INTRODUCTION: Not only muscle contraction, but also muscle relaxation plays an important role for performance of voluntary movements. Most of the studies have focused on muscle contraction rather than on their relaxation. Research on motor ability has established clearly that mental practice leads to improved execution of movement. AIM(S): Assessment of the effect of a four-week mental training on cortical activity related to decrease in grasping force in healthy, young people. METHOD(S): 15 healthy subjects (8 men and 7 women) between 23 to 33 years voluntarily participated in the study. Mental training (MT) lasted 4 weeks with 3 training sessions per week and cortical activity using 128‑channel EEG system was recorded in all subjects during two measurement sessions (before and after the MT). During sessions subjects performed: 3x maximal isometric voluntary contraction (MVIC) in grasp function, 40 repetitions at submaximal level of force (20% of MVIC) during the same task and 2 × MVIC. The amplitudes of motor related cortical potentials (MRCP) of the EEG signal were analyzed in the BESA software (BESA GmbH, Germany) for electrodes placed in the areas associated with the planning and execution of movements on averaged files from 20% MVIC part of the protocol and triggered around (from ‑3 s to 1 s) decrease in grasping force. To compare the MRCP values before and after MT, the Wilcoxon signed-rank test was performed in SPSS (IBM SPSS 22.0, USA) with the level of significant that was set at P≤0.05. RESULTS: Analysis of the MRCPs did show significant differences between relaxation amplitudes before and after the MT. CONCLUSIONS: Muscle relaxation is accompanied by activation of the premotor cortex (PM), primary motor (M1), primary and secondary somatosensory areas (S1, S2). The level of the cortical activity associated with relaxation of the muscles during precise grasp movement performed by right upper limb was higher after MT especially in the S1, S2 and PM areas compared to M1.