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George E. Carvell, PhD, PT   University of Pittsburgh
PD MOTOR DEFICITS: What It Teaches Us About Movement
PD Subjects have prolonged Reaction Times and prolonged Movement Times. Most of the delay is to an initial cue to move (simple vs choice reaction times). Motor Programs dissolve before your very eyes.
PD Subjects have difficulty sustaining a constant output or doing two motor tasks at once
Unilateral Repetitive Drawing
Repetitive Drawing with one hand while connecting dots with other
Marsden Fig 6C
 p 526
Marsden Fig 12  p 536
A resting (postural) tremor is one of the hallmarks of Parkinson's Disease (PD). It is seen when the body part is 'at rest' but disappears or is greatly attenuated when movement commences.  Resting tremor is a low frequency (~ 4-5 Hz) alternating activation of agonist and antagonist muscles. Motor units tend to fire synchronously, rather than being  activated with the typical staggered, asynchronous recruitment order. Resting tremor actually disappears when the person is totally recumbent and relaxed or sleeping. An Action Tremor may be seen in later stages of PD. Movement is slow (bradykinesia) but accurate (no dysmetria). The resting tremor and brady-kinesia (poverty of movement) are compounded by a leadpipe (plastic) or cogwheeling rigidity (not shown).
Marsden = C.D. Marsden, The Mysterious Motor Function of the Basal Ganglia: The Robert Wartenberg Lecture. Neurology 32: 514-539, 1982.
GMOMM  2001