Motor Control Laboratory

Gerald L. Gottlieb, Supervisor
Shannon McDermott

The control of movement must be simple or we would never have learned how to do it.

Some Publications:

• Strategies for the control of single degree of voluntary movements with one mechanical degree of freedom
• A Computational Model of the Simplest Motor Program
• " Adequate Control Theory " for human single-joint elbow flexion on two tasks (full text)
• Organizing principles for voluntary movement: Extending single joint rules (full text)
• Shifting Frames of Reference but the Same Old Point of View (full text)
• Coordinating movement at two joints: A principle of linear covariance (full text)
• A dialog on the Lambda Hypothesis from the Biomechanics List Service BIOMCH-L@NIC.SURFNET.NL (full text plus added hypertext of cited figures)
• On The Voluntary Movement of Compliant (Visco-elastic,inertial) Loads By Parcellated control mechanisms (full text)
• Coordinating two degrees of freedom during human arm movement: Load and speed invariance of relative joint torques (full text)
• What do we plan or control when we perform a voluntary movement? (full text)
• Rejecting the Equilibrium-Point Hypothesis: A Point of View (full text)
• Directional Control of Planar Human Arm Movement (full text)
• Muscle Activation Patterns During Two Types of Voluntary Single-Joint Movement (full text)
• On Planning Coordinated Movements (full text)
• An unlearned principle for Controlling Natural Movements (full text)

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

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, towards targets of different width over a wide range of experimentally manipulated velocities. The theory is based on three propositions. (1) Movements are planned according to "strategies" of which ther 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 movment speed and/or movement time (and hence appropropriate 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 montoneuron 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. Movement in which speed must be explicitly regulate are cntrolled by the SS strategy, which modulates pulse amplitude. The distinction between the two movement strategies reconciles many apparent conflicts in the motor control literature.

A Computational Model of the Simplest Motor Program

A computational procedure (program) is defined to generate control signals for the motoneuron pools of agonist and antagonist muscles that will move a limb segment from one stationary position to another. The program accounts for moving different distances with different inertial loads and the influence of instructions concerning movement speed and accuracy. These motor commands allow the program to produce EMG patterns as well as force and kinematic trajectories that are consistent with much of the data found in the literature of these movements. The program is premised on the notion that kinematically defined tasks are accomplished by programming commands to the motoneuron pools, based on only a few cognitively recognized kinematic and dynamic features of the task. Most of the features found in EMG and kinematic patterns can be considered consequences of the program's algorithmic procedures rather than specifically planned features of those movements.

" Adequate Control Theory " for human single-joint elbow flexion on two tasks.

The control of distance and speed during single-joint human elbow flexion is accomplished by different modes of activating the motoneuron pools. Distance is controlled by modulating the duration of activation while speed is controlled by modulating the intensity. The experiments reported on here compare movements of different distances under two sets of instructions: Subjects either moved as fast and accurately as possible or moved in a specified time. The first task showed duration modulation while the second, which required simultaneous control of distance and speed, showed both duration and intensity modulation. These results are interpreted in the context of a model for motor control, predicated on the existence of movement plans that use prior knowledge of the dynamics of the movement task to generate muscle activation patterns that produce joint torques. These plans use a simple algorithm based upon parameters of the task such as distance, load and speed. From this plan, the kinematic trajectory emerges.

Organizing principles for voluntary movement: Extending single joint rules.

1. Four subjects performed fast flexions of the elbow or shoulder over three different distances. Elbow flexions were done both in a horizontal, single degree of freedom manipulandum and in a sagittal plane with the unconstrained limb. Shoulder flexions were only performed in the sagittal plane by the unconstrained limb. We simultaneously recorded kinematic and EMG patterns at the "focal" joint, that which the subject intentionally flexed, and at the other, "non-focal" joint that the subject had been instructed to not flex.
   
2. Comparisons of the elbow EMG patterns across tasks show that agonist and antagonist muscles were similar in pattern but not size, reflecting the net muscle torque patterns. Comparisons at the shoulder, also revealed similar EMG patterns across tasks that reflected net muscle torques.
   
3. Comparisons of EMG patterns across joints show that elbow and shoulder flexors behaved similarly. This was not true of the extensors. The triceps EMG burst was delayed for longer distances but the posterior deltoid had an early, distance invariant onset.
   
4. Similarities in EMG reflect torque demands, required at the focal joint to produce flexion and at the non-focal joint to reduce extension induced by dynamical interactions with the focal, flexing joint. These similarities appear in spite of very different kinematic intentions and outcomes. This argues against a strong role for length sensitive reflexes in their generation.
   
