Changes in circulatory dynamics of upper extremity and saccadic reaction time during neck flexion

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The present study investigated changes in circulatory dynamics of upper extremity muscles during a task involving maintaining neck flexion and during the neck flexion task accompanied with an eye movement reaction task. Neck flexion angles were randomly set at 0°, 10° and 20°. Muscle oxygenation in the biceps brachii and triceps brachii, skin blood flow over the muscles, electrocardiograms, blood pressure and cardiac output were measured during both tasks. Furthermore, muscle sympathetic nerve activity (MSNA) was recorded from peroneal nerve during the neck flexion. During the neck flexion task, oxygenation in muscles increased significantly with angle of neck flexion. Oxygenation in muscles tended to be larger in the neck flexion task with the eye movement reaction task than in the neck flexion task alone. A significant difference between tasks was found at 20° flexion. Furthermore, skin blood flow over the biceps brachii tended to be smaller under the combined task than under the neck flexion task alone. A significant difference between tasks was found at 20° flexion. No significant task effects were found in electrocardiograms, blood pressure or cardiac output. Blood flow within upper extremity muscles was clearly increased while maintaining neck flexion. Blood flow increased more when the neck-flexion task accompanied the eye movement reaction task. MSNA was tendency to decrease during neck flexion. This suggests that brain activation associated with maintaining neck flexion influences the vasomotor center in the medulla oblongata controlling circulatory responses, leading to vasodilatation in muscles. Furthermore, additional brain activation appears to offer synergistic effects during the combined neck flexion and eye movement reaction tasks, and reduction of skin blood flow by skin vasoconstriction might be related to increased blood flow in addition to vasodilatation in muscles.