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Anticipatory postural adjustments in older adults: are changes in response characteristics due to changes in strategy?

The purpose of this study was to determine if (a) the aging process directly affects the ability to prepare postural adjustments in advance of voluntary movements, or (b) if aging leads to longer movement times which, in turn, lead to less need for advance preparation of the postural muscles. That is, is velocity a mediating variable between age and lack of postural preparation? Postural and prime mover muscle response characteristics of young (n = 16, age 27 +/- 5 years) and older (n = 16, age 71 +/- 8 years) adults were evaluated in an arm-raising paradigm across many arm movement velocity conditions. Older and young adults did not differ in arm movement velocities. Therefore, observed differences between age groups were directly related to the aging process. Muscle timing and sequencing were affected differentially between age groups depending upon which velocity condition was specified. Age-related differences were found in the timing of two postural muscles: in older subjects the contralateral erector spinae muscles were activated later, relative to the deltoid, than in younger subjects, with fewer older subjects showing preparatory activity. Also, older subjects showed more frequent activation of the quadriceps than young subjects.