EFFECTS OF PSYCHOPHYSIOLOGICAL STRESS ON TRAPEZIUS MUSCLES BLOOD-FLOWAND 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 us ed as a stress indicator in addition to an elevated heart rate. Muscle blood flow was recorded continuously by LDF using a single optical fi bre placed inside the muscle, and related to surface EMG. A group of 2 0 healthy women of different ages was examined. Recordings were made d uring a 50-min period in the following sequence: a 10-min series of al ternating 1-min periods of rest and stepwise increased contraction ind uced by keeping the arms straight and elevated at 30, 60, 90 and 135 d egrees with a 1-kg load carried in each hand; a 10-min recovery period without load; a repeated contraction series with simultaneous perform ance of the CW task; a second 10-min recovery period, and a second con traction series without CW task. Signal processing was done on line by computer. The LDF and root mean square (rms)-EMG values were calculat ed, 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 r ate 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 2 0% increase that persisted during the final contraction series perform ed without CW. There was no influence on MPF. This CW has previously b een shown to evoke an increased secretion of adrenaline from the adren al medullae to the blood. The increased blood flow in the exercising m uscle would therefore appear to have been caused by beta-adrenoceptor vasodilatation, and the fall ill the blood flow in the skin by cu-adre noceptor vasoconstriction. The findings may have implications for work situations characterized by repetitive static loads to the shoulder m uscles and psychological stress.