Final answer:
The muscle in question that is innervated by the spinal accessory nerve (XI) is the trapezius muscle. This nerve also controls the sternocleidomastoid muscle, both of which are essential for head and shoulder movements, and strength tests of these muscles help diagnose nerve function.
Step-by-step explanation:
The trapezius muscle is one of the muscles of the scapula served by the spinal accessory nerve (XI). This nerve, also known as the eleventh cranial nerve, is responsible for the contraction of neck muscles, and it particularly innervates the sternocleidomastoid and trapezius muscles. Both of these muscles are significant for changing the position of the head and contributing to the movement of the trunk and shoulders.
When testing the function of these muscles, procedures often involve asking patients to flex and extend their neck or shrug their shoulders against resistance. This allows for the assessment of muscle strength and to check for any asymmetry or weakness that could indicate damage to the spinal accessory nerve. This form of strength testing is a crucial part of the motor exam and is significant for diagnosing potential deficits that could impact the orientation of the head.