What principle states that muscles need above normal stress for adaptation?

The principle that states muscles need above-normal stress for adaptation is known as the Overload Principle. This principle emphasizes that in order for muscles to grow stronger, increase in endurance, or adapt to new physical demands, they must be subjected to levels of stress that exceed what they are accustomed to. When muscles are challenged beyond their normal capacity, they respond by adapting, which can include increasing in size or strength.

For instance, when someone begins a new exercise regimen that involves lifting weights, using weights that are heavier than what their muscles have previously experienced forces the body to adapt to cope with that extra load. This adaptation process is crucial for achieving fitness progress.

Other principles related to physical training include the principle of specificity, which states that training adaptations are specific to the type of exercise performed; the principle of progression, which focuses on the gradual increase in exercise demands over time; and the principle of reversibility, which highlights that fitness gains can be lost if training ceases or decreases. However, none of these principles directly address the necessity for exceeding normal levels of stress for adaptation, which is the core concept of the Overload Principle.