Exp Brain Res (2003) 153: 343–355 DOI 10.1007/s00221-003-1576-4 RESEARCH ARTICLE E. Tunik . H. Poizner . M. F. Levin . S. V. Adamovich . J. Messier . Y. Lamarre . A. G. Feldman Arm–trunk coordination in the absence of proprioception Received: 18 March 2003 / Accepted: 5 June 2003 / Published online: 19 September 2003 # Springer-Verlag 2003 Abstract During trunk-assisted reaching to targets placed within arms length, the influence of trunk motion on the hand trajectory is compensated for by changes in the arm configuration. The role of proprioception in this compen- sation was investigated by analyzing the movements of 2 deafferented and 12 healthy subjects. Subjects reached to remembered targets (placed ~80° ipsilateral or ~45° contralateral to the sagittal midline) with an active forward movement of the trunk produced by hip flexion. In 40% of randomly selected trials, trunk motion was mechanically blocked. No visual feedback was provided during the experiment. The hand trajectory and velocity profiles of healthy subjects remained invariant whether or not the trunk was blocked. The invariance was achieved by changes in arm interjoint coordination that, for reaches toward the ipsilateral target, started as early as 50 ms after the perturbation. Both deafferented subjects exhibited considerable, though incomplete, compensation for the effects of the perturbation. Compensation was more successful for reaches to the ipsilateral target. Both deafferented subjects showed invariance between condi- tions (unobstructed or blocked trunk motion) in their hand paths to the ipsilateral target, and one did to the contralateral target. For the other deafferented subject, hand paths in the two types of trials began to deviate after about 50% into the movement, because of excessive elbow extension. In movements to the ipsilateral target, when deafferented subjects compensated successfully, the changes in arm joint angles were initiated as early as 50 ms after the trunk perturbation, similar to healthy subjects. Although the deafferented subjects showed less than ideal compensatory control, they compensated to a remarkably large extent given their complete loss of proprioception. The presence of partial compensation in the absence of vision and proprioception points to the likelihood that not only proprioception but also vestibu- lospinal pathways help mediate this compensation. Keywords Motor equivalence . Reaching . Multijoint coordination . Kinematics . Compensatory synergy . Deafferentation . Vestibular system Introduction Due to the redundant number of degrees of freedom (DFs) of the body, a given movement can be performed using different kinematic and kinetic patterns. Redundancy underlies an essential property of neuromuscular control —motor equivalency, or the ability to reach the motor goal using different body configurations and environmental means (Lashley 1951; Bernstein 1967). For example, the hand and finger kinematics in reaching and grasping the same object in space can vary depending on the need to avoid obstacles and the shape of the object to be grasped. Understanding how the nervous system selects movement patterns from the redundant set of DFs while preserving the efficiency of the motor action is fundamental for the E. Tunik . H. Poizner (*) . S. V. Adamovich . J. Messier Center for Molecular and Behavioral Neuroscience, Rutgers University, 197 University Avenue, Newark, NJ 07102, USA e-mail: poizner@axon.rutgers.edu Tel.: +1-973-3531080 ext. 3231 Fax: +1-973-3531272 M. F. Levin . A. G. Feldman Center for Interdisciplinary Research in Rehabilitation (CRIR), Rehabilitation Institute of Montreal, Montreal, Quebec, Canada Y. Lamarre . A. G. Feldman Neurological Science Research Center, Department of Physiology, University of Montreal, Montreal, Quebec, Canada M. F. Levin School of Rehabilitation, Faculty of Medicine, University of Montreal, Montreal, Quebec, Canada Present address: J. Messier Department of Kinesiology, University of Montreal, Montreal, Quebec, Canada