Myotonic syndromes and periodic paralyses are rare disorders of skeletal muscle characterized mainly by muscle stiffness or episodic attacks of weakness. Familial forms are caused by mutations in genes coding for skeletal muscle voltage‐gated ion channels. Exercise is known to trigger, aggravate, or relieve the symptoms. Therefore, exercise can be used as a functional test in electromyography to improve the diagnosis of these muscle disorders. Abnormal changes in the compound muscle action potential can be disclosed using different exercise tests. We report the outcome of an inclusive electromyographic survey of a large population of patients with identified ion channel gene defects. Standardized protocols comprising short and long exercise tests were applied on 41 unaffected control subjects and on 51 case patients with chloride, sodium, or calcium channel mutations known to cause myotonia or periodic paralysis. These tests disclosed significant changes of compound muscle action potential, which generally matched the clinical symptoms. Combining the responses to the different tests defined five electromyographic patterns (I–V) that correlated with subgroups of mutations and may be used in clinical practice as guides for molecular diagnosis. We hypothesize that mutations are segregated into the different electromyographic patterns according to the underlying pathophysiological mechanisms. Ann Neurol 2004