A theory is presented to explain how accurate, single-joint movements are controlled. The theory applies to movements across different distances, with different inertial loads, toward targets of different widths over a wide range of experimentally manipulated velocities. The theory is based on three propositions. (1) Movements are planned according to “strategies” of which there are at least two: a speed-insensitive (SI) and a speed-sensitive (SS) one. (2) These strategies can be equated with sets of rules for performing diverse movement tasks. The choice between SI and SS depends on whether movement speed and/or movement time (and hence appropriate muscle forces) must be constrained to meet task requirements. (3) The electromyogram can be interpreted as a low-pass filtered version of the controlling signal to the motoneuron pools. This controlling signal can be modelled as a rectangular excitation pulse in which modulation occurs in either pulse amplitude or pulse width. Movements to different distances and with loads are controlled by the SI strategy, which modulates pulse width. Movements in which speed must be explicitly regulated are controlled by the SS strategy, which modulates pulse amplitude. The distinction between the two movement strategies reconciles many apparent conflicts in the motor control literature.

Strategies for the control of voluntary movements with one mechanical degree of freedom

Models for the speed and accuracy of aimed movements.

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.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0140525X00051268

Adaptability of innate motor patterns and motor control mechanisms.

The aim of this study has been to identify, in humans, the braking process underlying voluntary forearm movements performed at various velocities and amplitudes and against different inertial loads. The procedure used to estimate the parameters of the braking process involved kinematic analysis of the movement and measurement of alpha-motoneural input to flexor and extensor muscles by recording electromyographic (EMG) activity. We have shown that when the agonist force does not exceed the passive viscoelastic tension developed by the extensor and flexor muscles, the movement can be braked by the viscoelastic forces alone. In contrast, above this force threshold, it was found that the motor output in agonist and antagonist muscles consists of well defined bursts of EMG activity, first in agonist then in antagonist muscles. The timing of these two bursts (duration of the agonist activity and onset of the antagonist activity) are clearly correlated with the value of the peak velocity. For the same peak velocity, the addition of inertial loads increases the excitation level of both agonist and antagonist muscles but does not change the timing of the EMG bursts. The discussion of these results focuses on the concept of “unit of movement” organized to reduce the large number of possible patterns of activation of the muscles acting on the same joint.

Effects of inertial load and velocity on the braking process of voluntary limb movements.

Movements by a standing person are commonly associated with adjustments in the activity of postural muscles to cause a desired shift of the center of pressure (COP) and keep balance. We hypothesize that such COP shifts are controlled (stabilized) using a small set of central variables (muscle modes, M-modes), while each M-mode induces changes in the activity of a subgroup of postural muscles. The main purpose of this study has been to explore the possibility of identification of muscle synergies in a postural task using the framework of the uncontrolled manifold (UCM) hypothesis employing the following three steps in data analysis: (i) Identification of M-modes: Subjects were asked to release a load from extended arms through a pulley system, resulting in a COP shift forward prior to load release. Electromyographic (EMG) activity of eleven postural muscles on one side of the body was integrated over a 100 ms interval corresponding to the early stage of the COP shift, and subjected to a principal component (PC) analysis across multiple repetitions of each task. Three PCs were identified and associated with a ‘push-back M-mode’, a ‘push-forward M-mode’ and a ‘mixed M-mode’. (ii) Calculation of the Jacobian of the system, which relates changes in the magnitude of M-modes to COP shifts using regression techniques: Subjects performed three different tasks (releasing different loads at the back, voluntarily shifting body weight forward and backward, at different speeds) to verify if the relationship between magnitudes of M-modes and COP shifts is task or direction specific. (iii) UCM analysis: Three tasks were chosen (load release in the front, arm movement forward and backward) which were associated with an early shift in COP. A manifold was identified in the M-mode space corresponding to a certain average (across trials) shift of the COP and variance per degree of freedom within the UCM (VUCM) and orthogonal (VORT) to the UCM was computed. Across subjects, VUCM was significantly higher than VORT when analysis at the third step was performed using a Jacobian computed based on a set of tasks associated with a COP shift in the same direction but not in the opposite direction. This result confirms our hypothesis that the M-modes work together as a synergy to stabilize a desired shift of the COP. Forward and backward COP shifts are associated with different synergies based on the same three M-modes.

/pdf/muscle-synergies-during-shifts-of-the-center-of-pressure-by-13kd6lxhhk.pdf

Muscle synergies during shifts of the center of pressure by standing persons

The stability of synergy in agonists during the execution of a simple voluntary movement.

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The stability of synergy in agonists during the execution of a simple voluntary movement.