Final answer:
The best way to assess the strength of the sternocleidomastoid muscle is by having the patient turn their head against resistance (option C) . This evaluates each muscle's function and checks for asymmetry or weakness that indicates possible nerve damage.
Step-by-step explanation:
The strength of the sternocleidomastoid muscle can best be evaluated by having the patient turn his or her head against resistance. This movement is specifically controlled by the accessory nerve, which innervates the sternocleidomastoid and trapezius muscles. When assessing the strength of these muscles, a practitioner will look for any asymmetry or weakness that might suggest damage on the weaker side.
Movements such as shrugging the shoulders against resistance and flexing and extending the neck are also used for strength testing and provide insight into the function of the spinal accessory nerve. A clear understanding of how these muscles function and how they are tested provides valuable information during a motor exam and contributes to evaluating potential deficits associated with the accessory nerve.