The eect of timing of a perturbation on the execution of a lifting movement J.C.E. van der Burg * , J.H. van Die en Faculty of Human Movement Sciences, Institute for Fundamental and Clinical Human Movement Sciences, Vrije Universiteit Amsterdam, 1081 BT Amsterdam, Netherlands Abstract Anticipation to the mass magnitude is important in lifting, but underestimation of a mass, in contrast to overestimation, does not cause major movement disturbances. This may be caused by corrections in muscle activity before the object is actually lifted. This study was designed to assess the importance of these corrections during the loading phase for the exe- cution of a lifting movement when the mass is underestimated. Ten subjects lifted a box 1.6 kg), of which the mass was increased by 10 kg without them knowing so. The mass was added either before the box had been lifted from the ground perturbation before lift-o, PBL) or right perturbation after lift-o PAL). In the PBL condition back muscle activity was in- creased before lift-o. Even though this early corrective response could obviously not occur in the PAL condition, the lifting movement was executed without clear problems. In sum, cor- rections in muscle activity before lift-o are not necessary for adequate correction of a per- turbation induced by an unexpected heavier object. Ó 2001 Elsevier Science B.V. All rights reserved. PsycINFO classi®cation: 2330 Keywords: Motor coordination; Muscle contractions; Sensory feedback Human Movement Science 20 2001) 243±255 www.elsevier.com/locate/humov * Corresponding author. Tel.: +31-20-4448477; fax: 31-20-4448529. E-mail address: p_van_der_burg@fbw.vu.nl J.C.E. van der Burg). 0167-9457/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII:S0167-945701)00042-2