ON THE VOLUNTARY MOVEMENT OF COMPLIANT (INERTIAL-VISCOELASTIC) LOADS BY PARCELLATED CONTROL MECHANISMS

1. Experiments were performed to characterize the trajectories, net mu scle torques, and electromyogram (EMG) patterns when subjects performe d voluntary elbow flexions against different compliant loads. Subjects made movements in a single-joint manipulandum with different loads ge nerated by a torque motor. Some series of movements were performed und er entirely known and predictable load conditions. Other series were p erformed 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- a time integral) by prolonging the duration of the accelerating torque while keeping its rate of rise constant. Subjects modulate torque mos t for inertial loads, less for viscous loads, and least for elastic lo ads, and modulation is greater under predictable than unpredictable lo ad conditions. 3. Even when the loads are predictable, subjects move l arge inertial and viscous, but not elastic, loads more slowly than sma ll. Unpredictable changes in load have a larger effect on movement kin ematics than do known changes of the same magnitude. 4. Subjects prolo ng the duration and increase the area of the agonist EMG burst but do not change its rate of rise to move larger, predictable loads. Subject s change the area of the antagonist burst according to the torque requ irements of the load, increasing it only for increases in inertial loa ds. These effects are usually greater for predictable than unpredictab le loads but in either case, are highly variable across subjects. 5. P redictable loads that slow the movements delay the onset of the antago nist burst. When changes in load are unpredictable, only inertial chan ges affect antagonist latency. 6. The initial change in muscle force w hen there is an unexpected change in the external load is due to the v iscous properties of muscle tissue. Electromyographic evidence of refl ex changes in muscle activation follow this intrinsic mechanical respo nse by 50-70 ms. Elastic neuromuscular properties may also be importan t 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 th e muscle activation pattern expected to generate dynamic forces adequa te and appropriate to produce a satisfactory trajectory. This feed-for ward control program uses simple rules, based on an internal model of task dynamics constructed from prior experience. The second element (l ambda) is a kinematic plan or reference trajectory utilizing the negat ive feedback of reflex action to partially compensate for errors in al pha or for unexpected perturbations during the movement. It defines th e 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) forc es, lambda will become the dominant control signal. It is also used fo r correction and updating of the internal model used to generate alpha . The third element (gamma) modulates volitional set, the degree and m anner in which multiple reflex mechanisms can contribute to the muscle activation patterns if the actual trajectory deviates from the planne d one. Reflex mechanisms work in parallel with intrinsic muscle compli ance to provide partial adaptation of neuromuscular system dynamics to external load dynamics. These controlled compliant mechanisms maintai n the stability of the motor system, without which both posture and mo vement would not be possible.