Relations between the directions of vibration-induced kinesthetic illusions and the pattern of activation of antagonist muscles.

In humans, tendon vibration evokes illusory sensations of movement that are usually associated with an excitatory tonic response in muscles antagonistic to those vibrated (antagonist vibratory response, AVR), i.e., in the muscle groups normally contracted if the illusory movement had been performed. The aim of the present study was to investigate the relation between the parameters of the illusory sensation of movement and those of the AVR and to determine whether vectorial models could account for the integration of proprioceptive inputs from several muscles, as well as for the organization of the elementary motor commands leading to one unified motor response. For that purpose, we analyzed the relations between the anatomical site of the tendon vibration, the direction of the illusory movement, the muscles in which the AVR develops, and the characteristics of the AVR (surface EMG, motor unit types, firing rates, and activation latencies). This study confirmed the close relationship between the parameters of an AVR and those of the kinesthetic illusion. It showed that, during illusions of movements in different directions, motor units are activated according to a specific pattern correlated with their type, with the direction of the illusory movement and with the biomechanical properties of their bearing muscles. Finally, kinesthetic illusions and AVRs can be effectively represented using similar vectorial computations. These strong relations between the perceptual and motor effects of tendon vibration once again suggest that the AVR may result from a perceptual-to-motor transformation of proprioceptive information, rather than from spinal reflex mechanisms.