Slow Limb Movements Require Precise Control of Muscle Stiffness

TL;DR

This paper demonstrates that precise control of muscle stiffness, combined with muscle pre-tensioning, is essential for stable, accurate, and slow limb movements, highlighting the importance of muscle tone and neural control in motor functions.

Contribution

It introduces a neuromechanical model showing how muscle stiffness control enables stable slow movements, emphasizing the role of pre-tensioning and non-smooth control dynamics.

Findings

Stable postures require pre-tensioned muscles.

Slow movements are inherently non-smooth.

Muscle stiffness control suffices for accurate limb positioning.

Abstract

Slow and accurate finger and limb movements are essential to daily activities, but their neural control and governing mechanics are relatively unexplored. We consider neuromechanical systems where slow movements are produced by neural commands that modulate muscle stiffness. This formulation based on strain-energy equilibria is in agreement with prior work on neural control of muscle and limb impedance. Slow limb movements are driftless in the sense that movement stops when neural commands stop. We demonstrate, in the context of two planar tendon-driven systems representing a finger and a leg, that the control of muscle stiffness suffices to produce stable and accurate limb postures and quasi-static (slow) transitions among them. We prove, however, that stable postures are achievable only when muscles are pre-tensioned, as is the case for natural muscle tone. Our results further indicate, in accordance with experimental findings, that slow movements are non-smooth. The non-smoothness arises because the precision with which individual muscle stiffnesses need to be controlled changes substantially throughout the limb's motion. These results underscore the fundamental roles of muscle tone and accurate neural control of muscle stiffness in producing stable limb postures and slow movements.