1. The stretch reflex in the elbow flexor musculature was studied in 23 human subjects. The subjects were required to establish an initial force equivalent to 10% maximum at a prescribed initial length; mechanical disturbances delivered at random times increased load force to 15% or reduced it to 5%. We measured arm force, displacement, and EMG (usually biceps); acceleration was calculated from displacement, and average responses from sets of 10 like trials. 2. Modification of the stretch reflex was studied by comparing average responses obtained with different instructions, but with the same disturbance. The usual introductions were "compensate for arm deflection" and "do not intervene voluntarily". The initial response did not depend on instruction; changes in response that depended on instruction began abruptly after a latent period which ranged from 70 to 320 ms (measured from force and acceleration), depending on conditions and subject. The latency became longer (10-50 ms) and more variable when the subject did not know the direction of disturbance in advance. This and other observations indicate that modifications of the stretch reflex are not produced by servo actions. They are produced by triggered reactions, which occur at both short and long latencies and which have properties resembling the movements produced in a reaction-time task. 3. We confirmed that most subjects can suppress triggered reactions when the instruction calls for no intervention, leaving an unmodified reflex response. This response consists of a compliant deflection of the arm in the direction of the disturbance. 4. The compensatory actions associated with unmodified stretch (and unloading) reflexes were assessed from EMG responses of biceps. During a 300-ms transient phase, EMG changes were notably asymmetric when responses to symmetric disturbances were compared. Increased force stretched biceps and produced a prominent increase in EMG, whereas decreased force allowed biceps to shorten and produced either an EMG decrease of smaller magnitude or an actual increase. These asymmetric reflex actions produced quite symmetric mechanical responses (arm displacements and forces), which implies the existence of and compensation for nonlinear muscle mechanical properties. This result is discussed in relation to the hypothesis that the function of the stretch reflex is to compensate for variations in muscle properties, thus maintaining stiffness. 5. Effective control of muscle length or joint position does not result from servo action by the stretch reflex. Errors in position are corrected only when triggered reactions are superimposed on the reflex response.

These study findings suggest that chronic arsenic exposure may increase the risk of fetal and infant death.

1. A unified model of the properties of stretch responses of mammalian spindle endings is proposed. This model encompasses the disparity between sensitivity of spindle endings to small and to large stretch of the muscle as well as the disparity in their dynamic responsiveness for different amplitudes of stretch. 2. In the model the mechanical properties of intrafusal fibers include a property akin to friction, which is hypothesized on the basis of reported observations on amphibian muscle. Transducer and encoder processes are modeled in the light of recent observations on isolated spindles. The model involves five unknown parameters whose values are selected by reference to certain reported observations on deefferented primary and secondary endings. The model can be used to predict responses to length changes of arbitrary time course. 3. Predicted responses to large ramp-and-hold stretch are quantitatively comparable to observations over a wide range of stretch velocities. The quantities compared include the increment in response during ramp stretch as well as the dynamic index, which is a measure of adaptation at stretch plateau. 4. At a fixed frequency of sinusoidal stretch, the relation between amplitudes of stretch and response is predicted in quantitative agreement with measurements. As the frequency of stretch is decreased, the predicted phase lead decreases and then increases, while the sensitivity decreases monotonically, in accord with observations. 5. In the model the high sensitivity for small stretch is not specific to any particular length of the muscle. When stretch is large, the region of high sensitivity is gradually reestablished at the new length, a phenomenon referred to as resetting. The dynamic response to a large stretch can be seen as arising, for the most part, from the dynamic process of resetting. 6. The influences of static or dynamic fusimotor activation on stretch responses of the primary ending are simulated by modifying the parameter values in the model. The modifications are such that static (dynamic) fusimotor activity speeds up (slows down) the resetting of the high-sensitivity region. The predictions mimic qualitatively the observed fusimotor effects not only on the response to large ramp stretch but also the contrasting effects seen with smaller, sinusoidal stretch.

From among the diverse meanings of stability, the one the author adopts here is that the effects of a perturbation are opposed, and therefore small effects remain small. Except in linear systems, however, instability need not lead to unbounded motion and may actually be desirable when maneuverability is important. Moreover, properties of nerves, muscles, and tendons present serious challenges to stabilization. A review of observations from the motor control literature reveals that responses to perturbations in many common situations assist rather than resist the perturbation and are therefore presumably destabilizing. The observations encompass situations of position maintenance as well as impending or ongoing movement. The author proposes that the motor control system responds to a sudden perturbation by a pattern of muscle activity that mimics an accustomed voluntary movement, oblivious of stability considerations. What prevents runaway motion in the face of short-term instability appears to be voluntary intervention.

An attempt is made to integrate theoretically the mechanical, electromyographic, and psychophysical lines of inquiry into the control of movement by investigating the significance of joint stiffness in the reduction of effort. Attention is focused on single-joint, unperturbed movements of specified duration performed from one specified position to another in the presence of an inertial load. A theoretical measure of the sense of effort is formulated in the light of psychophysical observations and mechanical considerations. This measure is such that it is increased by reciprocal changes in the central drives to opposing sets of muscles, as well as by enhancement of joint stiffness. Mathematical analysis of the interplay of these factors reveals that, in any given condition, the minimization of this measure of effort necessitates a particular value of joint stiffness and a particular trajectory of movement. The predicted stiffness and trajectory are shown to be in quantitative agreement with available observations. In addition, the conditions in which a higher value of stiffness is predicted to be advantageous for reducing the effort are shown to be the conditions that are known to promote greater coactivation of the agonist and antagonist muscles. It is concluded that the seemingly wasteful coactivation may serve to optimize the stiffness. The stiffness, therefore, need not be viewed simply as a means of resisting imposed perturbations, but as a means of reducing the alterations in the central drives necessary for the performance of movement, thereby reducing the effort.