Upper motor neuron versus lower motor neuron has nothing to do with the upper body versus the lower body.
Upper motor neurons are so named because they come from the highest centers of the brain (i.e. the cerebral cortex) and project to neurons in the lower centers of the brain (including the brainstem) and spinal cord (the lowest part of the central nervous system). The lower motor neurons are the ones in the brainstem and the spinal cord that project to our muscles. Therefore, the upper motor neurons help control the lower motor neurons and the lower motor neurons directly control our muscles (the upper motor neurons indirectly control our muscles).
Upper motor neuron symptoms are things like weakness, slowed movements, hyper reflexes, primitive reflexes (e.g. Hoffman and Babinski). Upper motor neuron death will not cause atrophy . . . although you might get a little atrophy from disuse of muscle. As I stated, these are things that can only be diagnosed clinically and not with an EMG. The reason the EMG can't be used, is because the upper motor neurons don't make "contact" with our muscles.
Lower motor neuron symptoms are things like weakness and atrophy. Atrophy of the shoulder muscle will be from lower motor neuron death if it is caused by ALS and it would definitely show-up on an EMG.