Which of the following is a characteristic of undulating periodization?

Undulating periodization is characterized by the systematic variation of training variables, such as intensity, volume, and exercise selection, throughout the training cycle. This approach typically involves changing the acute variables on a weekly or even daily basis to allow for improved strength gains, enhanced muscle hypertrophy, and better overall performance. By implementing variations in a program, undulating periodization helps to prevent plateaus and reduces the risk of overtraining, making it a versatile and effective training strategy.

The other options do not reflect the principles of undulating periodization:

Using the same routine for months lacks variety and fails to take advantage of the benefits of fluctuating training intensity and volume that are central to undulating periodization. Focusing only on strength training limits the scope of training stimuli, whereas undulating periodization can incorporate various fitness components like endurance and hypertrophy. Doubling the repetitions every other day is not a recognized method within periodization strategies and does not align with the principles of varying training variables effectively.