Preprogrammed reactions in individuals with Down syndrome: the effects of instruction and predictability of the perturbation.
Preprogrammed reactions to unexpected loadings and unloadings of an elbow joint resisting an extending bias load were studied in subjects with Down syndrome. The subjects were instructed to either "let go" or "return back as fast as possible." Both predictable and unpredictable perturbation directions were used. Integrated electromyograms of the two elbow flexors and two elbow extensors were used to characterize the preprogrammed reactions. All the subjects were able to modulate their preprogrammed reactions in response to a change in the instruction. They were also able to grade the preprogrammed responses corresponding to the magnitude of the perturbation. Two basic patterns of modulation were observed, "reciprocal" and "coactivation." There were no effects of the predictability of the perturbation direction. Reconstructed joint compliant characteristics suggested an unchanged gain in the arc of the tonic stretch reflex. We propose that there are no abnormalities in the preprogrammed reactions of some individuals with Down syndrome. The apparent "clumsiness" of individuals with Down syndrome in a changing environment is assumed to reflect an adaptation to their impaired decision-making ability. The high variability of motor performance in subjects with Down syndrome may result from mixing different strategies for solving motor tasks.
Latash
M L
ML
Department of Physical Medicine, Rush-Presbyterian-St. Luke's Medical Center, Chicago, IL 60612.
Almeida
G L
GL
Corcos
D M
DM
D000293:Adolescent; D000328:Adult; D003657:Decision Making / Q000502:physiology; D004314:Down Syndrome / Q000503:physiopathology* / Q000523:psychology; D004550:Elbow / Q000503:physiopathology; D004576:Electromyography; D005260:Female; D006801:Humans; D008297:Male; D009048:Motor Skills; D009119:Muscle Contraction / Q000502:physiology; D011930:Reaction Time; D012026:Reflex, Stretch / Q000502:physiology