reaction forces (GRF) were acquired to control the impacts intensity. Participants were fitted with surface electromyographic sensors (EMG) on the Vastus Lateralis (VL), tri-axial accelerometers on the heel cup of the shoe and the VL. All signals were recorded during 30-sec every 50 impacts. Continuous wavelet transform was used to compute peak acceleration total magnitude of vibration (TMV) and settling time (ST) from acceleration data. The same approach was applied to EMG signal to calculate preactivation. Maximal voluntary contraction torque (MVC) and perceived fatigue were assessed before and after the exercise. Statistical analyses were performed on continuous and discrete data.
RESULTS:
The GRF recording confirmed that running impacts were accurately reproduced. Both discrete and continuous data statistical analyses indicated an increase in the TMV of heel impacts (+20%, p < 0.05) especially between 20 to 130 Hz, with no change in TMV at the VL. ST decreased at the heel (-10 %, p < 0.01) and increased at the VL. The mean preactivation tend to increase along the exercise (p = 0.05). This was associated with a reduction of MVC (-9%, p = 0.001) and elevation of perceived fatigue (p < 0.001).
CONCLUSION:
In line with existing literature, heel impact increased with the development of fatigue, especially in the high-frequency domain (3). Furthermore, the total magnitude of vibration did not change at the VL despite a higher impact at the heel. It is interesting to note that the ST at the VL gradually lengthened, indicating prolonged vibration, contrary to the observation of Khassetarash et al. (4). This ability to limit STV amplification even when impact increases, combined with increased preactivation, provides further evidence of the existence of an MTP in transient vibrations, even in cases of fatigue.
1) Nigg and Wakeling, 2001
2) Gassier et al., 2025
3) Gassier et al., 2025
4) Khassetarash et al., 2019