Kinematic and dynamic processes for the control of pointing movements in humans revealed by short-term exposure to microgravity

doi:10.1016/j.neuroscience.2005.06.063

Neuroscience

Volume 135, Issue 2, 2005, Pages 371-383

https://doi.org/10.1016/j.neuroscience.2005.06.063 Get rights and content

Abstract

The generation of accurate motor commands requires implicit knowledge of both limb and environmental dynamics. The action of gravity on moving limb segments must be taken into account within the motor command, and may affect the limb trajectory chosen to accomplish a given motor task. Exactly how the CNS deals with these gravitoinertial forces remains an open question. Does the CNS measure gravitational forces directly, or are they accommodated in the motor plan by way of internal models of physical laws? In this study five male subjects participated. We measured kinematic and dynamic parameters of upward and downward arm movements executed at two different speeds, in both normal Earth gravity and in the weightless conditions of parabolic flight. Exposure to microgravity affected velocity profiles for both directions and speeds. The shape of velocity profiles (the ratio of maximum to mean velocity) and movement duration both showed transient perturbations initially in microgravity, but returned to normal gravity values with practice in 0×g. Differences in relative time to peak velocity between upward versus downward movements, persisted for all trial performed in weightlessness. These differences in kinematic profiles and in the torque profiles used to produce them, diminished, however, with practice in 0×g. These findings lead to the conclusion that the CNS explicitly represents gravitational and inertial forces in the internal models used to generate and execute arm movements. Furthermore, the results suggest that the CNS adapts motor plans to novel environments on different time scales; dynamics adapt first to reproduce standard kinematics, and then kinematics patterns are adapted to optimize dynamics.

Section snippets

Experimental procedures

Data presented in this study were taken from experiments made in a normal gravitational environment (1×g) and in microgravity (0×g); the latter achieved during parabolic flights. The experiments were performed on three flights executed on each of three successive flight days. Flights were composed of 30 parabolas, each consisting of three successive phases with respect to normal gravity: i) hypergravity ∼1.8×g, ii) microgravity, ∼0×g and iii) hypergravity ∼1.8×g. Each phase lasted ∼20–25 s, the

General characteristics

Fig. 1 shows stick diagrams of the upper arm and forearm during upward motion, and the paths of the fingertip in the sagittal plan for the two movement directions and speeds conditions in both normal and microgravity environments. Prior to the onset of an upward movement, subjects upper arm elevation was ∼30° and elbow anatomical angle was ∼120° and prior to the onset of a downward movement upper arm elevation was ∼90° and elbow anatomical angle was ∼125°. Fingertip paths were curved with

Discussion

Both short-lived perturbations with rapid re-adaptation and long-term quasi-permanent alterations in hand kinematics were observed over the course of the experiment for each of the participating subjects. These observations provide insight into how the CNS treats gravitational and inertial forces in the planning and execution of point-to-point arm movements.

Acknowledgments

This work was supported by CNES (Centre National d’Etudes Spatiales).

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Kinematic and dynamic processes for the control of pointing movements in humans revealed by short-term exposure to microgravity

Abstract

Section snippets

Experimental procedures

General characteristics

Discussion

Acknowledgments

Curr Opin Neurobiol

Brain Res Brain Res Rev

Neurosci Lett

Acta Astronaut

Brain Res Brain Res Rev

Kinematic features of unrestrained vertical arm movements

J Neurosci

Learning arm kinematics and dynamics

Annu Rev Neurosci

The effects of a change in gravity on the dynamics of prehension

Exp Brain Res

Recovery of the locomotor function after prolonged microgravity exposure. I. Head-trunk movement and locomotor equilibrium during various tasks

Exp Brain Res

Computation of inverse dynamics for the control of movements

Biol Cybern

Gravitoinertial force level influences arm movement control

J Neurophysiol

The role of internal models in motion planning and controlevidence from grip force adjustments during movements of hand-held loads

J Neurosci

Two components of muscle activationscaling with the speed of arm movement

J Neurophysiol

The coordination of arm movementsan experimentally confirmed mathematical model

J Neurosci

Signal-dependent noise determines motor planning

Nature

Object size effects on initial lifting forces under microgravity conditions

Exp Brain Res