Recent reports have shown task-related changes in oscillatory activity in the 15-30 Hz range in the sensorimotor cortex of human subjects and monkeys during skilled hand movements. In the monkey these oscillations have been shown to be coherent with oscillatory activity in the electromyographic activity of hand and forearm muscles. 2. In this study we investigated the modulation of oscillations in the electromyogram (EMG) of human volunteers during tasks requiring precision grip of two spring-loaded levers. 3. Two tasks were investigated: in the 'hold' task, subjects were required to maintain a steady grip force (ca 2.1 N or 2.6 N) for 8 s. In the 'ramp' task, there was an initial hold period for 3 s (force ca 2.1 N) followed by a linear increase in grip force over a 2 s period. The task ended with a further steady hold for 3 s at the higher force level (ca 2.6 N). 4. Surface EMGs were recorded from five hand and forearm muscles in 12 subjects. The coherence of oscillatory activity was calculated between each muscle pair. Frequencies between 1 and 100 Hz were analysed. 5. Each subject showed a peak in the coherence spectra in the 15-30 Hz bandwidth during the hold task. This coherence was absent during the initial movement of the levers. During the ramp task the coherence in the 15-30 Hz range was also significantly reduced during the movement phase, and significantly increased during the second hold period, relative to the initial hold. 6. There was coherence between the simultaneously recorded magnetoencephalogram (MEG) and EMG during steady grip in the hold task; this coherence disappeared during the initial lever movement. Using a single equivalent current dipole source model, the coherent cortical activity was localized to the hand region of the contralateral motor cortex. This suggests that the EMG-EMG coherence was, therefore, at least in part, of cortical origin. 7. The results are discussed in terms of a possible role for synchrony in the efficient recruitment of motor units during maintained grip.
