Mark L. Latash

Intrathecal baclofen for severe spinal spasticity

We studied the effect of the intrathecal infusion of baclofen, an agonist of gamma-aminobutyric acid, on abnormal muscle tone and spasms associated with spinal spasticity, in a randomized double-blind crossover study. Twenty patients with spinal spasticity caused by multiple sclerosis or spinal-cord injury who had had no response to treatment with oral baclofen received an intrathecal infusion of baclofen or saline for three days. The infusions were administered by means of a programmable pump implanted in the lumbar subarachnoid space. Muscle tone decreased in all 20 patients (mean [+/- SD] Ashworth score for rigidity, from 4.0 +/- 1.0 to 1.2 +/- 0.4; P less than 0.0001), and spasms were decreased in 18 of the 19 patients who had spasms (mean [+/- SD] score for spasm frequency, from 3.3 +/- 1.2 to 0.4 +/- 0.8; P less than 0.0005). Tests for motor function, neurologic examination, and assessments by the patients correctly indicated when baclofen was being infused in all cases. All patients were then entered in an open long-term trial of continuous infusion of intrathecal baclofen. During a mean follow-up period of 19.2 months (range, 10 to 33), muscle tone has been maintained within the normal range (mean Ashworth score, 1.0 +/- 0.1) and spasms have been reduced to a level that does not interfere with activities of daily living (mean spasm score, 0.3 +/- 0.6). No drowsiness or confusion occurred, one pump failed, and two catheters became dislodged and had to be replaced. No infections were observed. Our observations suggest that intrathecal baclofen is an effective long-term treatment for spinal spasticity that has not responded to oral baclofen.

Motor Control and Learning

Control of Movement and Posture.- The Nature of Voluntary Control of Motor Actions.- Plans for Grasping Objects.- Adherence and Postural Control: A Biomechanical Analysis of Transient Push Efforts.- Control of Rhythmic Action.- Trajectory Formation in Timed Repetitive Movements.- Stability and Variability in Skilled Rhythmic Action-A Dynamical Analysis of Rhythmic Ball Bouncing.- The Distinctions Between State, Parameter and Graph Dynamics in Sensorimotor Control and Coordination.- Motor Learning and Neural Plasticity.- Stabilization of Old and New Postural Patterns in Standing Humans.- The Role of the Motor Cortex in Motor Learning.- Feedback Remapping and the Cortical Control of Movement.- How Cerebral and Cerebellar Plasticities May Cooperate During Arm Reaching Movement Learning: A Neural Network Model.- Motor Performance and Regional Brain Metabolism of Four Spontaneous Murine Mutations with Degeneration of the Cerebellar Cortex.- Development and Aging.- Development and Motor Control: From the First Step on.- Changes in Finger Coordination and Hand Function with Advanced Age.Control of Movement and Posture.- The Nature of Voluntary Control of Motor Actions.- Plans for Grasping Objects.- Adherence and Postural Control: A Biomechanical Analysis of Transient Push Efforts.- Control of Rhythmic Action.- Trajectory Formation in Timed Repetitive Movements.- Stability and Variability in Skilled Rhythmic Action-A Dynamical Analysis of Rhythmic Ball Bouncing.- The Distinctions Between State, Parameter and Graph Dynamics in Sensorimotor Control and Coordination.- Motor Learning and Neural Plasticity.- Stabilization of Old and New Postural Patterns in Standing Humans.- The Role of the Motor Cortex in Motor Learning.- Feedback Remapping and the Cortical Control of Movement.- How Cerebral and Cerebellar Plasticities May Cooperate During Arm Reaching Movement Learning: A Neural Network Model.- Motor Performance and Regional Brain Metabolism of Four Spontaneous Murine Mutations with Degeneration of the Cerebellar Cortex.- Development and Aging.- Development and Motor Control: From the First Step on.- Changes in Finger Coordination and Hand Function with Advanced Age.

Motor control strategies revealed in the structure of motor variability.

LATASH, M.L., J.P. SCHOLZ, and G. SCHÖNER. Motor control strategies revealed in the structure of motor variability. Exerc. Sport Sci. Rev., Vol. 30, No. 1, pp 26–31, 2002. We describe an uncontrolled manifold hypothesis, which suggests a particular solution for the notorious problem of motor redundancy. A body of recent experiments supports the uncontrolled manifold hypothesis and shows its ability to discover biological strategies of the coordination of apparently redundant motor systems. The hypothesis and associated computational apparatus have great potential for application in the areas of motor rehabilitation and motor skill acquisition.

