This article presents a method for discovering how the defining values of forthcoming body movements are specified. In experiments using this movement precuing technique, information is given about some, none, or all of the defining values of a movement that will be required when a reaction signal is presented. It is assumed that the reaction time (RT) reflects the time to specify those values that were not precued. With RTs for the same movements in different precue conditions, it is possible to make detailed inferences about the value specification process for each of the movements under study. The present experiments were concerned with the specification of the arm, direction, and extent (or distance) of aimed hand movements. In Experiment 1 it appeared that (a) specification times during RTs were longest for arm, shorter for direction, and shortest for extent, and (b) these values were specified serially but not in an invariant order. Experiment 2 suggested that the precuing effects obtained in Experiment 1 were not attributable to stimulus identification. Experiment 3 suggested that subjects in Experiment 1 did not use precues to prepare sets of possible movements from which the required movement was later selected. The model of value specification supported by the data is consistant with a distinctive-feature view, rather than a hierarchical view, of motor programming.

Human Movement Initiation: Specification of Arm, Direction, and Extent

The Hoffmann reflex: a means of assessing spinal reflex excitability and its descending control in man.

A nonlinear eighth-order agonist-antagonist muscle model is identified, based on engineering analysis and design criteria, as the desired structure for the broad-range study of a variety of fundamental human joint movements. To complement this structure, systematic protocols, that combine material and geometrical information for each muscle, are developed to obtain the model parameter values needed for the various muscle constitutive equations. The parameters describing the four basic nonlinear relations are easy to visualize, representing the peak curve values and "shape" parameters. Elbow, knee, wrist, and ankle fiexion-extension and eye, wrist, and head rotation are simulated by this same model structure.

Analysis of Fundamental Human Movement Patterns Through the Use of In-Depth Antagonistic Muscle Models

Recent work on the dynamics of interactions between humans and machined is reviewed. An experimental apparatus for simulating virtual environments and investigating the man/machine interface is described. Relevant experiments on the mechanical impedance of the human arm are reviewed. Despite active neuromuscular feedback control, the human arm exhibits the impedance of a passive object. Preliminary results of some experiments on the adaptability of human arm impedance are presented. These results indicate limitation in the rapidity of parameter adaptation in humans. >

Controlling impedance at the man/machine interface

The aim of this study was to evaluate corticospinal excitability of both hemispheres during the reaction time (RT) using transcranial magnetic stimulation (TMS). Nine right-handed subjects performed right and left thumb extensions in simple (SRT), choice (CRT) and go/no-go auditory RT paradigms. TMS, inducing motor-evoked potentials (MEPs) simultaneously in the extensor pollicis brevis muscles bilaterally, was applied at different latencies from the tone. For all paradigms, MEP amplitudes on the side of movement increased progressively in the 80-120 ms before EMG onset, while the resting side showed inhibition. The inhibition was significantly more pronounced for right than for left thumb movements. For the left SRT, significant facilitation occurred on the right after EMG onset. Initial bilateral facilitation occurred in SRT trials with slow RT. After no-go tones, bilateral inhibition occurred at a time corresponding to the mean RT to go tones. The timing of the corticospinal rise in excitability on the side of movement was independent of task difficulty and RT. This suggests that corticospinal activation is, to some extent, in series and not in parallel with stimulus processing and response selection. Corticospinal inhibition on the side not to be moved implies that suppression of movement is an active process. This inhibition is more efficient for right- than for left-side movements in right-handed subjects, possibly because of left hemispheric dominance for movement.

Human corticospinal excitability evaluated with transcranial magnetic stimulation during different reaction time paradigms.

The passive elastic torque function and the passive viscous torque function of the muscles and connective tissues spanning the elbow joint have been determined in three adult male subjects. The procedure for estimating the passive elastic torque involved measurement of the torque required to passively move the forearm-hand segment, at a constant angular velocity, throughout a complete range of elbow joint motion. The “suspension method” (Hatze, 1975) was used to obtain the estimate of passive viscous torque. Both of these torque functions were shown to be nonlinear functions of the angular displacement of the joint. In order to assess whether myotatic reflex activity was contributing to the damping of the segment, and thereby biasing the value of the torque contributed by passive viscous elements, the surface electromyograms of the biceps brachii and brachioradialis muscles were examined while the suspended body segment was oscillated. In one subject there was firm evidence of involuntary muscle activity in the brachioradialis muscle which tended to distort the oscillogram. This activity was enhanced when the subject maintained a voluntary isometric contraction in a remote muscle group (Jendrassik's manoeuvre). Based upon these observations, recommendations have been made for reducing unwanted myotatic reflex activity while using the “suspension method” to obtain estimates of various biomechanical parameters.

Passive visco-elastic properties of the structures spanning the human elbow joint

Effects of three different fatiguing local muscular exercises upon plantar flexion reaction time and achilles tendon reflex time have been studied in 24 normal males. The Exercise Conditions, each involving a series of 30 maximal voluntary isometric contractions (MVC) of the plantar flexors, differed by allowing either 5, 10 or 20 sec rest interval between each MVC. Decrements in strength ranged from 15% to 34% MVC. Trend analysis of the fatigue patterns revealed that a cubic orthogonal polynomial equation was sufficient to describe the profile of MVC decrement for all conditions (Table 3). Following the fatiguing exercise, simple visual reaction time (plantar flexion), and its two components, premotor and motor time, failed to demonstrate any change from Pre Exercise Conditions. Achilles Tendon Reflex Times, however, demonstrated a marked augmentation, as manifest in reduced total reflex times, contraction times and half relaxation times. These results may suggest the differential fatigue of motor units employed in the three motor tasks,viz. MVC, voluntary reaction and achilles tendon reflex. A plausible explanation for the augmentation of the reflex contraction resides in the known potentiating effect of elevated intramuscular temperature. Alternatively, one might postulate a neurally mediated increase in gain of the stretch servomechanism. The possibility of both mechanisms being operative is not excluded.

Effects of fatiguing isometric exercise upon achilles tendon reflex and plantar flexion reaction time components in man

Facilitation of late reflexes in humans during the preparatory period of voluntary movement.

Accuracy of skilled movement was investigated in a serial positioning task with the index finger reversibly deafferented by an ischemic nerve block. The reflex response to sudden muscle stretch was monitored at systematic intervals following cuff application. Serial positioning in the verified absence of stretch reflexes was significantly less accurate and more variable than under normal conditions. These findings were discussed in terms of the “servo-assistance” hypothesis for the control of movement and the limits of accuracy that can be achieved in the absence of kinesthesis.

Keith C. Hayes

Papers

Passive visco-elastic properties of the structures spanning the human elbow joint

Effects of fatiguing isometric exercise upon achilles tendon reflex and plantar flexion reaction time components in man

Facilitation of late reflexes in humans during the preparatory period of voluntary movement.

The Ischemic Nerve Block and Skilled Movement

Learning effects in human muscular responses to proprioceptive stimuli.