EMG hysteresis patterns in human elbow muscles under simultaneous force and length changes

This study investigated signal transmission in the human motor system, focusing on the influence of coinciding and opposing directions of change in key mechanical parameters - muscle length and force - on electromyograms (EMGs). The goal is to understand how these mechanical changes affect muscle activation during movement, a crucial aspect for developing a comprehensive model of motor control. Eight participants performed movement tests using a robotic-mechatronic device. Each test was repeated ten times to obtain averaged EMG signals from the elbow muscles. The participants developed active elbow flexor contractions while pressing a force vector-measuring manipulator in biofeedback mode. The manipulator's movement trajectory and visual biofeedback were preprogrammed to create time profiles of muscle length and force changes using a double trapezoid waveform. The EMG data were compared for opposing pattern (OP) and coinciding pattern (CP) movements in terms of length and force changes. During OP movements, the averaged EMG waveforms followed programmed force changes, with some natural distortions. However, CP movements strongly compensated for the differences in force and length, reducing EMG variation. Statistical analysis revealed that the amplitude of the force factor did not significantly affect the integrated areas of EMGs or their peak amplitudes, but the directional force factor and interaction of factors were effective. Interestingly, muscle hysteresis effects, where higher EMG activity typically corresponds to muscle shortening, were violated by some agonists but preserved by others. This study highlights the complex mechanical interactions and redistribution of forces among synergistic muscles during different movement patterns. These findings may help refine the "thermodynamic" model of muscle activation, providing insights into how central commands are modulated during motor tasks involving both coinciding and opposing mechanical changes in muscle length and force.