Unloading Reaction

We have been discussing in neural control the role of reflexes in the control of voluntary movement.  Yesterday we discussed a hypothesis that believes that reflexes play a very minor role in movement, and the central, voluntary commands sent from the brain control almost all aspects of voluntary movement.  There is a pretty cool reflex known as the unloading reaction that disproves this theory.

Imagine that you are holding a dumbbell in your right hand, and you are attempting to keep your elbow flexed (bent) at about 90 degrees.  You are instructed to continuously try and contract the elbow flexors (think biceps brachii, brachialis) in order to maintain this position, and you are instructed to continue to try and activate the muscles no matter what happens.  As you are holding the dumbbell, it is quickly removed from your hand.  What happens to the activity of the elbow flexors?

When the dumbbell is removed from your hand, there is a period of almost complete silence (no activity) in the elbow flexors.  Why does this occur even when you are trying to activate the elbow flexors?  When the dumbbell is in your hand, it activates the muscle spindles in the biceps (the dumbbell is trying to lengthen the biceps, which activates the muscle spindles), which causes a reflexive contraction of the biceps.  When the dumbbell is removed from your hand, muscle spindles are no longer activated, so there is no reflexive contraction of the elbow flexors.  This demonstrates that reflexes play more than a minor role in the control of voluntary movement.