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DECELERATION. Rate of slowing down a motion. A negative acceleration. Very significant in biomechanics in the control of end points of a motion. Adequate coordination and voluntary control requires accurate deceleration. Many drugs (e.g., muscle relaxants) interfere with deceleration control and thus destroy coordination.
DELTOID. The muscle of the shoulder, responsible for extending the arm sideways, and for swinging the arm at the shoulder. Overuse of the deltoid muscle may cause fatigue or pain in the shoulder.
DEMANDED MOTIONS INVENTORY. The nature and quality of motions which are required to perform a specific task. The demanded motions inventory may be limited by constraints imposed by the working environment including machinery and tools. Also used to denote the nature and sequence of motions demanded by a specific task design (See AVAILABLE MOTIONS INVENTORY.)
DE QUERVAIN'S DISEASE. A stenosing tenosynovitis involving the abductor pollicis longus and extensor pollicis brevis tendons of the thumb. This disease appears as a weakness that affects the gripping of objects. A pain over the radial styloid process is felt. One occupational cause is the forceful gripping of the type found in clothes wringing.
DIAPHYSIS. The central portion or shaft of a long bone.
DISTAL. Away from the central axis of the body; opposite of proximal.
DOPPLER EFFECT. An apparent signal frequency change caused by relative motion of transmitter and observer. Characteristically, a light color change or sound pitch change. Sometimes used in work measurement to investigate motion patterns. Current devices are not always reliable in the response ranges of interest.
DORSIFLEXION. Bending upwards around an axis. For example, when the ankle flexion takes place upwardly it is called dorsiflexion and the downward flexion is called plantar flexion.
DYNAMIC MOMENT. A force-distance relationship dealing with dynamic (resulting from inertia and acceleration) forces as opposed to static (resulting from gravitational accelerations only) forces. An example of dynamic moment is that experienced during a lifting task. At the instant of lift-off (load moves off platform), the acceleration of the load is caused by the dynamic moment on the lumbo-sacral joint. The consequences of this phenomenon are not always clearly predictable.
DYNAMIC WORK. Muscle work performed when one end of a muscle moves with respect to the other end and external movement is produced. (See ISOMETRIC WORK).
DYNAMOMETER. Apparatus for measuring force or work output external to a subject. Often used to compare external output with associated physiological phenomena (electromyography, spirometry, etc.) to assess physiological work efficiency.
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