Force tremor analysed using Fourier techniques has revealed low frequency volitional fluctuations, with a peak around 1 Hz. Furthermore, the major part of spectral power is contained below 4 Hz (Vaillancourt et al. 2002). Higher frequency components also exist within the data, but due to their small magnitude and aperiodic nature are difficult to assess using the fast Fourier transform (FFT) method and have therefore received little attention. These ‘noise’ components presumably represent unfused events during contractions and thus provide indirect information regarding muscle state during force maintenance tasks. Assuming this is the case, then altering the contractile properties of the muscle should modulate their amplitude.

We tested this by recording isometric force tremor during abduction of the index finger, in eight subjects at 20 % of maximal voluntary force, before and after 10 min of hand cooling (10 °C) or warming (44 °C) by water immersion. This has previously been shown to rapidly change the contractile properties of the first dorsal interosseous muscle (Ranatunga et al. 1987). The high frequency ‘noise’ was decoupled from the volitional component and a time-domain interval technique was used to analyse the data.

The results revealed that, in all subjects, muscle cooling significantly reduced the amplitude of the noise by ~70% conversely heating significantly increased the amplitude by ~50 %. In addition, some subjects were tested with eyes open or closed and this was found to have no effect on the amplitude of the noise (P < 0.05 in all cases, repeated measures analysis). Changes produced by cooling lasted longer than those produced by heating. These results support the suggestion by Lakie et al. (1994) that temperature related changes in isotonically recorded postural hand tremor were due to altered muscle properties.

We suggest that applying a simple interval technique to look at the instantaneous changes in the high frequency ‘noise’ component of force tremor can assist in understanding of the mechanisms by which tremor size can be altered and allow them to be differentiated.