Simplify the following text for a Grade 13 native English-speaking person.
Original: The basic principles of strength training involve a manipulation of the number of repetitions ( reps ) , sets , tempo , exercises and force to cause desired changes in strength , endurance , size or shape by overloading of a group of muscles .
Simplified: The basic principles of strength training involve a number of repetitions ( reps ) , sets , tempo , exercises and force to cause desired changes in strength , endurance , size or shape by overloading of a group of muscles .