On the voluntary movement of compliant (inertial-viscoelastic) loads by parcellated control mechanisms

Abstract

1. Experiments were performed to characterize the trajectories, net muscle torques, and electromyogram (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 the same known loads, interspersed, just before movement onset with occasional, unpredictable changes in the magnitude of the load. 2. To move a larger load, subjects increase the impulse (torque-time integral) by prolonging the duration of the accelerating torque while keeping its rate of rise constant. Subjects modulate torque most for inertial loads, less for viscous loads, and least for elastic loads, and modulation is 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 changes in load have a larger effect on movement kinematics than do known changes of the same magnitude. 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 increases in 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 change in muscle force when there is an unexpected change in the external load 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 should be described by three elements. The first element (alpha) 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 on an internal model of task dynamics constructed from prior experience. The second element (lambda) is a kinematic plan or reference trajectory utilizing the negative feedback of reflex action to partially compensate for errors in alpha or for unexpected perturbations during the movement. It defines the locus of a moving, instantaneous equilibrium position of the limb, a "template" for the intended trajectory. As movements become slower and require smaller dynamic (velocity and acceleration dependent) forces, lambda will become the dominant control signal. It is also...

Control of velocity and position in single joint movements

“…Our current findings integrate and expand the pulse-step model of Ghez and colleagues (Ghez, 1979;Gordon & Ghez, 1984, 1987a with the model of Gottlieb and colleagues Gottlieb, 1996Gottlieb, , 1998Gottlieb et al, 1989Gottlieb et al, , 1990Gottlieb et al, , 1995Gottlieb et al, , 1996. Our findings provide support for the idea that during targeted movement tasks, pulse-height mechanisms specify movement speed, whereas pulse-width mechanisms appear to be adapted for modulation of final limb position.…”

“…Although the terminology used by Gottlieb and colleagues Gottlieb, 1996Gottlieb, , 1998Gottlieb, Chen, & Corcos, 1995, 1996Gottlieb et al, 1989;Gottlieb, Corcos, Agarwal, & Latash, 1990) was different than that of the Ghez group, the control models proposed by both groups were quite similar to one another. Both described a pulsatile output that can be modulated in both amplitude and duration.…”

Control of Fast-Reaching Movements by Muscle Synergy Combinations

“…Such a mechanism might represent an implementation of a feedforward controller based on simple rules expressed in terms of dynamic joint torques (Gottlieb et al, 1997). In this way, the CNS might simplify the problem of constructing an inverse model of the arm dynamics by building a map of goals into a low-dimensional set of synergy recruitment parameters; however, a synergy-based feedforward controller may operate together with a feedback controller and a postural controller (Gottlieb, 1996;Bhushan and Shadmehr, Example of reversal and via-point muscle pattern reconstruction with point-to-point synergies. Trajectories, averaged phasic muscle patterns, synergy reconstruction, synergy combination coefficients, endpoint tangential velocity, and angular accelerations are shown (different rows) for one reversal movement (3rd column) and two via-point movements (5th and 7th columns) with the same first phase (as in the point-to-point movement in the 1st column) and different second phases (as in the point-to-point movements in the 2nd, 4th, and 6th columns).…”

“…Knowledge of musculoskeletal dynamics is necessary to implement a feedforward controller that can launch the arm in the appropriate direction before sensory information can drive error-correcting controls through feedback loops (Gottlieb, 1996;Gribble and Ostry, 1999;Sainburg et al, 1999). How this knowledge is incorporated in the motor commands is a long-standing question in motor control.…”

Motor learning through the combination of primitives

“…Studies by Wolpert et al (1998) proposed that the neural structures within the cerebellum perform sensorimotor operations equivalent to a combination of multiple forward and inverse models. Strong experimental evidence for the biological and behavioural relevance of internal models has been o¡ered by numerous recent experiments (Brashers-Krug et al 1996;Flanagan & Wing 1997;Flash & Gurevich 1992;Gottlieb 1996;Sabes et al 1998;Shadmehr & Mussa-Ivaldi 1994). In particular, the experimental results obtained by Shadmehr & Mussa-Ivaldi (1994) demonstrate clearly the formation of internal models.…”