If you decide to work on cardiorespiratory endurance four days a week, you are applying __________.

The concept of frequency in physical fitness refers to how often an individual engages in a specific activity or exercise within a given timeframe, such as a week. By choosing to work on cardiorespiratory endurance four days a week, you are emphasizing the frequency of your training sessions. This is crucial because regular engagement in cardiovascular exercise is necessary to improve and maintain cardiorespiratory endurance over time.

The other concepts, such as time, specificity, and intensity, pertain to different aspects of an exercise program. Time focuses on the duration of each workout session, specificity relates to the particular areas of fitness being targeted, and intensity refers to the difficulty level of the workouts. However, in the context of this question, the focus is solely on the number of days you commit to training, which aligns perfectly with the definition of frequency.