An implantable variable rate pacemaker adaptive to patient exercise detects movements of the patient, and discriminates between detected movements indicative of true physical exercise by the patient and detected movements which are not true physical exercise. An activity sensor converts mechanical movements of the patient to an electrical signal representative thereof. Only a portion of the electrical signal in a frequency range below 4 Hz is processed by the pacemaker's electrical circuitry, to discriminate against signal components arising from other than physical exercise of the patient. As a result of this signal processing, a predetermined variable signal component is recovered, other than frequency of amplitude crossings relative to a threshold value, to produce an output signal representative of true physical body exercise. The pacemaker responds to this output signal by generating pulses at a rate that varies depending on the predetermined signal component, to stimulate the patient's heart at a rate according to the state of true physical exercise of the patient.