[231] Simulation of surface EMG and muscle force during maximal voluntary contraction and ballistic contraction

「Objective」 The amplitude of the surface-recorded EMG supplies information about muscle force or muscle activity. During static muscle contraction, there is a linear relationship between the muscle force and the amplitude of the surface EMG. Therefore, under conditions of static contraction, the value of surface EMG normalized by the amplitude recorded during static maximal voluntary contraction (MVC), which is defined as %MVC, is a significant index for the general evaluation of muscle force or muscle activity. We often determine %MVC, even in the surface EMG recorded during dynamic contraction. However, in ballistic contraction, the amplitude of the surface EMG is often larger than that of static MVC. In order to investigate the difference between MVC and ballistic contraction, the following analysis was undertaken in a model, using numerical simulation for the different types of contraction. 「Methods」 In the model, sixty F-type motor units (MUs) with a radius of 5 mm and sixty S-type MUs with a radius of 2.5 mm were distributed randomly over a range of 50 mm in transverse direction and 30 mm in depth. F-type MUs and S-type MUs comprised 393 fibers and 98 fibers, respectively. For simulation, the inputs to the model consisted of the following parameters for each MU: peak force, contraction time, firing rate, gain of force, conduction velocity, and current intensity of each MU. The output of the simulation was evaluated for both MVC and ballistic contraction. 「Results」 According to the variable location and depth of active MUs, the amplitude of the surface EMG recorded during static contraction was always different, even if the muscle force was equal. For the case of ballistic contraction, the amplitude of the simulated surface EMG for synchronous firing was larger than that obtained when MUs were firing asynchronously, even when the muscle forces were equal. 「Discussion」 The simulation shows that the difference in the surface EMG amplitude between MVC and ballistic contraction is due to the timing of MU firing, i.e., the degree of MU synchronization. When the ballistic contraction was normalized according to the %MVC index, the force was evaluated at 10 times the force during MVC. The simulation therefore shows that, in an actual muscle, to correctly evaluate the surface EMG recorded during ballistic contraction using the %MVC index, it is essential to obtain physiological information concerning the degree of synchronization in the surface EMG.