The effect of exercise on the cardiovascular system has been studied extensively using a wide range of physiological sensors. Athletes now commonly use EKG-based monitors to ascertain heart rate, but these devices cannot directly monitor the level of physical stress. We hypothesize that the low frequency spindle waves seen in the photoplethysmographs (PPG) of exercising individuals may be useful for noninvasively detecting hemodynamic stressors to the human vascular system. In a clinical trial with nine healthy subjects performing the Bruce Protocol treadmill test these low frequency spindle waves were observed in the forehead and ear PPG in all subjects before the onset of volitional fatigue. As volitional fatigue approached, the spindle waves become more pronounced, decreased in period and then within several seconds of the cessation of the protocol they disappeared. Using a software-based detector, these distinct spindle waves can be reliably detected. This technique holds promise for the automatic detection and characterization of exercise induced physical stress.