Effects of foreperiod duration on anticipatory postural adjustments: determination of an optimal preparation in standing and sitting for a raising arm movement.

The purpose of the present experiment was to examine the motor and postural preparation processes evolving during the foreperiod (known to be optimal at 500 ms in sitting) in a voluntary upper limb movement executed while standing. The foreperiod duration (300, 500, 700 and 900 ms) and the postural conditions (sitting versus standing) were manipulated using a priming procedure. Two types of prime were provided: (1) a prime giving valid information on the side of the raising arm movement to execute, and (2) a prime giving non-valid information (prime opposite to the required side). In a sitting and standing condition, eight normal subjects performed a raising arm movement with a 1 kg load at the level of the wrist. Premotor time and acceleration of arm movement were recorded in the two postural conditions. In the standing condition, anticipatory postural adjustments (APAs) were also quantified using the relative latency of electromyographic (EMG) data and the centre of foot pressure (CoP) displacements. Results (1) showed that the optimal foreperiod duration (i.e., leading to the shortest premotor time) increased as a function of postural conditions (500 versus 700 ms in the sitting and standing conditions, respectively), and (2) emphasized the existence of a temporal modulation in the central organization of the postural and focal commands according to the foreperiod duration.