Proceedings of The Physiological Society

University College London December 2005 (2006) Proc Physiol Soc 1, PC8

Poster Communications

Excitability of inhibitory neurons in the human motor cortex during static and dynamic contractions

Zuur, Abraham T.; Perez, Monica; Nielsen, Jens Bo; Sinkjaer, Thomas; Grey, Michael James;

1. Center for Sensory-Motor Interaction, Aalborg University, Aalborg, Denmark. 2. Department of Medical Physiology, University of Copenhagen, Copenhagen, Denmark.

We investigated the nature of the supra-spinal contribution to the motoneuronal drive of the Soleus (SOL) muscle during dynamic and static contractions using sub-threshold transcranial magnetic stimuli to suppress the corticospinal output (Davey et al. 1994). Eight healthy subjects with no history of neuromuscular disorder participated in the study. Subjects were seated with the right foot fixed to a stationary manipulandum. Surface EMG was recorded from the right SOL. Visual feedback of the ankle torque was provided via strain gauges fixed to the manipulandum. Subjects were instructed to follow a four-segment ramp and hold force trajectory using their plantar flexor muscles. The ramp was defined as a per cent of the maximum voluntary contraction (MVC) as follows: hold 5% MVC (1 s), ramp up (0.6 s), hold 15% MVC (0.9 s), ramp down (0.6 s). Magnetic stimuli were delivered via a custom-made 90 mm double coil (batwing design) placed on the scalp and over the hot-spot of the SOL muscle. The stimulation intensity was decreased until a clear suppression of the mean rectified SOL EMG could be seen without evidence of facilitation at 15% MVC. Stimuli were delivered in the middle of each trajectory section. At least 90 stimulated trials and 90 control trials were recorded for each section in a fully randomised fashion. The onset of the suppression was defined as the point where the ensemble-averaged SOL EMG for the stimulated trial was less than that of the control EMG for at least 5 ms. For each subject, the same analysis window was used for each trajectory section (13-24 ms wide). The suppression was expressed as a per cent change from the control EMG. A one-way ANOVA and Tukey-Kramer post-hoc test was used to compare the magnitude of the suppressions at each trajectory section (significance level

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