Author(s): BAYER, P., DEL VECCHIO, A., THOMAS, H.J., PERJU-DUMBRAVA, L., FARINA, D., FERRI, A. , Institution: VICTORIA UNIVERSITY, Country: AUSTRALIA, Abstract-ID: 1382

INTRODUCTION:
Individuals with Amyotrophic Lateral Sclerosis (ALS) experience progressive degeneration and eventually death of their motor neurons, leading to muscle atrophy and weakness [1]. As the disease advances, surviving motor neurons partially compensate for this loss through collateral sprouting, resulting in the formation of enlarged motor units (MUs) [2]. These structural adaptations alter the shape and discharge patterns of motor unit action potentials (MUAPs) [3]. This study aims to investigate differences in MU characteristics during voluntary contractions at three force intensities between individuals with ALS and sedentary healthy controls using a novel, non-invasive technique called high-density surface electromyography (HDsEMG).
METHODS:
Participants (ALS: n=9, females=11%, age=60±13 years; healthy: n=9, females=56%, age=60±14 years) performed maximal (MVC) and submaximal isometric voluntary contractions (35%, 50%, and 70% of MVC) of the ankle dorsiflexors on an isokinetic dynamometer. HDsEMG recordings were obtained from the tibialis anterior using a 64-electrode grid. MUs were identified using a convolutive blind source separation method. MUAP peak-to-peak values, area, and MU spatial and discharge characteristics were assessed and compared between the two populations.
RESULTS:
There was no significant difference in the MVC between groups (ALS: 21.9±13.2 Nm, healthy: 23.7±8.5 Nm; p=0.747). On average, 9 MUs were identified in participants with ALS, compared to 10 in healthy controls across all force levels. There was no significant difference between groups for MU activity within the electrode grid (ALS: 454±89 mm2, healthy: 486±168 mm2; p=0.911). MU characteristics were significantly altered in participants with ALS compared to healthy controls including larger MUAP peak-to-peak amplitudes (391±143 µV vs. 277±202 µV; p=0.027), greater MUAP areas (2689±1111 µV·ms vs. 1843±12587 µV·ms; p=0.007) and higher MU discharge rates (17±5 pps vs. 13±3 pps; p<0.001). The MU recruitment thresholds were increased in ALS (58±21 %MVC vs. 48±13 %MVC; p=0.048) while the de-recruitment thresholds did not differ between groups.
CONCLUSION:
The increased MUAP peak-to-peak amplitude, MUAP area, discharge rate, and higher recruitment threshold in the participants with ALS indicate MU remodelling, where collateral sprouting leads to enlarged MUs and require a greater synaptic input. These findings suggest that HDsEMG could serve as a powerful tool for future studies, allowing more detailed identification of MU changes and a deeper understanding of ALS progression and the effects of interventions.

[1] Gordon, P.H. (2013), Aging and disease.
[2] Nijssen, J. et al (2017), Acta Neuropathologica.
[3] Krarup, C. et al (2011), Clinical Neurophysiology.