A tremor is an involuntary, rhythmic twitching of opposing muscles. They usually are visible to the naked eye and sometimes can even be felt when touching affected areas. Tremors are caused by the synchronous contraction of muscles that are antagonistic to one another, which means that they normally pull in opposite directions. When they pull in the same direction at the same time, it causes a to-and-fro movement in the affected area of the body. What causes most tremors is unknown. Sometimes, they involve genetic or developmental abnormalities but can also be caused by trauma, spinal cord lesions, inflammation, exposure to toxins, heart disease and immune system disorders. Tremors are fairly easy to differentiate from seizures because of their regularity. They can happen during rest or activity, but tend to worsen with excitement and lessen with rest. Owners often describe tremors as “shaking” or “shuddering”.