Descending and reflex pathways usually converge on common interneurons and motoneurons. This implies that active movements may result from changes in reflex parameters produced by control signals conveyed by descending systems. Specifically, according to the λ-model, a fast change in limb position is produced by a rapid change in the threshold of the stretch reflex. Consequently, external perturbations may be ineffective in eliciting additional reflex modifications of electromyographic (EMG) patterns unless the perturbations are relatively strong. In this way, the model accounts for the relatively weak effects of perturbations on the initial agonist EMG burst (Ag1) usually observed in fast movements. On the other hand, the same model permits robust reflex modifications of the timing and shape of the Ag1 in response to strong perturbations even in the fastest movements. To test the model, we verified the suggestion that the onset time of the Ag1, even in the fastest movements, depends on proprioceptive feedback in a manner consistent with a stretch reflex. In control trials, subjects (n = 6) made fast unopposed elbow flexion movements of ~60°(peak velocity 500-700°/s) in response to an auditory signal. In random test trials, a brief (50 ms) torque of 8-15 Nm either assisting or opposing the movement was applied 50 ms after this signal. Subjects had no visual feedback and were instructed not to correct arm deflections in case of perturbations. In all subjects, the onset time of the Ag1 depended on the direction of perturbation: it was 25-60 ms less in opposing compared with assisting load conditions. Assisting torques caused, at a short latency of 37 ms, an additional antagonist EMG burst preceding the Ag1. The direction dependent effects of the perturbation persisted when cutaneous feedback was suppressed. It was concluded that the direction-dependent changes in the onset time and duration of the Ag1 as well as the antagonist activation preceding the Ag1 resulted from stretch reflex activity elicited by the perturbations rather than from a change in the control strategy or cutaneous reflexes. The results support the hypothesis on the hierarchical scheme of sensorimotor integration in which EMG patterns and movement emerge from the modification of the thresholds and other parameters of proprioceptive reflexes by control systems.
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