Movement detection thresholds at the human elbow joint.

Publisher Summary This chapter reports experiments that show three things. First, there are systematic changes in movement detection thresholds, observable at the human elbow joint, that cannot be assigned to signals from skin or joint receptors and that must therefore be attributed to muscle spindles in elbow muscles. Second, when only one of the pairs of antagonist muscles generates proprioceptive signals, movement detection thresholds are no higher than when the two antagonists are in a mechanically identical state and both are therefore potentially able to generate signals. Third, significant rises in the movement detection thresholds are observed when they are measured at the active contraction of the elbow muscles. This finding of a rise in the detection threshold during a contraction, compared to that in a passive muscle, is contrary to some earlier observations, and a different result can probably be explained by the way the muscle was conditioned before each test. When considered more broadly, this result is also unexpected. When limbs are moved passively, position sense is poor. In a forearm-matching task, for example, subjects are much more accurate when they place their arms voluntarily than when the passive arm is positioned by the experimenter. Perhaps there is an additional cue, that of gravity, providing positional information.