Directional specificity of postural muscles in feed-forward postural reactions during fast voluntary arm movements

Healthy subjects performed bilateral fast shoulder movements in different directions while standing on a force platform. Anticipatory postural adjustments were seen as changes in the electrical activity of postural muscles as well as displacements of the center of pressure and center of gravity. Postural muscle pairs of agonist-antagonist commonly demonstrated triphasic patterns starting prior to the first electromyographic (EMG) burst in the prime-mover muscle. Proximal postural muscles demonstrated the largest anticipatory increase in the background activity during movements in one of the two opposite directions (forward or backwards). These changes progressively decreased when movements deviated from the preferred direction and frequently disappeared during movements in the opposite direction. The patterns in distal muscles varied across subjects and could demonstrate larger anticipatory changes during movements forward and backwards as compared to movements in intermediate directions. Bilateral addition of inertial loads to the wrists did not change the general anticipatory patterns, while making some of their features more pronounced. Anticipatory postural adjustments were followed by later changes in the activity of postural muscles, also reflected in the mechanical variables. Changes in leg joint angles revealed a „hip-ankle strategy” during shoulder flexions and an „ankle strategy” during shoulder extensions. The study demonstrates different behaviors of proximal and distal muscles during anticipatory postural adjustments in preparation for fast arm movements. We suggest that the proximal muscles produce a general pattern of postural adjustments, while distal muscles take care of fine adjustments that are more likely to vary across subjects.

Enslaving effects in multi-finger force production

Abstract. When a person produces isometric force with one, two, or three fingers, the other fingers of the hand also produce a certain force. Enslaving is the involuntary force production by fingers not explicitly involved in a force-production task. This study explored the enslaving effects (EE) in multi-finger tasks in which the contributions of the flexor digitorum profundus (FDP), flexor digitorum superficialis (FDS), and intrinsic muscles (INT) were manipulated. A new experimental technique was developed that allows the redistribution of the muscle activity between the FDP, FDS, and INT muscles. In the experiment, ten subjects were instructed to perform maximal voluntary contractions with all possible one-, two-, three-, and four-finger combinations. The point of force application was changed in parallel for the index, middle, ring, and little fingers from the middle of the distal phalanx, to the distal interphalangeal joint, and then to the proximal interphalangeal joint. It was found that: (1) the EE of similar amplitude were present in various experimental conditions that involved different muscle groups for force production; (2) the EE were large on average – the slave fingers could produce forces reaching 67.5% of the maximal forces produced by themselves in a single-finger task; (3) the EE were larger for neighboring fingers; and (4) the EE were non-additive – in most cases, the EE from two or three fingers were smaller than the EE from at least one finger. EE among different muscles suggest a widespread neural interaction among the structures controlling flexor muscles in the hand as the main mechanism of finger enslaving.

Joint stiffness: Myth or reality?

Abstract The notion of joint stiffness as commonly studied in biomechanics and motor control is compared with the physical definition of stiffness. The importance of elastic deformation and storage of elastic energy is stressed. Different terms are suggested in order to differentiate between experimentally observed relations between joint angle and torque that are likely to have different nature. A review of studies measuring stiffness of joint subcomponents and intact joints is presented. We suggest to either abandon the term ‘joint stiffness’ as misleading or to state up front stiffness of which of the joint components or subsystems is analyzed in each particular study. We also suggest that each study of ‘joint stiffness’ should clearly state to what extent the results are defined by the systems properties and to what extent they are reflections of the particular experimental procedure.

Force sharing among fingers as a model of the redundancy problem.

Abstract The aim of this study was to test Bernstein’s idea that motor synergies provide solutions to the motor redundancy problem. Forces produced by individual fingers of one hand were recorded in one-, two-, three-, and four-finger tasks. The subjects (n=10) were asked to produce maximal total force (maximal voluntary contraction, MVC) and to match a ramp total force profile using different combinations of fingers. We found that individual finger forces were smaller in multifinger MVC tasks than in single-finger tasks. The deficit increased with the number of fingers involved. A saturation effect was observed: when several effectors were involved, adding a new effector did not significantly change the total force output. The data confirmed the idea that the central neural drive arriving at the level of synergies has a certain limit, a ceiling, that cannot be exceeded. The central nervous system cannot maximally activate the muscles serving all the fingers at the same time. Secondly, during the course of ramp trials, forces produced by individual fingers were linearly related to each other. Hence, a force sharing pattern was established at the beginning of the trial and did not change during the ramp period. A hypothesis is suggested that force distribution among fingers may be organized so as to minimize unnecessary rotational moment with respect to the functional longitudinal axis of the hand. Finally, in the four-finger trials, variance of the total maximal force output in ten consecutive attempts was smaller than the sum of variances of the maximal individual finger forces. The finding suggests that the control system of the motor tasks studied involves at least two levels, a central neural drive level and a synergy level. At the synergy level, an intercompensation in individual finger force production is observed.

