may contribute to slower, more-variable movements and reduced strength observed in older adults. Changes in posture and balance are then discussed, as a stable base of support is necessary to execute precise motor skills as well as being important for mobility of older adults. Finally, an overview of motor learning research as well as a discussion of improvements in motor function with generalized and specific training programs are presented. As is apparent, changes in control and coordination of movement significantly affect the type of activities that older adults can efficiently perform and often determine whether they can live independently. Thus, those involved in enhancing the performance capabilities of these individuals need to have a good understanding of how the aging processes diminish motor performance.

RESPONSE INITIATION

Reaction time is defined as the time required to initiate a movement response following a visual, auditory, or other sensory signal and is thought to reflect the speed of transmission of the central nervous system (Stelmach and Goggin, 1988). Experiments are conducted to measure the time it takes to initiate a response when an imperative stimulus is presented. The imperative stimulus is usually visual, but may be auditory or tactile. Such reactions can be to a single stimulus, multiple stimuli, or may include incompatible responses. In a simple reaction-time task, where one stimulus is given and one response is required, it has been demonstrated that reaction time increases in range from 0.5 ms/yr (5 ms/decade) (Fozard, Vercryssen, Reynolds, Hancock, and Quilter, 1994) to 2 ms/decade (Gottsdanker, 1982). It has been widely shown in the research that the speed of processing information decreases (i.e., the time increases) with advanced age on the order of 26 percent (264 ms in the young—20 years old—versus 327 ms in older adults—60 years old) (Welford, 1984). Similar findings have been reported for auditory and tactile simple reaction times as well (Redfern, Muller, Jennings, and Furman, 2002; Walhovd and Fjell, 2001; Liu, 2001; Walker, Alicandri, Sedney, and Roberts, 1991). This approximately 50-ms increase in simple reaction times is consistent across studies that have examined such changes across the life span (Fozard et al., 1994) as well as those that compare groups of young and older adults on the same reaction-time tasks (Amrhein, Stelmach, and Goggin, 1991; Walker, Philbin, and Fisk, 1997; Stelmach and Goggin, 1988; Cerella, 1985; Cerella, Poon, and Williams, 1980; Bashore, Ridderinkhof, and van der Molen, 1997; Gottsdanker, 1982; Stelmach and Goggin, 1988). See Schaie (in this volume) for a similar discussion of response speed with respect to cognitive changes that occur with advanced age.



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