Neurophysiological correlates of passive movements are speed- and type-dependent

Purpose: The supraspinal involvement in the control of passive movements remains elusive. Previous studies provided electromyographic evidence for speed and –type dependent changes in muscle activity. Based on mechanoreceptor properties, their age-related changes and the somatotopically organized connections between sensory and motor systems, this study aimed to provide electrophysiological evidence for the involvement of frontal cortex inhibition and corticomotor interactions in the control of passive movements. Methods: Twenty healthy younger and older adults performed passive elbow flexion and extension movements at three metronome-based speeds (20, 60, and 100 beats per minute) in continuous and discontinuous fashion. The continuous condition included movements with no inter-movement pauses and the discontinuous conditions with pauses between flexion–extension transitions. The order of movement speeds increased progressively to prevent carryover effects in hypothesized resistance to passive movements from higher to lower speeds. In all conditions, electro-encephalographic and electromyographic data were acquired. Alpha power and beta corticomuscular coherence were used to quantify frontal cortex inhibition and brain-muscle connectivity, respectively. Results: Frontal cortex inhibition decreased (p = 0.036) and brain-muscle connectivity increased (p < 0.001) with increasing movement speeds. In addition, frontal cortex inhibition was 17% higher in the discontinuous condition as compared to the continuous condition (p = 0.005) while corticomuscular coherence was 25.9% higher in the continuous vs. the discontinuous condition (p < 0.001). These effects were independent of age. Conclusion: Frontal cortex inhibition and brain-muscle interactions depend on passive movement speed and -type. The current data may provide insights into the processes underlying pathological muscle tone during passive movements.