RELATIONSHIP BETWEEN INTER-SPIKE INTERVAL AND AMPLITUDE OF MECHANOMYOGRAPHIC SIGNAL FROM SINGLE MOTOR UNITS DURING VOLUNTARY MUSCLE CONTRACTION

• Published in: Japanese Journal of Physical Fitness and Sports Medicine Vol. 60; no. 2; pp. 207 - 216
• Main Author: MORIMOTO, SHIGERU
• Format: Journal Article
• Language: English; Japanese
• Published: The Japanese Society of Physical Fitness and Sports Medicine 2011
• Subjects: firing interval; MMG; motor unit; motor unit mechanical signal; prolonged activity
• ISSN: 0039-906X; 1881-4751; 1881-4751
• DOI: 10.7600/jspfsm.60.207

To investigate factors that increase the mechanomyographic (MMG) signal during voluntary prolonged muscle contraction at relatively low tension levels, the MMG signal from the motor unit (MU mechanical signal: MUMS) was analyzed. In the present study, the author focused on the interval dependency of MUMS amplitude as one of the factors to increase MMG. From the m. vastus medialis or m. vastus lateralis, MUAP and myoelectrical signal (MES) were recorded by Ag/AgCl disc electrode and MMG and MUMS were respectively recorded by condenser microphone during various types of muscle contractions; brief isometric constant contraction (BICC), prolonged isometric constant contraction (PICC) and prolonged isometric contraction under constant MU firing interval (PIC-CFI). The amplitude of positive phase in MUMS (MS-Vpositive) increased proportionally with the firing interval of the MU in BICC. The firing interval of MU showed an initial elongation, followed by shortening during PICC. During PICC and PIC-CFI, MS-Vpositive sustained in an initial part of the contraction and then abruptly increased. There was no meaningful relationship between the firing interval and MS-Vpositive during PICC and PIC-CFI. However, the increment ratio of the MS-Vpositive in PIC-CFI was smaller than that in PICC. The present results on MS-Vpositive suggested that the increment of MMG during PICC did not depend on the firing interval change in the activated MUs. Based on findings with different increment ratios in MS-Vpositive between PICC and PIC-CFI, it is suggested that the hysteresis of muscle contraction is one of the factors causing an increase in MUMS and MMG.