Modulation of anticipatory postural adjustments in a reactive and a self-triggered mode in humans.

Five subjects raised their preferred arm until horizontal in one of two experimental conditions: (1) in response to a visual signal (reactive condition), and (2) in a self-triggered condition. In both conditions, subjects were instructed to execute the movement at approximately 80% of their maximal velocity, such that all movements were nearly similar. EMG activity was recorded in a focal muscle (Anterior Deltoidus) and in three postural muscles (ipsi-lateral Gastrocnemius, contra-lateral Tensor Fascia Latae and ipsi lateral Semitendinosus) known to be activated prior to the focal muscle. Results showed greater anticipatory postural adjustments in the self-triggered than in the reactive condition for the three postural muscles. It is suggested that similar intentional movements can be triggered by different timing strategies.