Lee, K. S., Waikar, A. M. and Wu, L.
Physical stress evaluation of microscope work using objective and subjective methods.  International Journal of Industrial Ergonomics 2: 203-209 (1988).

This paper presents a laboratory study which evaluated physical stress resulting from prolonged microscope use. Electromyography (root-mean-square EMG value) was employed as an objective measure for the quantitative assessment of physical stress. A discomfort rating was used as a subjective measure for the qualitative assessment. Twelve subjects were observed during two four-hour periods of intensive microscope work. The study revealed significant increases in physical stress over time of the experiment by the subjective measure (very comfortable to very uncomfortable) as well as the objective measure (25–65% increase in EMG values). At low stress levels, the performance seemed to increase despite the increased stress indicated by EMG level and subjective feeling of discomfort. However, the performance declined when the muscles were significantly stressed. The validity of EMG as a measure of physical stress in microscope work is evidenced by the high correlation between the two measures. The average correlation coefficient r for objective and subjective measures was approximately 0.93.