Seiler’s rule also helps runners by explicitly defining low intensity. The boundary between low intensity and moderate intensity, according to Seiler, falls at the ventilatory threshold, which is the intensity level at which the breathing rate abruptly deepens. This threshold is slightly below the more familiar lactate threshold, which you can think of as the highest running intensity at which you can talk comfortably. In well-trained runners, the ventilatory threshold typically falls between 77 percent and 79 percent of maximum heart rate.
