actually I think neither field stole EMG from the other...
The original electromyographers evolved from physicians who
started studying electricity as a "cure all" zapping
patients for anything from chronic pain to "female troubles"
It was quite fashionable for a certain time and physicians wrote up
cases in the journal called The Journal of Electrotherapeutics.
They were neither physiatrists nor neurologists. If anything they
were closely aligned with radiologists!!
The journal at one time was called The Journal of Electrotherapeutics
and Radiology. These were doctors interested in physical modalities to treat patients. Especially electricity and roentgen rays. Yes even radiation was considered beneficial at one point.
Back to the point... tracing the lineage of the journals
The Archives of Physical Therapy, X-ray and Radium, was changed to Archives of Physical Therapy. In 1945 the journal became simply Archives of Physical Medicine. Later it became the Archives of Physical Medicine and Rehabilitation, a premiere journal in the field of rehabilitation.
SO what does this mean? In my opinion, PM&R can trace it's roots back to the original electrophysiologists!! The field eventually evolved into modern application of EMG and nerve conduction study.
Also, Look at the way the residency programs arrange education in EMG... PM&R docs are REQUIRED to do a couple hundred EMG's to graduate. Neurologists who want to do EMG's do an elective electrophysiology fellowship after they graduate.
In conclusion, electrotherapeutics was the origin, neither PM&R nor neurology at that time. Naturally, PM&R doctors, who evolved from interns treating patients with paraplegia and other disabilities mostly from wartime injuries, became fundamental to PM&R to help these patients. Their interest in physical modalities and similar mindset to their predecessors as well as the modern applications of nerve conduction to help diagnose functional impairments made EMG fundamental to the field of PM&R.