Tremor is an involuntary, rhythmical, forwards and backwards movement of a body part. Caused by the rapid alternating contraction and relaxation of muscles, it is a phenomenon found in all animal species.
Tremors fall into two main categories: those which are considered to be normal, occurring to most people (physiological tremor) and those which represent dysfunction in the nervous system (pathological tremor) due to conditions such as Parkinson’s and Essential Tremor (ET). The continuity and rhythmic pattern of tremors separate them from other abnormal involuntary movements, such as tics, jerks, and spasms etc.
Physiological tremor (normal tremor)
From time to time physiological tremor occurs in most people and is quite normal, for example when an outstretched limb is held against gravity. Tremor is also a common reaction to stress, anxiety or fear, and many people experience this when speaking in public or taking an exam. Certain medications and fatigue can worsen it and when normal tremor becomes exaggerated (enhanced physiological tremor), the random contractions of the muscle fibres become more organised so that they contract together. This results in a more forceful tremor – a tremor of greater amplitude.
Pathological tremor (abnormal tremor)
Pathological tremor represents dysfunction arising within the nervous system, i.e. the brain and the peripheral nerves. These tremors are usually classified, or described, according to the behavioural circumstances in which they arise or the situations in which they are seen, for example rest tremor or postural tremor (see below). They can further be described by the name of the condition that underlies them - Parkinsonian rest tremor, or Parkinsonian postural tremor.
Rest tremor
This is defined as a tremor that occurs in a body part which is not voluntarily activated and is completely supported against gravity - for example, when sitting watching television with the arm resting at the side, the hand will be seen to shake from time to time. When the arm is moved voluntarily or carries out an intended action, the rest tremor usually, but not always, attenuates or ‘switches off’.
Action tremor
The most common group of tremors are known as ‘action tremors’, and they occur when there is voluntary contraction of muscles in a body part. For example, moving a limb from one point to the other or maintaining posture. Action tremors include:
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postural tremor - a tremor which is evident whilst voluntarily maintaining a position against gravity, for example standing with arms outstretched
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kinetic tremor - a tremor which occurs during any type of movement
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intention tremor - a tremor which becomes exaggerated at the end of a guided movement.
Other tremors
Rarer types of tremor include ‘task specific tremor’ which, as the name suggests, occurs only when carrying out specific tasks - for example, when writing. There are also tremors that only come on when muscles are contracted firmly against a rigid object - for example, when gripping a ball or squeezing the doctor’s finger’.
It should be remembered that almost any condition affecting the nervous system may result in the manifestation of a pathological tremor and most medications that affect the nervous system can produce tremor as a side effect. Tremors can also be caused by psychological illness and its treatments.
