A vibrating object moves back and forth from its normal stationary position. A complete cycle of vibration occurs when the object moves from one extreme position to the other extreme, and back again. The number of cycles that a vibrating object completes in one second is called frequency. The unit of frequency is hertz (Hz). One hertz equals one cycle per second.

A vibrating object moves to a certain maximum distance on either side of its stationary position. Amplitude is the distance from the stationary position to the extreme position on either side and is measured in metres (m). The intensity of vibration depends on amplitude.

The speed of a vibrating object varies from zero to a maximum during each cycle of vibration. It moves fastest as it passes through its stationary position to an extreme position. The vibrating object slows down as it approaches the extreme, where it stops and then moves in the opposite direction through the stationary position toward the other extreme. Speed is expressed in units of metres per second (m/s).

Acceleration is a measure of how quickly speed changes with time and therefore, acceleration is expressed in units of (metres per second) per second or metres per second squared (m/s²). The magnitude of acceleration changes from zero to a maximum during each cycle of vibration. It increases as vibrating object moves further from its normal stationary position.

Every object tends to vibrate at one particular frequency that depends on the composition of the object, its size, structure, weight and shape. This frequency of natural vibration is called the resonant frequency. A vibrating machine transfers the maximum amount of energy to an object when the machine vibrates at the object's resonant frequency.

Contact with a vibrating machine transfers vibration energy to a person's body. Depending on how the exposure occurs, vibration may affect a major part of the worker's body or only a particular organ. The effect of vibration exposure also depends on the frequency of vibration. Each organ of the body has its own resonant frequency. If exposure occurs at or near any of these resonant frequencies, the resulting effect is greatly increased.

Segmental vibration exposure affects an organ, part or "segment" of the body. The most widely studied and most common type of segmental vibration exposure is hand-arm vibration exposure which affects the hands and arms. Hand-arm vibration affects operators of chain saws, chipping tools, jackhammers, jack leg drills, grinders and many other workers who operate hand-held vibrating tools.

Whole body vibration energy enters the body through a seat or the floor, it affects the entire body or a number of organs in the body. Exposed groups include operators of trucks, buses, tractors and those who work on vibrating floors. Table 1 lists examples of vibration exposure in various industries.