An active implantable medical device for cardiac resynchronization therapy with effort based rate-responsive pacing is described. The device calculates a rate-responsive stimulation frequency based on output signal of an effort sensor between a base frequency (fbase) and a maximum frequency (fmax). The device determines a target stimulation frequency based on the difference between a first frequency and the maximum frequency (fmax). The first frequency is the higher frequency of the base frequency (fbase) and the spontaneous frequency of the patient. The device calculates a stimulation frequency that has an immediate increase in the pacing rate from the higher of the initial value of the current stimulation frequency, or the spontaneous frequency to the target stimulation frequency, within a predetermined time, or a predetermined number of cardiac cycles. A plurality of consecutive effort zones (Z1-Z4) is defined over the extent of the dynamic range of the output signal of the effort sensor.