A number of studies suggest that the maximum voluntary grip force produced in the hand varies throughout the day; typically being low in the morning and high in the evening (see, for example, Callard et al. 2000). In this study we have measured maximum voluntary contraction (MVC) and several indices of corticospinal excitability over a 24 h period.

With local ethical approval and informed consent, six healthy subjects (mean ± S.E.M. age 47.3 ± 6.1 years; 5 males; 2 left handed) took part in this study. Subjects were investigated every 3 h between 09.00 h and 06.00 h the following morning and were not permitted to sleep. In each session electromyographic (EMG) recordings were made from the thenar muscles using self-adhesive surface electrodes; the skin was marked so that the electrodes could be placed accurately in subsequent sessions. Transcranial magnetic stimulation (TMS) of the motor cortex was achieved using a MagStim 200 stimulator connected to a 9 cm circular stimulating coil centred on the vertex. Ten stimuli were delivered at a strength of 1.2 X threshold (T) with the muscle relaxed and further trials of ten stimuli were delivered at strengths of 1.2 T, 1.0 T and 0.8 T whilst the subject maintained a 10 % MVC contraction. Latency, duration and area of the motor-evoked potentials (MEPs) were measured. In trials with the muscle contracted, the duration and extent of inhibition during the silent period were measured. MVC was measured in the thenar muscles (highest of 3 readings) of the dominant hand.

Data from the recording sessions were compared over time (ANOVA). MVC of the thenar muscle did not change over the 24 h (P > 0.05). The mean areas, latencies and durations of MEPs did not change (P > 0.05) over the 24 h test period with the muscle relaxed or contracted. Furthermore, the extent and duration of the silent period seen after the MEP in the contracted muscle did not change (P > 0.05) over the 24 h of the experiment at any stimulus intensity.

Previous work (Manganotti et al. 2001) during sleep deprivation found no differences in MEP amplitude, or duration of the silent period over the 24 h test period, except at a stimulus strength 1.3 T when they found a small increase in the duration of the silent period at 03.00 h. We have further shown that the magnitude of the silent period (relative to background EMG) does not change over 24 h. This study eliminates time of day as a significant variable to scientists planning future electrophysiological investigations of corticospinal function.

This work was supported by the ARC and ISRT.

All procedures accord with current local guidelines and the Declaration of Helsinki.