Abstract
Recent studies in the field of nonlinear dynamics have shown that the motion of a damped, driven pendulum transits between non-chaotic and chaotic states. This has raised an important question to the field of human movement control: why are human rhythmical movements apparently capable of avoiding chaos? In the present study, it is hypothesized that readjustment of the stiffness with increasing movement frequency in human rhythmical movement is necessary to maintain the order of rhythmical movement avoiding chaotic states. In other words, movement-generated afferent signals are used for readjustment of the stiffness so as to maintain the order of rhythmical movement. The stiffness change with increasing movement frequency was checked experimentally using EMG signals. Subjects extended and flexed their elbow joints rhythmically at frequency ranging from 1.0 to 3.0 Hz in step of 0.5 Hz. It was found that the joint stiffness was readjusted with increasing forcing frequency. A simulation study of the elbow joint motions confirmed that if the readjustment of the joint stiffness was not done the stability of rhythmical movement was lost. These experimental data and simulation data supported the hypothesis.
