This is a video from a middle-aged man with myotonic dystrophy type 1, an autosomal dominant multi-systemic disorder that not only affects the muscles, but also causes other problems such as cataracts, arrhythmias and diabetes mellitus. The video shown here is typical of myotonia which can be seen in these patients. The tone is usually normal when muscles are relaxed. However, with muscle contraction, and in this case tested by asking the patient to form a tight fist, there is a delayed and slow relaxation of the hand muscles. This phenomenon is therefore termed grip myotonia. Patients may also have percussion myotonia. This can be elicited by tapping over a muscle bulk (most commonly tested by tapping the thenar eminence), which would result in a prolonged tonic abduction and opposition movement of the thumb lasting for several seconds.