Effects of kinematics constraints on hand trajectory during whole-body lifting tasks

Effects of Parkinson's disease on proprioceptive control of posture and reaching while standing

Although previous studies have shown pointing errors and abnormal multijoint coordination in seated subjects with Parkinson's disease (PD) who cannot view their arm, the extent to which subjects with PD have problems using proprioception to coordinate equilibrium maintenance and goal-oriented task execution has not been adequately investigated. If a common motor program controls voluntary arm pointing movements and the accompanying postural adjustments, then impairments of proprioceptive integration in subjects with PD should have similar effects on pointing and body center of mass (CoM) control with eyes closed. Ten standing subjects with PD (OFF-medication) and 10 age-matched control (CTR) subjects pointed to a target with their eyes closed and open. Although pointing accuracy was not significantly different between groups, body CoM displacements were reduced in subjects with PD, but not in CTR, when eyes were closed. In addition, with eyes closed, PD subjects showed reduced temporal coupling between pointing and CoM velocity profiles and reduced spatial coupling between pointing and CoM endpoints. This poor coupling with eyes closed could be related to the PD subjects' increased jerkiness of CoM displacements. The different effects of eye closure between CTR and PD subjects on the CoM displacements, but not pointing accuracy, are consistent with separate motor programs for the pointing and postural components of this task. Furthermore, the decoupling between the two movement components in subjects with PD when they could not use vision, suggests that the basal ganglia are involved in the integration of proprioceptive information for posture-movement coordination.

A mathematical tool to generate complex whole body motor tasks and test hypotheses on underlying motor planning

In spite of the complexity of human motor behavior, difficulties in mathematical modeling have restricted to rather simple movements attempts to identify the motor planning criterion used by the central nervous system. This paper presents a novel-simulation technique able to predict the "desired trajectory" corresponding to a wide range of kinematic and kinetic optimality criteria for tasks involving many degrees of freedom and the coordination between goal achievement and balance maintenance. Employment of proper time discretization, inverse dynamic methods and constrained optimization technique are combined. The application of this simulator to a planar whole body pointing movement shows its effectiveness in managing system nonlinearities and instability as well as in ensuring the anatomo-physiological feasibility of predicted motor plans. In addition, the simulator's capability to simultaneously optimize competing movement aspects represents an interesting opportunity for the motor control community, in which the coexistence of several controlled variables has been hypothesized.

Human whole-body reaching in normal gravity and microgravity reveals a strong temporal coordination between postural and focal task components

Previous experiments by our group in normal gravity (1 G) have revealed spatial relationships between postural and focal components of whole-body reaching and pointing movements. We suggested that these relationships could be explained partly through the use of gravity to displace the CoM and attain the object or target position. In this study we compared human whole-body reaching in 1 G and microgravity (0 G) in order to more fully investigate how gravity contributes to strategies adopted for task execution and to determine possible invariant temporal relationships between multiple segments. Whole-body reaching movements made from the standing position in two experimental conditions of execution speed (naturally paced and as fast as possible) were recorded during periods of 1 G and 0 G in parabolic flight. Overall, at each speed of reaching, movement times were significantly slower when performed in 0 G than in 1 G for two of the three subjects, but all subjects were able to produce significantly faster movements in 0 G than in 1 G. Despite similar general trends across subjects observed in 1 G, angular displacements of reaching movements performed in 0 G differed greatly between subjects. There were changes at all joints, but above all at the shoulder and the ankle. However, despite a high intersubject and intratrial variability in 0 G, in both gravity conditions all subjects demonstrated times to peak curvilinear velocity for the finger (end effector) and the whole-body centre of mass (CoM) that coincided, regardless of the speed of execution. Moreover, cross-correlations between multiple segment curvilinear velocities and those of the CoM revealed tight, highly correlated temporal relationships between segments proximal to the CoM (which was expected). However, for more distal segments, the correlations were weaker, and the movements lagged behind movements of the CoM. The major and most interesting finding of this study was that although the finger was the most distal within the segment chain, with respect to the CoM, it was highly correlated with the CoM (0.99--0.98, all conditions) and with no time lag. Despite the large intersubject and inter-environmental variability recorded in this study, temporal relationships between postural task components (CoM displacements) and those of the focal movement (end-effector trajectory) were consistently conserved.