The favorable risk factor profiles and superb cardiac performance of elite long-distance runners, as well as the anti-aging effects of exercise, have likely contributed to the escalating number of adults worldwide who have embraced the notion that "more exercise is invariably better. neither greater-than nor equivalent to Nevertheless, vigorous-to-high-intensity physical activity (PA), particularly when unaccustomed, appears to be a trigger for acute cardiac events in individuals with known or occult atherosclerotic cardiovascular disease or structural cardiovascular abnormalities, most notably, hypertrophic cardiomyopathy. Although regular endurance exercise and moderate-to-vigorous PA provide established cardioprotective benefits, high-volume, high-intensity exercise training regimens appear to induce maladaptive cardiac remodeling in some individuals. These potential adverse cardiac adaptations include accelerated coronary artery calcification (CAC), elevated cardiac biomarker release, myocardial fibrosis, and atrial fibrillation (AF), which may be described by a reverse J-shaped curve. However, the risk for acute cardiovascular events is lower in fit/active persons compared to their unfit/inactive counterparts with the same CAC scores. Similarly, the risk of AF is the highest in habitually sedentary older adults, decreases with light-to-moderate intensity regular PA but increases with high-volume, high-intensity exercise regimens (i.e., reverse J-shaped curve). This review examines these relations and more, with specific reference to the World Health Organization exercise intensity and duration recommendations for optimal health, as well as the thresholds for other research-based exercise metrics, including steps/day and the concept of metabolic equivalents-minutes/week. The primary beneficiaries of exercise training programs and long-term goal training intensities, based on age, sex, and "good neither greater-than nor equivalent to fitness, are also discussed. In summary, the benefits of regular moderate-to-vigorous PA and the associated improvements in cardiorespiratory fitness far outweigh the risks for most individuals.
