Modulation of stretch reflexes to environmental dynamics and perturbation properties

Abstract

Reflexes and co-contraction are the two mechanisms used for effective limb control when humans face unexpected perturbations in their daily activities. When the environment has reduced stability margins, reflexes are tempered due to the oscillations caused by the neural time delay of the reflexive pathways. An explanation is that reflexes adapt to the dynamics of the environment and stability margins are the constraint. This view requires that humans assess stability margins, which could happen by detecting changes (i.e. oscillations) in the perturbation eliciting the reflexes. The perturbation perceived by the human is the actual perturbation filtered by the dynamics of the environment. Therefore, changes in the stability of the environment influence the perceived perturbation. The goal of this study is to determine whether reflex modulation is triggered by the environmental dynamics (i.e. damping and stability margins) or by the properties of the perturbation eliciting a reflexive response. An experiment was designed where participants were asked to minimize the displacements caused by continuous force perturbations applied to the hand while interacting with different environmental dynamics. Some of the perturbations were prefiltered to mimic the filtering effect of the environmental dynamics. Variations in the dynamics of the shoulder joint were quantified through the estimated arm admittance (i.e. displacements in response to force perturbations). The results show variations in the admittance between the perturbations mimicking the environment and the true environment at low frequencies (below 5Hz); and for different prior knowledge about the environmental dynamics (below 2Hz). These variations indicate that perturbations can be designed to mimic the environmental dynamics, and that perturbation properties and stability constraints cause changes in the motor behaviour.