Differentiate between the application of double progression and undulating periodization for a hardgainer aiming to consistently apply progressive overload without excessive neural fatigue.

Double progression and undulating periodization are distinct strategies for achieving progressive overload, the gradual increase of stress placed upon the body during training to continue making gains, while managing neural fatigue, which is the exhaustion of the nervous system impacting force production and coordination. For a hardgainer, an individual who struggles to gain muscle mass and strength due to factors like recovery capacity, the choice between these methods significantly impacts their ability to consistently progress without overtraining. Double progression is a straightforward method where an individual first aims to increase repetitions, or reps, within a predetermined target range for a given weight, or load. Once the upper limit of the repetition range is consistently achieved across all sets, the weight is then increased, and the repetitions are reset to the lower end of the range. For example, if a hardgainer is targeting 3 sets of 8-12 repetitions with a squat, they would perform 3 sets of 8 repetitions at a specific weight. Over subsequent workouts, they would strive to increase the repetitions (e.g., 9, 10, 11, then 12 reps per set) with that same weight. Only after consistently achieving 3 sets of 12 r....