5. These results support the hypothesis that movements are controlled by muscle activation patterns that are planned for the expected torque requirements of the task. This general rule is true whether we are performing single joint or multiple joint movements, with or without external constraints. The similarities between single and multijoint movement control may be a consequence of ontogenetic development of multijoint movement strategies that prove useful and are therefore also expressed under the constrained conditions of specialized tasks such as those performed in single-joint manipulanda.
   

Shifting Frames of Reference but the Same Old Point of View

A Commentary on "The origin and use of positional frames of reference in motor control ", A. Feldman & M. Levin, 18:723-806 (1995)

Models of central control variables (CVs) that are expressed in positional reference frames and that rely on proprioception as the dominant specifier of muscle activation patterns have not yet been shown to be adequate for the description of fast, voluntary movement, even of single joints. An alternative model with illustrative data is proposed.

Coordinating movement at two joints: A principle of linear covariance

1. Six subjects performed fast, "single-joint" flexions of either the elbow or shoulder over three angular distances in a sagittal plane. Movement endpoints were located to require flexion of only a single, "focal" joint without any external, mechanical constraint on the other, "nonfocal" joint. Three subjects performed another series of movements between two targets while moving along different paths and in which both joints were flexed.
2. We compared the torque patterns that were produced at the two joints. For single-joint movements, they were both biphasic pulses that accelerated and then decelerated the limb.
3. The torque at the nonfocal joint of a single joint movement was very close to linearly proportional to that at the focal joint throughout the movement. Elbow and shoulder torques differed by a linear scaling constant and went through extrema and zero crossings almost simultaneously.
4. In contrast, during movements in which subjects were explicitly instructed to use a hand path they would not naturally use, the linear interjoint torque scaling rule did not apply. This demonstrated that when we wish to move along a path between two targets that is not produced by linear torque covariation, we are able to modify that rule at will.
5. We speculate that linear, dynamic covariation of the torque patterns >across two joints may be an important principle for reducing the number of degrees of freedom that the nervous system must independently control in performing unconstrained limb movements over naturally chosen paths.

On The Voluntary Movement of Compliant (Visco-elastic,inertial) Loads By Parcellated control mechanisms

1. Experiments were performed to characterize the trajectories, net muscle torques and EMG patterns when subjects performed voluntary elbow flexions against different compliant loads. Subjects made movements in a single-joint manipulandum with different loads generated by a torque motor. Some series of movements were performed under entirely known and predictable load conditions. Other series were performed with occasional, unpredictable changes in the magnitude of the load, just before movement onset.
2. To move a larger load, subjects increase the impulse by prolonging the duration of the accelerating torque while keeping its rate of rise constant. Prolongation is greatest for inertial and least for elastic loads and greater under predictable than unpredictable load conditions.
3. Even when the loads are predictable, subjects move large inertial and viscous (but not elastic) loads more slowly than small. Unpredictable loads have a larger effect on movement kinematics than do predictable.
4. Subjects prolong the duration and increase the area of the agonist EMG burst but do not change its rate of rise to move larger, predictable loads. Subjects change the area of the antagonist burst according to the torque requirements of the load, increasing it only for inertial loads. These effects are usually greater for predictable than unpredictable loads but in either case, are highly variable across subjects.
5. Predictable loads that slow the movements delay the onset of the antagonist burst. When changes in load are unpredictable, only inertial changes affect antagonist latency.
6. The initial resistance to and unexpected external force is due to the viscous properties of muscle tissue. Electromyographic evidence of reflex changes in muscle activation follow this intrinsic mechanical response by 50-70 ms. Elastic neuromuscular properties may also be important but only late in the movement as the final position is approached.
7. We propose that the central command for a voluntary movement can be described by three elements; The first element (a) specifies the muscle activation pattern expected to generate dynamic forces adequate and appropriate to produce a satisfactory trajectory. This feed-forward control program uses simple rules, based upon an internal model of task dynamics constructed from prior experience. The second element (l) is a kinematic plan or reference trajectory utilizing the negative feedback of reflex action to compensate for errors in a or for unexpected perturbations during the current movement. It defines the locus of a moving, instantaneous equilibrium position of the limb, assumed to be very similar to the actual trajectory and independent of limb and load dynamic characteristics. As movements become slower and require smaller dynamic forces, it may become the dominant control signal. It is also used for correction and updating of the internal model used to generate a. The third element (g) modulates volitional set, the degree and manner in which multiple reflex mechanisms can contribute to the muscle activation patterns if the actual trajectory deviates from the planned one. Reflex mechanisms work in parallel with intrinsic muscle compliance to provide partial adaptation of neuromuscular system dynamics to external load dynamics and to help create stable posture at the movement endpoints.

