The fitness principle that emphasizes the need for progression is known as the principle of overload. This principle asserts that to enhance physical abilities, individuals must challenge their bodies by gradually increasing stress on muscles through higher weight, intensity, or volume. Progressive overload allows the body to adapt, leading to improved strength, endurance, and overall fitness. Failing to apply overload can result in plateaus, where no further improvements occur. It's key to adjust workouts periodically to continue making gains and foster adaptability in physical capabilities.