The aim of this study was to discriminate on a single-trial basis the cortical activity associated to two rates of torque development (RTDs) in imaginary isometric plantar flexions. Electroencephalographic (EEG), electrooculographic (EOG), and electromyographic (EMG) signals were recorded while ten healthy subjects imagined right-sided isometric ankle plantar-flexion tasks at moderate [from 0% to 60% of the maximal voluntary contraction (MVC) in 4 s] and ballistic (from 0% to 60% MVC as fast as possible) RTDs. The EEG signals were classified using feature extraction based on the marginal distribution of a discrete wavelet transform with optimization of the mother wavelet. The classifier was based on support vector machines (SVMs). Minimum misclassification rate for the best case was 8.3%. Average minimum misclassification rate over the ten subjects was (17.4 +/- 8.4)%. The two RTDs could be best differentiated from channel C4 on average. In conclusion, different RTDs could be differentiated in imaginary isometric plantar-flexion by only using cortical potentials recorded with surface EEG. This result constitutes the first step for the development of a new type of brain-computer interfaces that rely on kinetic parameters of a single limb rather than movements of opposite limbs.