Oscillator mechanisms in the human motor system: investigating their properties using the aftercontraction effect

It is proposed that the human motor system is organized to use hardware and/or software non-linear oscillator mechanisms, the output of these oscillators being responsible for driving the limbs via signals to muscle groups. Following earlier theoretical development, it is argued that these muscle groupings act as a unit and themselves are likely to behave as a non-linear system. The attributes of non-linear oscillators are many, and they are potentially significant for the explanation of motor behavior. This paper reviews and presents recent experiments that investigated the properties of muscular aftercontraction. The basic finding shows that subsequent to a period of moderate strain against a fixed surface the treated limb exhibits prolonged involuntary molar oscillations in the plane of the treatment. These results provide for the presence of driving oscillator mechanisms in the human motor apparatus. The mechanisms show generality of action in that directed attention can lead to oscillation of untreated limbs. Overall, the experiments showed that the movements exhibited the mutual interaction, synchronization, and preservation of phase relationships that are fundamental properties of non-linear oscillators. the picture that emerges is that these mechanisms can drive involuntary movements that are richly patterned: like slow versions of voluntary movements. The aftercontraction phenomenon proves to be an excellent tool for research on the oscillatory substrate of human motor organization.