Final answer:
The principle of fitness referring to working your body harder than it is used to is called the Overload principle. It's a method to improve fitness by forcing the body to adapt, and it's applied by increasing exercise intensity or volume.
Step-by-step explanation:
The principle of fitness that involves working your body at a level greater than what it is accustomed to is known as the Overload principle. This key principle is based on the idea that in order to improve your fitness, you must increase the amount of exercise or the intensity beyond what your body is used to. Essentially, by overloading your system, you encourage your body to make adaptations that lead to improved muscle strength, endurance, and overall performance.
In practice, the Overload principle can be applied by increasing the weight lifted, lengthening the duration of a workout, raising the intensity, or modifying the type of exercise to challenge the body further. It's important to apply this principle sensibly and with proper progression to avoid injury.
Other related principles of fitness include Specificity, which pertains to training specific to the sport or the goal; Reversibility, which warns that fitness gains can be lost if training ceases; and Individuality, which acknowledges that everyone's body responds differently to exercise.