Extension and adduction of the thumb. The thumb waggle. Tenosynovitis, causes and effects among keyboard users.
Touch typing requires that the thumb be held ready over the space bar.
This posture is a combination of extension and adduction of the thumb, while depression of the space bar requires abduction followed by a return to adducted posture. De Quervain's tenosynovitis. The tendons of muscles which achieve this, the abductor pollicis longus and extensor pollicis brevis, run closely parallel in the confined space under the extensor retinaculum as they enter the hand. At this junction they may become inflamed and damaged by the sustained tension required for maintenance of the thumb ready posture, and the relatively small motions required for repeated depression of the space bar. This debilitating condition, once commonly referred to as washer woman's thumb is known as De Quervain's tenosynovitis.