Swim CSS, or Critical Swim Speed, is a metric used in swimming to determine an athlete's sustainable pace over various distances. It represents the swimming speed at which an individual transitions from aerobic to anaerobic energy systems. By calculating the time taken to swim a certain distance, typically 400 meters or 200 meters, swimmers can establish their CSS. This valuable metric helps in structuring training programs, optimizing performance, and achieving efficient and effective swim workouts. It serves as a benchmark for endurance and plays a key role in enhancing overall swim performance.