Different types of tremor in the human thumb.

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Abstract
1. The upper limbs of normal subjects were immobilized in a way that allowed measurement of forces and movements at the thumb interphalangeal joint without significant movement elsewhere in the limb. 2. When the subject attempted to maintain a steady flexing force at the joint against a rigid stop, the actual force showed the irregular 8‐11 Hz fluctuations characteristic of a ‘physiological tremor'. This force fluctuation increased when the mean flexing force increased. 3. If the subject exerted his flexing force against a light complaint spring, there was an analogous irregular 8‐11 Hz movement at the joint. 4. When, however, an extra inertial load was added to the terminal phalanx, flexion against a complaint spring was often accompanied by a different type of tremor. This was a more regular oscillation, of lower frequency (3‐6 Hz), and of much larger amplitude. 5. The precise frequency and amplitude of this type of tremor depended on the characteristics of the added inertia and spring, in a way that could have been predicted from the responses of the joint to an imposed sinusoidal movement (Brown, Rack & Ross, 1982a). The movement appeared to arise from re‐excitation within stretch reflex pathways. 6. The irregular 8‐11 Hz tremor at this joint could not be attributed to reflex re‐excitation, since the responses to sinusoidal movement indicated a stretch reflex whose timing would not support a movement at that frequency. It is, however, emphasized that other joints of the hand and fingers may behave in different ways.

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