Effect of recruitment of new MUs on mechanomyographic signal during prolonged isometric constant contractions at low level

The present study focused on the effect of the newly motor unit (MU) recruitment on mechanomyographic signal (MMG) by the analysis on motor unit mechanical signal (MUMS) during prolonged isometric constant contraction (PICC) at low torque levels of the knee extension. The mechanical and myoelectric signals (MES) from m. vastus medialis or lateralis were recorded by condenser microphone and disc electrode, respectively. In order to recruit the objective MU during the PICC, the target torque set at several levels below the recruitment threshold torque of the MU (≦ 7.4 %MVC). 1) iMMG and iMES sustained constant for initial several minutes and then increased during the PICC. 2) MUMS superimposed on MMG from back ground MUs activities and iMUMS increased significantly at the timing of MU recruitment. Subsequent iMUMS decreased according to the decrement of MUMS amplitude depend on the discharge trend of the MU. 3) Amplitude of MUMS (MS-Vpositive) showed different trend depended the recruitment timing during PICC. At the iMMG constant phase, MS-Vpositive sustained constant followed by the increment similar to iMMG trend. In contrast, at iMMG increment phase, MS-Vpositive showed increment trend without the constant phase. The present results suggested that the newly MU recruitment increase the iMMG during the PICC. IMMG increment at later period of the PICC could interpret from the MU recruitment and MS-Vpositive increment. It is necessary to investigate the factors to increase the MS-Vpositive from the muscle and muscle fibers conditions.