Observation of an action evokes in the observer the subliminal activation of the same motor program needed to execute it (motor resonance, MR). To perform an action, the motor program encodes both the primary movement and the anticipatory postural adjustments (APAs), acting to prevent the segmental or the whole-body equilibrium disturbances due to the primary movement. Should MR be the hidden replica of the entire motor program behind an action, a resonant activation would then be expected for both primary movement and APAs.

To test this hypothesis, 15 subjects observed a video showing a rapid wrist flexion performed with the supine hand, which is known to activate a strong APA in arm muscles. Motor evoked potentials (MEPs) were recorded in Flexor Carpi Radialis (FCR, prime mover) and Biceps Brachii (BB, postural muscle) by means of transcranial magnetic stimulation applied over the left primary motor cortex. MEPs amplitudes were measured at different intervals before and after the onset of the observed movement. Results show a significant modulation of excitability both in FCR and in BB muscles, the last occurring in advance with respect to the FCR, as it happens during a real wrist flexion.

These data demonstrate that the resonant response replicates both the primary movement and APAs. Given that MR reflects aspects intrinsic to motor programming, this result strongly reinforces the oneness of the neural process producing both the primary movement and the APAs.