Illusions of forearm displacement during vibration of elbow muscles in humans.

Position matching ability at the forearm in young adults was measured after arm muscles had been placed in a defined mechanical state, called conditioning. With flexion conditioning, elbow flexors were contracted isometrically with the arm held flexed; with extension conditioning, extensors were contracted with the arm held extended. When both arms were flexion conditioned, vibration of the reference biceps produced significant position matching errors as shown by placement of the indicator arm. When the reference arm was flexion conditioned and the indicator arm extension conditioned, vibration no longer produced significant errors. Vibrating elbow flexors of the indicator arm produced significant illusions in the opposite direction from illusions produced by vibrating flexors of the reference arm. These observations show that in an arm matching task the way in which muscles of both arms are conditioned can have an influence on matching performance, including the ability to indicate a perceived illusion.