What are the principle of overload, the principle of progression, and the principle of specificity?

These are 3 of the 4 priniciples of training.

Overload: fitness and/or physical adaptation can only be improved upon by training more than you would otherwise.

Progression: slow and gradual increases to the amount and intensity of training are necessary to effect change

Specificity: the conditions and parameters of training should use and meet those that are required by the specific activity to achieve improvement in the body parts that is necessitated by the task, sport, or activity. "practice like you're going to play"

Primary effectit is the most accurate.

Nail fungus, which can cause the nails to crack, peel and change color and texture.