Central control and reflex regulation of mechanical impedance: The basis for a unified motor-control scheme

Adaptability of innate motor patterns and motor control mechanisms

The following factors underlying behavioral plasticity are discussed: (1) reflex adaptability and its role in the voluntary control of movement, (2) degrees of freedom and motor equivalence, and (3) the problem of the discrete organization of motor behavior. Our discussion concerns a variety of innate motor patterns, with emphasis on the wiping reflex in the frog.

It is proposed that central regulation of stretch reflex thresholds governs voluntary control over muscle force and length. This suggestion is an integral part of the equilibrium-point hypothesis, two versions of which are compared.

Kinematic analysis of the wiping reflex in the spinal frog has shown that each stimulated skin site is associated with a group of different but equally effective trajectories directed to the target site. Such phenomena reflect the principle of motor equivalence -the capacity of the neuronal structures responsible for movement to select one or another of a set of possible trajectories leading to the goal. Redundancy of degrees of freedom at the neuronal level as well as at the mechanical level of the body's joints makes motor equivalence possible. This sort of equivalence accommodates the overall flexibility of motor behavior.

An integrated behavioral act or a single movement consists of dynamic components. We distinguish six components for the wiping reflex, each associated with a certain functional goal, specific body positions, and motor-equivalent movement patterns. The nervous system can combine the available components in various ways in forming integrated behavioral sequences. The significance of command neuronal organization is discussed with respect to (1) the combinatory strategy of the nervous system and (2) the relation between continuous and discrete forms of motor control. We conclude that voluntary movements are effected by the central nervous system with the help of the mechanisms that underlie the variability and modifiability of innate motor patterns.

Feedback control of limb stiffness and scaled phase invariance properties of skilled high-speed arm flexion movements of a loaded manipulator

In this and a previous experiment it has been observed that subjects produce innervation patterns (EMG) that are load specific, i.e., they produce concentration patterns for movements made with inertial loads and triphasic patterns for movements made with elastic loads. The protocol of these experiments prevented any adaptive responses to the load changes, therefore, it was assumed that pattern matching to load type was a real time updating response of the peripheral feedback systems. This updating response was assumed to be a mechanism for fine tuning the muscle torque by regulation of the mechanical impedance (stiffness) of the limb. Using a standard equation of motion, it was shown that the velocity is equal to the ratio of the muscle torque to the mechanical impedance. Substitution of this ratio for the ordinate of the scaled phase diagrams was then suggested as a real time updating mechanism to account for the scaled phase invariance recorded in this experiment and in the experiment reported by Ruitenbeek, J.C., Biol. Cybernetics 51 (1984) 11-20.

Peripheral regulation of stiffness during arm movements by coactivation of the antagonist muscles

Two experiments investigated whether unexpected and differential loading of a rapid, unsighted arm movement resulted in the central nervous system (CNS) regulating limb stiffness by modifying the associated neuromuscular activity. In Experiment 1, subjects completed multiple, spring-loaded training trials until a prespecified criterion of learning was attained. On selected trials, the spring load was unexpectedly replaced by an inertial load. Results indicated that to maintain positional accuracy during this inertial load trial, limb stiffness was increased by coactivating the antagonist muscles, i.e. by changing the associated neuromuscular activity from a predominantly triphasic pattern to one of coactivation. In Experiment 2, the sequence of loading was reversed producing a change in the required limb stiffness from a relatively high to low level. This change was observed as a pattern of coactivation being replaced by a triphasic activity pattern. These results support the notion that limb stiffness is regulated primarily through modification of the neuromuscular activity pattern prior to movement termination. It was also demonstrated that the size of the unexpected load did not affect the basic activation pattern selected by the CNS. It is proposed that the signal which triggers the CNS to regulate limb stiffness is based on peripheral information generated as a result of agonist activity occurring during the first part of the movement.