From MedlinePlus Genetics
Potassium-aggravated myotonia is a disorder that affects muscles used for movement (skeletal muscles). Beginning in childhood or adolescence, people with this condition experience episodes of sustained muscle tensing (myotonia) that prevent muscles from relaxing. Myotonia causes muscle stiffness that worsens after exercise. In this disorder, episodes of myotonia may also be triggered (aggravated) by eating foods that are high in the mineral potassium, such as bananas and potatoes. During these episodes, stiffness occurs in skeletal muscles throughout the body. 

Potassium-aggravated myotonia ranges in severity from mild episodes of muscle stiffness (myotonia fluctuans) to severe, disabling disease with frequent attacks (myotonia permanen). Unlike some other forms of myotonia, potassium-aggravated myotonia is not associated with episodes of muscle weakness.

  https://medlineplus.gov/genetics/condition/potassium-aggravated-myotonia