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George E. Carvell, PhD, PT   University of Pittsburgh
All of the hallmarks of cerebellar ataxia are illustrated. Timing errors contribute to delays in initiation and termination of movement, to deficits when changing direction of movement, to poor control of velocity (dysmetria) and timing problems contribute to poor ability to coordinate muscle coupling in multijoint movements (dyssynergia). In addition, when challenged to generate rapid rhythmic movement alterations the rhythm is slow, small in amplitude and degenerates over a few cycles (dysdiadochokiesia) - see fig 2.
Fig 2A  V.B. Brooks and W.T. Thach, Cerebellar Control of Posture and Movement Chapter 18 In: Handbook of Physiology, The Nervous System II  V.B. Brooks, ed, Bethesda, American
Fig 9-7 p 187 W.T. Thach and E.B. Montgomery, Motor System, Neurobiology of Disease, L. Pearlman and R.C. Collins, eds, Chapter  9, pp 186-196, New York: Oxford Univ Press, 1990.
Fig 9-7
Fig 2
Traces of Rapid Alternating Movements produced by an individual with a left cerebellar lesion (from G. Holmes).
Rapid cyclic movements are thought to be generated by a motor program that may run with or without feedback. Fig 2 shows a motor program dissolving before your very eyes.
Physiol Soc, 1981.
GMOMM  2001