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%. The authors measured arm force, displacement, and EMG (usually biceps); acceleration was calculated from displacement, and average responses from sets of 10 like trials. 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 (10-15 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. The authors 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. 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.
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