Preparatory trunk motion accompanies rapid upper limb movement

Evaluation of trunk movements, trunk muscle activation, intra-abdominal pre ssure and displacement of centres of pressure and mass was undertaken to de termine whether trunk orientation is a controlled variable prior to and dur ing rapid bilateral movement of the upper limbs. Standing subjects performe d rapid bilateral symmetrical upper limb movements in three directions (fle xion, abduction and extension). The results indicated a small (0.4-3.3 degr ees) but consistent initial angular displacement between the segments of th e trunk in a direction opposite to that produced by the reactive moments re sulting from limb movement. Phasic activation of superficial trunk muscles was consistent with this pattern of preparatory motion and with the directi on of motion of the centre of mass. In contrast, activation of the deep abd ominal muscles was independent of the direction of limb motion, suggesting a non-direction specific contribution to spinal stability. The results supp ort the opinion that feedforward postural responses result in trunk movemen ts, and that orientation of the trunk and centre of mass are both controlle d variables in relation to rapid limb movements.