Non-Linearity in Motor Unit Velocity Twitch Dynamics: Implications for Ultrafast Ultrasound Source Separation

• Published in: IEEE transactions on neural systems and rehabilitation engineering 2023, Vol.31, p.3699-3710
• Main Authors: Lubel, Emma, Sgambato, Bruno Grandi, Rohlen, Robin, Ibanez, Jaime, Barsakcioglu, Deren Y., Tang, Meng-Xing, Farina, Dario
• Format: Article
• Language: English
• Subjects: Average velocity; B-mode; Basic Medicine; Decomposition; Electrodes; Electromyography; Engineering and Technology; Force; Fysiologi; intramuscular electromyography; Legged locomotion; Man-machine interfaces; Mathematical analysis; Mathematical models; Medical and Health Sciences; Medical Engineering; Medical Image Processing; Medicin och hälsovetenskap; Medicinsk bildbehandling; Medicinska och farmaceutiska grundvetenskaper; Medicinteknik; Motor units; Muscles; Penetration depth; Physiology; Recording; Spatial discrimination; Spatial resolution; surface electromyography; Teknik; Tracking; Ultrasonic imaging; Ultrasound; Velocity
• ISSN: 1534-4320; 1558-0210; 1558-0210
• DOI: 10.1109/TNSRE.2023.3315146

Ultrasound (US) muscle image series can be used for peripheral human-machine interfacing based on global features, or even on the decomposition of US images into the contributions of individual motor units (MUs). With respect to state-of-the-art surface electromyography (sEMG), US provides higher spatial resolution and deeper penetration depth. However, the accuracy of current methods for direct US decomposition, even at low forces, is relatively poor. These methods are based on linear mathematical models of the contributions of MUs to US images. Here, we test the hypothesis of linearity by comparing the average velocity twitch profiles of MUs when varying the number of other concomitantly active units. We observe that the velocity twitch profile has a decreasing peak-to-peak amplitude when tracking the same target motor unit at progressively increasing contraction force levels, thus with an increasing number of concomitantly active units. This observation indicates non-linear factors in the generation model. Furthermore, we directly studied the impact of one MU on a neighboring MU, finding that the effect of one source on the other is not symmetrical and may be related to unit size. We conclude that a linear approximation is partly limiting the decomposition methods to decompose full velocity twitch trains from velocity images, highlighting the need for more advanced models and methods for US decomposition than those currently employed.