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George E. Carvell, PhD, PT   University of Pittsburgh
 
WHERE DO VOLUNTARY MOVEMENTS BEGIN?   Who’s On First?
At first blush one might suspect that voluntary movements begin with activity in the motor cortex. That might seem to be a logical place to set the motor apparatus into action according to a Jacksonian top-down hierarchical system. W.T. Thach provided early evidence that subcortical areas may be involved in the earliest aspects of generating voluntary actions. In addition, he provided evidence that neurons do not all act alike within a distributed motor control system. About 1/3 of the neurons code for muscle force regardless of joint position (MPAT), 1/3 code for joint position regardless of force (JPOS), and yet
another 1/3 code for the direction of the upcoming event or motor set (DSET) in over-trained monkeys performing a well rehearsed serial progression of ‘move and hold’ actions.
Who’s On First?
W.T. Thach, Correlation of Neural Discharge with Pattern and Force of Muscular Activity, Joint Position, and Direction of Intended Next Move-ment in Motor Cortex and Cerebellum. J Neurophysiol 41: 654-676, 1978.
Fig. 1, p 656
Fig. 16, p 668
Movement
Onset (time 0)
Peak
of EMG Activity
Overlearned Sequence
Muscle-Like Pattern
Joint Position Pattern
Motor Set (Predictive) Pattern