Imagine you’re two hours into a long run and your pace—one that felt easy and controlled an hour ago—now requires more effort to maintain. This feeling of working harder to maintain the same pace is actually a phenomenon called physiological decoupling, and a growing body of research suggests it may be one of the most underappreciated determinants of endurance performance. A study published in the European Journal of Applied Physiology examines whether simple, measurable signals—specifically heart rate and breathing rate—can predict how much an athlete’s aerobic fitness degrades during prolonged exercise. The findings validate an observation coaches (including myself) have made for years, and give athletes a concrete, field-based tool for measuring their own long-distance durability .…