Circadian variation of physiological functions related to physical work capacity

The circadian variation of selected physiological functions related to physical work capacity was studied in four trained young male subjects. The twofold objective was to describe, in parametric terms, the variation of these functions and estimate the times and values of their maximum and minimum responses. Statistically significant periodic regression effects were found for the following functions: oral temperature; systolic blood pressure; heart rate at rest, during work (245 and 270 W) and after a 5-min recovery; ratings of perceived exertion; strength of hand grip; and neuromuscular coordination measured with a seesaw board balance test. The Fourier curve (sine curve) accounted for 63 % of the variation in oral temperature and from 13 to 38 % of the variation in the other significant harmonic intensities. In phase with the rhythm of oral temperature, but with larger amplitude, were systolic blood pressure, heart rate at rest and heart rate during work; roughly in phase were strength of hand grip and neuromuscular coordination. Ratings of perceived exertion and heart rate after a 5-min recovery showed a rhythm which was out of phase with the oral temperature. The number of disturbances in neuromuscular coordination was greatest at 2400 and smallest at 1200, the difference being as large as 44 %. The results suggest that the measured capacities of muscle strength and neuromuscular coordination are the lowest, that physical work is perceived as the heaviest, and recovery after work is the slowest at night. These findings appear to support the notion that performance capacity is reduced during the night.