Anticipatory postural adjustments during load catching by standing subjects

Objectives: (1) To study differences in the generation of anticipatory postural adjustments (APAs) in arm and trunk/leg muscles prior to catching a load released either by the subject him-/herself or by the experimenter. (2) To study the importance of different mechanical characteristics of the load at impact for the generation of APAs prior to load catching. Methods: Standing subjects were asked to catch loads dropped onto the left hand from different heights either by the experimenter or by the subjectʹs right hand. The load mass and release height were manipulated to keep either the mass or the momentum of the load at impact constant. APAs were quantified with integral electromyographic indices. Results: APAs were observed in leg, trunk and arm muscles prior to load impact for both self- and experimenter-release trials. Kinetic energy showed higher correlations with the magnitude of APA than momentum, but only in experimenter-release trials. Conclusions: Subjects can generate APAs in both arm and trunk/leg muscles in the absence of an explicit voluntary action. The relative importance of kinetic energy and momentum for defining the magnitude of APAs can reflect the difference in the sources of information used to prepare for the forthcoming perturbation during self- and experimenter-released load catch.