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Slow and accurate finger and limb movements are essential to dailyactivities, but their neural control and governing mechanics are relativelyunexplored. We consider neuromechanical systems where slow movements areproduced by neural commands that modulate muscle stiffness. This formulationbased on strain-energy equilibria is in agreement with prior work on neuralcontrol of muscle and limb impedance. Slow limb movements are driftless in thesense that movement stops when neural commands stop. We demonstrate, in thecontext of two planar tendon-driven systems representing a finger and a leg,that the control of muscle stiffness suffices to produce stable and accuratelimb postures and quasi-static (slow) transitions among them. We prove,however, that stable postures are achievable only when muscles arepre-tensioned, as is the case for natural muscle tone. Our results furtherindicate, in accordance with experimental findings, that slow movements arenon-smooth. The non-smoothness arises because the precision with whichindividual muscle stiffnesses need to be controlled changes substantiallythroughout the limb's motion. These results underscore the fundamental roles ofmuscle tone and accurate neural control of muscle stiffness in producing stablelimb postures and slow movements.

Slow Limb Movements Require Precise Control of Muscle Stiffness