Effects of psychophysiological stress on trapezius muscles blood flow and electromyography during static load

Mental stress was induced by the Stroop colour word task (CW task) and the effects on the microcirculation and electromyography (EMG) in the upper portion of the trapezius muscle were studied during a series of fatiguing, standardized static contractions. A lowered blood flow of the skin recorded continuously by laser-Doppler flowmetry (LDF) was used as a stress indicator in addition to an elevated heart rate. Muscle blood flow was recorded continuously by LDF using a single optical fibre placed inside the muscle, and related to surface EMG. A group of 20 healthy women of different ages was examined. Recordings were made during a 50-min period in the following sequence: a 10-min series of alternating 1-min periods of rest and stepwise increased contraction induced by keeping the arms straight and elevated at 30, 60, 90 and 135 degrees with a 1-kg load carried in each hand; a 10-min recovery period without load; a repeated contraction series with simultaneous performance of the CW task; a second 10-min recovery period, and a second contraction series without CW task. Signal processing was done on line by computer. The LDF and root mean square (rms)-EMG values were calculated, as well as the EMG mean power frequency (MPF) for fatigue. The CW-task added to the contraction series caused an increase in the heart rate accompanied by a decrease in the blood flow to the skin and a 30% increase in the blood flow in the exercising muscle. Both returned to normal during the subsequent recovery period and showed normal levels during the final contraction series without CW. The rms-EMG showed a 20% increase that persisted during the final contraction series performed without CW. There was no influence on MPF. This CW has previously been shown to evoke an increased secretion of adrenaline from the adrenal medullae to the blood. The increased blood flow in the exercising muscle would therefore appear to have been caused by beta-adrenoceptor vasodilatation, and the fall in the blood flow in the skin by alpha-adrenoceptor vasoconstriction. The findings may have implications for work situations characterized by repetitive static loads to the shoulder muscles and psychological stress.