ATP2A1 and myopathy: There are myopathies with muscle stiffness and cramps, such as myotonic dystrophy type 2, Brody disease (caused by a mutation of the sarcoplasmic/endoplasmic reticulum Ca2+ ATPase type 1 [SERCA1], the main protein involved in muscle relaxation), McArdle disease (glycogenosis type 5) and nemaline myopathy type 6, in which TMS-induced muscle relaxation is slower than healthy controls or patients with myopathies without muscle stiffness.