Coordinating two degrees of freedom during human arm movement: Load and speed invariance of relative joint torques

1. Eight subjects performed three series of pointing tasks with the unconstrained arm. Series one and two required subjects to move between two fixed targets as quickly as possible with different weights attached to the wrist. By specifying initial and final positions of the finger tip, the first series was performed by flexion of both shoulder and elbow and the second by shoulder flexion and elbow extension. The third series required flexion at both joints and subjects were instructed to vary movement speed. We examined how variations in load or intended speed were associated with changes in the amount and timing of the EMG activity and the net muscle torque production.
   
2. EMG and torque patterns at the individual joints varied with load and speed according to most of the same rules we have described for single-joint movements.
   a) Movements were produced by biphasic torque pulses and biphasic or triphasic EMG bursts at both joints.
   b) The accelerating impulse was proportional to the load when the subject moved "as fast and accurately as possible" or to speed if that was intentionally varied.
   c) The area of the EMG bursts of agonist muscles varied with the impulse.
   d) The rates of rise of the net muscle torques and of the EMG bursts were proportional to intended speed and insensitive to inertial load.
   e) The areas of the antagonist muscle EMG bursts were proportional to intended movement speed but showed less dependence on load, which is unlike what is observed during single joint movements.
   
3. Comparisons across joints showed that the impulse produced at the shoulder was proportional to that produced at the elbow as both varied together with load and speed. The torques at the two joints varied in close synchrony, achieving maxima and going through zero almost simultaneously.
   
4. We hypothesize that "coordination" of the elbow and shoulder is by the planning and generation of synchronized, biphasic muscle torque pulses that remain in near linear proportionality to each other throughout most of the movement. This linear synergy produces movements with the commonly observed kinematic properties and that are preserved over changes in speed and load
   

What do we plan or control when we perform a voluntary movement?

We argue here for an alternative to kinematic planning. Voluntary movement is accomplished by the execution of motor programs for planned forces and corresponding EMG patterns in the muscles. However, rather than directly solving inverse dynamic equations, the CNS uses relatively simple coordination rules among muscles and joints that greatly simplify the problem of finding muscle activation patterns to satisfactorily approximate our kinematic goals. The well known kinematic features of movements result from a trial and error tuning of force profiles based upon visual and kinesthetic feedback. In this chapter we will present illustrative data for this hypothesis. Some data have been presented at greater length in.

Directional Control of Planar Human Arm Movement
Gerald L. Gottlieb, , Qilai Song, Gil L. Almeida, Di-an Hong, and Daniel Corcos.
Journal of Neurophysiology 78:2985-2998, 1997;

We examined the patterns of joint kinematics and torques in two kinds of sagittal plane reaching movements. One consisted of movements from a fixed initial position with the arm partially outstretched, to different targets, equidistant from the initial position and located according to the hours of a clock. The other series added movements from different initial positions and directions and ¦ 40­80 cm distances. Dynamic muscle torque was calculated by inverse dynamic equations with the gravitational components removed. In making movements in almost every direction, the dynamic components of the muscle torques at both the elbow and shoulder were related almost linearly to each other. Both were similarly shaped, biphasic, almost synchronous and symmetrical pulses. These findings are consistent with our previously reported observations, which we termed a linear synergy. The relative scaling of the two joint torques changes continuously and regularly with movement direction. This was confirmed by calculating a vector defined by the dynamic components of the shoulder and elbow torques. The vector rotates smoothly about an ellipse in intrinsic, joint torque space as the direction of hand motion rotates about a circle in extrinsic Cartesian space. This confirms a second implication of linear synergy that the scaling constant between the linearly related joint torques is directionally dependent. Multiple linear regression showed that the torque at each joint scales as a simple linear function of the angular displacement at both joints, in spite of the complex nonlinear dynamics of multijoint movement. The coefficients of this function are independent of the initial arm position and movement distance and are the same for all subjects. This is an unanticipated finding. We discuss these observations in terms of the hypothesis that voluntary, multiple degrees of freedom, rapid reaching movements may use rule-based, feed-forward control of dynamic joint torque. Rule-based control of joint torque with separate dynamic and static controllers is an alternative to models such as those based on the equilibrium point hypotheses that rely on a positionally based controller to produce both dynamic and static torque components. It is also an alternative to feed-forward models that directly solve the problems of inverse dynamics. Our experimental findings are not necessarily incompatible with any of the alternative models, but they describe new, additional findings for which we need to account. The rules are chosen by the nervous system according to features of the kinematic task to couple muscle contraction at the shoulder and elbow in a linear synergy. Speed and load control preserves the relative magnitudes of the dynamic torques while directional control is accomplished by modulating them in a differential manner. This control system operates in parallel with a positional control system that solves the problems of postural stability.

