ANTICIPATORY POSTURAL ADJUSTMENTS BEFORE LOAD PICKUP IN A BI-MANUAL WHOLE-BODY LIFTING TASK

Balance regulation and movement control were examined in the context o f bi-manual lifting. Subjects picked up a load (20% body mass) after s everal unloaded cycles using the leg-lift technique. The addition of t he load to the body caused the system center of mass to shift forward and thus presented the subject with an expected perturbation of balanc e. To examine whether the disturbances to balance were counteracted by anticipatory postural adjustments, the last cycle, in which the barbe ll was grasped and lifted, was compared with the preceding unloaded cy cle. Using a global mechanical analysis of the movement, we found that anticipatory postural adjustments were present before load pickup in bi-manual lifting. These anticipatory postural adjustments were charac terized by a backward directed horizontal momentum, a backward directe d horizontal component of the ground reaction force accompanied with a forward shift of the center of pressure, and a backward shift of the center of mass (CoM). These characteristics could all be understood fr om the mechanical consideration that adding a load in front of the bod y induces a forward shift of the CoM. However, major compensations of the position of the CoM were also observed after bar grasp. It is ther efore proposed that commands giving rise to postural adjustments are c losely tied to commands controlling the ongoing movement. On the basis of this insight the strict dichotomy in the control of posture and mo vement is being questioned.