From MedlinePlus Genetics
Rippling muscle disease is a condition in which the muscles are unusually sensitive to movement or pressure (irritable). The muscles near the center of the body (proximal muscles) are most affected, especially the thighs. In most people with this condition, stretching the muscle causes visible ripples to spread across the muscle, lasting 5 to 20 seconds. A bump or other sudden impact on the muscle causes it to bunch up (percussion-induced muscle mounding) or exhibit repetitive tensing (percussion-induced rapid contraction). The rapid contractions can continue for up to 30 seconds and may be painful.

People with rippling muscle disease may have overgrowth (hypertrophy) of some muscles, especially in the calf. Some affected individuals have an abnormal pattern of walking (gait), such as walking on tiptoe. They may experience fatigue, cramps, or muscle stiffness, especially after exercise or in cold temperatures.

The age of onset of rippling muscle disease varies widely, but it often begins in late childhood or adolescence. Rippling muscles may also occur as a feature of other muscle disorders such as limb-girdle muscular dystrophy.  https://medlineplus.gov/genetics/condition/rippling-muscle-disease