Rejecting the Equilibrium-Point Hypothesis: A Point of View

The title says it. Needless to say, some may disagree. Tant pis.

Muscle Activation Patterns During Two Types of Voluntary Single-Joint Movement

We have examined the systematic variations in the EMG patterns during two types of single joint elbow movements. These patterns may be interpreted as exhibiting rules by which the CNS controls movement parameters. Normal human subjects performed two series of fast elbow flexion movements of 20°-100° in a horizontal plane manipulandum. The first series consisted of pointing movements (PM) from an initial position to a target, the second of reversal movements (RM) to the same targets with an immediate return to the starting position. Both series showed kinematic and EMG patterns that followed our previously described Speed Insensitive strategy for controlling movement distance. Kinematic patterns of PMs and RMs were identical to about the time of peak PM deceleration. Agonist EMG bursts were also initially the same but RM bursts ended abruptly in a silent period while PM bursts declined more gradually. Antagonist EMG bursts of RMs were later than those of PMs but were not bigger, contrary to our prior expectation and despite the larger net extension torque during RMs. The increase in net RM extension-directed torque that takes the limb back to its initial position appears to be a consequence of reduced flexor muscle torque rather than increased extensor muscle torque. We propose that rules for movement control may be similar for different kinds of movements as long as they are functionally sufficient for the task. However, even in a single-joint movement paradigm, physics alone, that is the knowledge of net muscle torque and limb kinematics is not adequate to fully predict those rules or the muscle activation patterns they produce. These must be discovered by experiment. The simplest expression of such rules may not be in terms of joint torque or limb trajectory but rather, explicitly in terms of muscle activation patterns.

On Planning Coordinated Movements
Gerald L. Gottlieb
Cognitive Studies 6:290-308, 1999;

Human arm movements display characteristic patterns in their trajectories, the torques at the joints and in the EMG patterns in the various muscles. The relationship between the torques and the kinematics is described by physics but the relationship between these properties and the activation of the muscles that produce both is complex and far from well understood. The paper discusses some of the motor control theories that help us understand why we perform our movements in the way we do and how the central nervous system activates the muscles towards those ends. This paper argues that in spite of the well known regularities that are found in simple movement kinematics, an understanding of how muscles are activated will emerge from studying the regularities of the joint torques. However, because of the complex anatomical relations by which muscle forces are converted into joint torques, a full understanding of how the patterns of muscle activation are created may emerge only from the study of the EMG patterns themselves.

An unlearned principle for Controlling Natural Movements
FRANK T.J.M. ZAAL,KRISTIN DAIGLE, GERALD L. GOTTLIEB, AND ESTHER THELEN
Journal of Neurophysiology82:255-259,1999;

Recently, Gottlieb and colleagues discovered a linear relation between elbow and shoulder dynamic torque in natural pointing movements in the sagittal plane. The present study investigates if the process of learning to reach involves discov-ering this linearity principle. We inspected torque data from four infants who were learning to reach and grab a toy in front of them. In a longitudinal study, we collected data both in the period before and after they performed their first successful reaches. Torque profiles at the shoulder and elbow were typically multipeaked and became more and more biphasic toward the end of the first year of life. Torques at the shoulder and elbow were correlated tightly for movements in the prereaching period as well as for reaches later in the year. Further-more, slopes of a regression of shoulder dynamic torque on elbow dynamic torque were remarkably constant at a value ;2.5­3.0. If linear synergy is used by the nervous system to reduce the controlled degrees of freedom, it will act as a strong constraint on the complex of possible coordination patterns for arm movement early in life. Natural reaching movements can capitalize on this constraint because it simplifies the process of learning to reach.

A Test of Torque-Control and Equilibrium-Point Models of Motor Control
GERALD L. GOTTLIEB
Human Movement Science,(in press) 2001

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Established November, 1995
Revised January, 2001