Age-associated changes in the modulation of afferent feedback during voluntary actions
Older adults tend to use greater amounts of antagonist muscle activity than young adults when performing the same movements. As antagonist muscles oppose the actions of agonist muscles, heightened levels of antagonist coactivation are typically considered undesirable due to the increase in metabolic cost and the potential to compromise movement accuracy. The greater coactivation used by elderly adults has been attributed to age-associated declines in the neuromotor system. Recent evidence suggests that young adults tend to rely on afferent feedback to produce corrective actions when performing voluntary contractions, whereas older adults prefer a feedforward strategy of increasing antagonist coactivation to stiffen the joint. This observation suggests that aging may be accompanied by a shift in the preferred control of voluntary movements from feedback to feedforward strategies. This dissertation comprised four studies that tested this hypothesis. Consistent with the hypothesis, the first study found that the synaptic input received by motor neurons innervating an arm muscle of young and old subjects differed when they performed steady contractions. The second study investigated the interaction between coactivation and modulation of afferent input during the functionally relevant task of stepping up down from a platform and the differences between young and old subjects. The results indicated that older adults coactivated leg muscles more and modulated afferent input less during stepping. The other two studies comprised an approach whereby the activity level of the agonist muscle was controlled and the capacity of the nervous system to augment afferent input was measured. The findings indicated that older adults, in contrast to young adults, did not modulate afferent input across conditions. Despite similar values for muscle strength as young adults, middle-aged adults also did not change afferent input across task conditions. These results indicate that the decline in the ability of the nervous system to modulate sensory input begins during middle age and precedes the age-associated decline in muscle mass and strength.