This study was designed to gain more insight into the mechanisms underlying motor imagery (MI). While there is ample evidence that motor performance and MI share common central neural mechanisms, the question whether MI is accompanied by subliminal electromyographic (EMG) activity remained unsolved. Thirty right-handed volunteers were asked to lift or to imagine lifting a weighted dumbbell using different types of muscle contraction, i.e. heavy concentric, light concentric, isometric and eccentric contractions. EMG activity from 9 muscles of the dominant arm (agonist, antagonist, synergist and fixator muscles) was monitored. Autonomic nervous system responses were also recorded on the non-dominant hand, thus attesting mental activity at the peripheral level. A significant increased pattern of EMG activity was recorded in all muscles during MI, when compared to the rest condition, while the goniometric data did not reveal any movement. Although being subliminal, the magnitude of this activation was found to be correlated to the mental effort required to lift a weight mentally, as more EMG activity was recorded during imaginary lifting of heavy than light concentric contractions. When considering the different types of contraction, our results provided evidence of selective changes in EMG activity. Especially, the imagined eccentric condition elicited a significant weaker muscular activity than all other conditions. In addition, the changes in the EMG pattern mirrored those usually observed during physical movement. These findings support the hypotheses of a selective effect of MI at the level of muscular activity and of incomplete inhibition of the motor command during MI.