Effect of time of day on force variation in a human muscle.

The effect of time of day on the neural activation and contractile properties of the human adductor pollicis muscle was investigated in 13 healthy subjects. Two different times of day were chosen, corresponding to the minimum (7 h) and maximum (18 h) levels of strength. The force produced was compared with the associated electromyographic (EMG) activity during voluntary and electrically induced contractions in order to determine whether peripheral or central mechanisms play a dominant role in diurnal force fluctuation. The results indicated that the force produced during a maximum voluntary contraction (MVC) was significantly higher (+8.9%) in the evening than the morning. Since the increase in force of the MVC and the tetanic contraction (100 Hz) were similar, it is suggested that peripheral mechanisms are responsible for diurnal fluctuations in force. This conclusion is supported by the observation that central activation, tested by the interpolated twitch method during an MVC, did not change, and that the EMG was less per unit force in the evening. In addition to the increase in maximum twitch and tetanus force, significant changes in muscle contractile kinetics were also observed. The maximum rate of tension development and the relaxation of the twitch and tetanus increased in the evening, and the twitch contraction time (CT) and the time to half-relaxation (TR(1/2)) were reduced. Because the mean range of variation in skin temperature (2. 6 degrees C) observed over the course of the day was very low, this change cannot entirely explain those observed in muscle contractile properties.