Which muscle is NOT typically affected by a lower motor neuron injury to C5?

A lower motor neuron injury at the C5 level primarily affects the muscles innervated by the C5 spinal nerve root. The deltoid, biceps brachii, and coracobrachialis are all innervated by branches of the brachial plexus that originate from the C5 nerve root. As such, injuries at this level would impact both motor function and muscle tone in those muscles, potentially leading to weakness or atrophy.

On the other hand, the triceps muscle is primarily innervated by the radial nerve, which originates from the C7 spinal cord level. Since the triceps is not directly controlled by the C5 spinal cord segment, it typically remains unaffected in cases of lower motor neuron injuries at C5. This differentiates it from the other muscles mentioned, illustrating why it is the correct choice for this question. Understanding the innervation of these muscles helps clarify the implications of spinal cord injuries at specific levels.