Stretch reflexes in the elbow flexors were studied in 23 human subjects by applying small disturbances to preloaded limbs. Arm force, displacement and EMG [electromyogram] (usually biceps) were measured and acceleration was calculated. Modification of the stretch reflex was studied by comparing average responses obtained with different instructions, but with the same disturbance. Initial response did not depend on instruction; differences began abruptly after a latent period (70-320 ms in force). Latency became longer (10-50 ms) and more variable when the subject did not know the direction of disturbance in advance, indicating that the modifications were produced by triggered reactions, rather than by servo actions. Most subjects suppressed triggered reactions when instructed "do not intervene", leaving an unmodified reflex response consisting of a compliant arm deflection. During a 300 ms transient phase, EMG changes (unmodified reflex) were notably asymmetric when responses to loading and unloading were compared. The asymmetric reflex actions produced symmetric mechanical responses, which implies the existence of the compensation for nonlinear muscle mechanical properties, thus maintaining stiffness. Effective control of muscle length was achieved only when triggered reactions were superimposed.
