Muscles

The muscular system is the body's network of tissues that controls movement both of the body and within it (such as the heart's pumping action and the movement of food through the gut). Movement is generated through the contraction and relaxation of specific muscles.
The muscles of the body are divided into two main classes: skeletal (voluntary) and smooth (involuntary). Skeletal muscles are attached to the skeleton and move various parts of the body. They are called voluntary because a person controls their use, such as in the flexing of an arm or the raising of a foot. There are about 650 skeletal muscles in the whole human body. Smooth muscles are found in the stomach and intestinal walls, vein and artery walls, and in various internal organs. They are called involuntary muscles because a person generally cannot consciously control them. They are regulated by the autonomic nervous system (part of the nervous system that affects internal organs).

Tremor

Tremors are unintentional trembling or shaking movements in one or more parts of your body. Most tremors occur in the hands. You can also have arm, head, face, vocal cord, trunk and leg tremors. Tremors are most common in middle-aged and older people, but anyone can have them.
Tremors commonly occur in otherwise healthy people. Sometimes, however, nerve diseases such as Parkinson's disease or dystonia, an overactive thyroid, or certain medicines can cause them. Heavy drinking, because it kills certain nerve cells, can cause tremors, too. Some forms are inherited and run in families. Others have no known cause.