Effects of mental and physical demands on heart rate variability during computer work

The aim of the study was to evaluate the effect of physical and mental demands on heart rate variability- (HRV-) derived indices of autonomic activity. Ten healthy, female subjects performed two computer tasks: one with combined mental and physical demands and a reference task primarily consisting of physical demands. The combined task, which was performed once with a keyboard and once with a computer mouse, was a computerized version of the colour word conflict task (CWT). The CWT is highly mentally demanding due to the inherent perceptual conflict between a word stimulus and a colour stimulus. In the reference task (REF) the physical demands were comparable to CWT, while the mental demands were low. Finally, the subjects rested at the workplace (REST). Data on performance, heart rate (HR), mean arterial blood pressure (MAP), HRV, and urinary concentrations of catecholamines were obtained. The following frequency bands were applied for HRV: very low frequency (VLF, 0.00-0.04 Hz), low frequency (LF, 0.05-0.15 Hz), high frequency (HF, 0.16-0.40 Hz) and total power (TP, 0.00-0.40 Hz). Indices of sympathetic nervous activity (I(SNS)) and parasympathetic nervous activity (I(PNS)) were estimated as normalized powers in LF and HF bands: I(SNS) = LF/(TP-VLF) and I(PNS) = HF/(TP-VLF). Values are expressed as normalised units (nu). There was an increase in I(SNS) during CWT [mouse: 0.490 (0.052) nu [ave (SEM)] and keyboard: 0.476 (0.039) nu] and REF [mouse: 0.453 (0.059) nu and keyboard: 0.489 (0.047) nu] compared to REST [0.397 (0.047) nu], but no difference between CWT and REF. Corresponding decreases were observed for I(PNS). HR and MAP were higher during CWT compared to REST. No effects were observed for excreted amounts of catecholamines. There were no differences between the computer mouse and the keyboard condition for I(SNS) and I(PNS). In conclusion, an increase in I(SNS) and a decrease in I(PNS) were found in response to a physically demanding reference computer task. Addition of mental demands did not elicit any further effect on I(SNS) and I(PNS), suggesting a significant influence of the physical rather than the mental demands during computer work.