Abstract: The heart rate of a subject during strenuous physical exercise is monitored accurately by feeding the plurality of electric signals of diverse amplitudes generated by transducers engaging the skin of the subject, to a variably controlled gain amplifier to adjust the amplitudes to within a predetermined amplitude range, feeding the adjusted amplitude signals to a slew rate filter formed of a pair of slew rate limiters to compare the slew rates of the adjusted signals, feeding the compared signals to a slew rate limit comparator to separate from the QRS waveforms representing heartbeat of the subject those interfering electric signals having slew rates outside the range of slew rates representing QRS waveforms, and monitoring the QRS waveforms.