Final answer:
Shortness of breath may occur in a client with a strained trapezius muscle because the trapezius muscle is an accessory muscle of respiration that may need to assist with breathing when the primary muscles are strained or injured.
Step-by-step explanation:
Shortness of breath in a client with a strained trapezius muscle may be due to the fact that the trapezius muscle is an accessory muscle of respiration. During normal breathing, the primary muscles involved are the diaphragm and intercostal muscles. However, when these muscles are strained or injured, other muscles, such as the trapezius, may need to assist with breathing. This can lead to a feeling of shortness of breath.