Final answer:
The principle applied in the recommended exercises for strengthening Cesar's rotator cuff muscles is specificity, which focuses on targeted training to produce a specific desired effect.
Step-by-step explanation:
The principle at work in this situation where Cesar has weak rotator cuff muscles and a physical therapist has recommended a set of exercises to help strengthen them is specificity. According to the principle of specificity, training should be relevant and appropriate to the sport for which the individual is training to produce the desired effect. In Cesar's case, the exercises are specifically designed to target and improve the strength of the rotator cuff muscles.