Complex sentence: In LEMS, the CMAP amplitude increases greatly (over 200%, often much more) in response to either exercising the muscle under investigation or the administration of a rapid burst of stimuli (20 impulses per second for ten secords).
Split into simpler sentences: In LEMS, in response to exercising the muscle, the CMAP amplitude increases greatly (over 200%, often much more). This also occurs on the administration of a rapid burst of stimuli (20 impulses per second for ten secords).