Final answer:
To facilitate continued strength gains in a client who has a fair plus muscle grade, the best modification is to include isotonic exercises with resistance in the physical therapy plan.
Step-by-step explanation:
The best modification an Occupational Therapist (OTR) can make to the client's plan of care to facilitate continued strength gains in a client recovering from a shoulder injury would be isotonic exercise with resistance. Isotonic exercises involve the lengthening and shortening of the muscle through a range of motion against resistance, which is well-suited to building muscular strength when a fair plus muscle grade is identified. Since the client is already performing range of motion exercises and passive stretches at home, adding resistance will help progress their strength gains beyond what they are currently achieving with their exercises.