The aim of the study was to learn cortical signal characteristics for controlling voluntary deactivation of human skeletal muscles with different speeds. This study investigated whether MRCP measures for fast knee extensor (KE) deactivation were different from those for slow deactivation. Twenty-seven healthy volunteers (22.4 ± 4.5 years) participated in the study. They performed isometric KE contractions [2 sets (slow and fast) of 45 trials] using the right leg at a target level of 20% maximal voluntary contraction (MVC) force. In each trial, subjects held the force at the target for 10s after reaching it and then relaxed the force slowly (rate of force descending ~4% MVC/s) or as quick as possible to the baseline during fast set. Cortical signals for deactivating the muscles were quantified by estimating amplitude of the MRCP derived by force-triggered averaging of the 45-trial EEG data. Mapping of the MRCP based on data of the 128 channels was obtained to evaluate distribution of brain activity associated with slow and fast KE deactivation. MRCP controlling KE deactivation was significantly higher for fast than slow deactivation (P<0.05). A higher level of cortical activation is required for controlling fast muscles deactivation. These findings indicated that control of KE deactivation at the cortical level is modulated by a rate of force decrease. This could be related to a varied impact of feedback and feedforward mechanisms during slow and fast deactivation and this may result in differential corticospinal projections to the motoneuron pools of the synergist and antagonist muscles during both conditions. This work was supported in part by NIH 1T32 AR050959 and R01 NS 35130 grants, and Ministry of Science and Higher Education, Republic of Poland 10/MOB/2007/0 grant
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