That's a good question. I will admit that it has been tempting for me to take an anti anxiety pill prior to a procedure. Its not a good idea tho unless doc says so. During a lot of procedures your BP & heart rate are monitored closely. (Not sure on the emg)
If it drops too low they may end up not doing the test and rescheduling it. A test like this one relies on your motor reflexes, and if your senses are dulled b/c of the meds it probably will affect the diagnostic procedure and end up.working against you.

Sometimes some form of sedation is given on certain procedures. That would be a good question to ask your medical team.
Self medicating is never a good idea unless cleared by doctor.

Wishing you the best for your outcome!
Caring,
Rae