Elbow position can make a difference when mixed latency difference (MLD) is measured. MLD is a method of measuring the difference between the latency of the ulnar and the median mixed nerve action potentials (MNAP).
Their method entails recording MNAP of both nerves from the same position in the upper arm following the stimulation of these nerves at the same level at the wrist. The MLD increases when the elbow is bent. A small amount of flexion (less than 15 degrees) is acceptable for this test. Elbow position isn't a problem if both tests are done with the exact same position.
It's best to keep the patient's forearm palm up (supination) during the test. It's easier to stimulate the median and ulnar nerves in this position. It's also easy to overstimulate the ulnar nerve in the supinated position, so extra care is required during the testing.
Don't be afraid to ask your doctor this question. Your observations of test differences may make a difference in how the results of your tests are interpreted.