The principle of training that involves adapting to the intensity of workouts is referred to as the progressive overload principle. This principle dictates that to continue making gains in strength, endurance, or overall fitness, individuals must gradually increase the stress placed on their bodies through higher weights, more repetitions, or increased workout intensity. This ongoing challenge promotes adaptation and capacity improvements, while maintaining proper recovery to avoid injury. Adhering to this principle is crucial for achieving long-term fitness goals and progress.