What are “normal movements” in atypical populations?

Redundancy of the motor control system is an important feature that gives the central control structures options for solving everyday motor problems. The choice of particular control patterns is based on priorities (coordinative rules) that are presently unknown. Motor patterns observed in unimpaired young adults reflect these priorities. We hypothesize that under certain atypical conditions, which may include disorders in perception of the environment and in decision making, structural or biochemical changes within the central nervous system (CNS), and/or structural changes of the effectors, the central nervous system may reconsider its priorities. A new set of priorities will reflect the current state of the system and may lead to different patterns of voluntary movement. Under such conditions, changed motor patterns should be considered not pathological but rather adaptive to a primary disorder and may even be viewed as optimal for a given state of the system of movement production. Therapeutic approaches should not be directed toward restoring the motor patterns to as close to “normal” as possible but rather toward resolving the original underlying problem. We illustrate this approach using, as examples, movements in amputees, in patients with Parkinsons disease, in patients with dystonia, and in persons with Down syndrome.

Structure of motor variability in marginally redundant multifinger force production tasks.

Abstract. The framework of the uncontrolled manifold hypothesis (UCM hypothesis) was applied to the analysis of the structure of finger force variability during oscillatory force production tasks. Subjects produced cycles of force with one, two (index and middle), or three (index, middle, and ring) fingers acting in parallel against force sensors mounted inside a small frame. The frame could be placed on the top of a table (stable conditions) or on a 4-mm-wide supporting surface (unstable conditions). Subjects were less variable when they used two fingers than when using one finger; adding the third finger did not change indices of variability of the performance. Components of finger force variance that did (VUN) or did not (VCOMP) change the value of a particular functional variable were computed for two control hypotheses: (1) at each time, the subjects tried to stabilize the total value of force (force-control); and (2), at each time, the subjects tried to stabilize the total moment produced with respect to an axis parallel to the hand/forearm (moment-control). Most subjects showed selective stabilization of moment and destabilization of force throughout most of the force cycle, in both stable and unstable conditions. The shapes of VUN and VCOMP suggested a possibility of selective compensation of timing errors across fingers within force cycles. One subject showed different relations between VUN and VCOMP, suggesting that these relations did in fact reflect particular central strategies of solving the tasks. The UCM method is applicable to force production tasks. It allows the comparison of control hypotheses in a quantitative way and unveils central strategies of control of redundant motor systems. Within this approach, redundancy (rather, abundance) is not a problem but an inherent part of a solution for natural motor tasks.

Coordinated force production in multi-finger tasks: finger interaction and neural network modeling

Abstract During maximal voluntary contraction (MVC) with several fingers, the following three phenomena are observed: (1) the total force produced by all the involved fingers is shared among the fingers in a specific manner (sharing); (2) the force produced by a given finger in a multi-finger task is smaller than the force generated by this finger in a single-finger task (force deficit); (3) the fingers that are not required to produce any force by instruction are involuntary activated (enslaving). We studied involuntary force production by individual fingers (enslaving effects, EE) during tasks when (an)other finger(s) of the hand generated maximal voluntary pressing force in isometric conditions. The subjects (n = 10) were instructed to press as hard as possible on the force sensors with one, two, three and four fingers acting in parallel in all possible combinations. The EE were (A) large, the slave fingers always producing a force ranging from 10.9% to 54.7% of the maximal force produced by the finger in the single-finger task; (B) nearly symmetrical; (C) larger for the neighboring fingers; and (D) non-additive. In most cases, the EE from two or three fingers were smaller than the EE from at least one finger (this phenomenon was coined occlusion). The occlusion cannot be explained only by anatomical musculo-tendinous connections. Therefore, neural factors contribute substantially to the EE. A neural network model that accounts for all the three effects has been developed. The model consists of three layers: the input layer that models a central neural drive; the hidden layer modeling transformation of the central drive into an input signal to the muscles serving several fingers simultaneously (multi-digit muscles); and the output layer representing finger force output. The output of the hidden layer is set inversely proportional to the number of fingers involved. In addition, direct connections between the input and output layers represent signals to the hand muscles serving individual fingers (uni-digit muscles). The network was validated using three different training sets. Single digit muscles contributed from 25% to 50% of the total finger force. The master matrix and the enslaving matrix were computed; they characterize the ability of a given finger to enslave other fingers and its ability to be enslaved. Overall, the neural network modeling suggests that no direct correspondence exists between neural command to an individual finger and finger force. To produce a desired finger force, a command sent to an intended finger should be scaled in accordance with the commands sent to the other fingers.