The University Record, November 26, 1996

Older men need less time to stop than older women

Joseph Datsko, professor emeritus of mechanical engineering, participated in recent U-M experiments to identify gender and age differences in the ability to stop suddenly while walking.

Photo by Albert Schultz


By Sally Pobojewski
News and Information Services

 

Seventy-year-old John and Mary Smith are striding briskly through the park on their daily two-mile walk when a bicyclist suddenly skids and crashes to the ground directly in front of them. Who is most likely to stop before colliding with the bike?

John is, according to an experiment conducted by University researchers and reported Nov. 18 at the Gerontological Society of America's meeting in Washington, D.C.

For reasons that remain unclear, healthy and active older women need more time to stop when they are suddenly confronted with an obstacle while walking than do healthy, active men in the same age range, says Cheng Cao, a graduate student in mechanical engineering and applied mechanics.

When the 40 adults in Cao's study were suddenly alerted to stop walking six-tenths of a second before reaching a designated line on the experimental track, for example, 84 percent of the young adults (both men and women) and 72 percent of the older men were able to stop before crossing the line, while only 57 percent of the older females could do so.

"To achieve a stopping success rate of 50 percent, young adults had to react approximately 520 milliseconds before reaching the barrier," Cao said. "To achieve the same rate of success, older males required approximately 530 milliseconds and older females 590 milliseconds." This corresponds to young adults needing to reach about 68 centimeters before they reach the barrier, with older males needing 69 centimeters and older females 77 centimeters.

"It seems like a very small difference, but the consequences to older women of not having the additional warning time are not trivial," Cao added.

Cao tested 20 young adults (mean age 23.4 years) and 20 older adults (mean age 72.6 years) with 10 men and 10 women in each group. Subjects in all categories were screened by a geriatrician and described themselves as healthy and physically active. While attached to a safety harness, the test subjects walked down an experimental track at a comfortable pace of about 1.3 meters per second. All were told to stop as quickly as possible whenever they saw lights at any of five possible barrier locations on the track. In his experiment, Cao randomly varied the available response time, or the time between the visual cue to stop and the subject's passage through the array of lights on the barrier, from 375 to 825 milliseconds (almost four-tenths to about eight-tenths of a second).

"Young adults had a significantly higher mean rate of success than did the older adults," Cao said. "At all available response times, older female subjects had a significantly lower rate of success than either older males or young adults. No significant gender differences were found among young adults."

Cao's study is part of an ongoing research program focused on mobility issues in the elderly, which is directed by Albert B. Schultz, the Vennema Professor of Mechanical Engineering and Applied Mechanics and a research scientist at the Institute of Gerontology. Schultz says he and his colleagues have detected significant gender differences in the amount of time it takes older adults to turn quickly to avoid an obstacle and to recover balance after an external support is removed.

According to Schultz, differences in gender response times in older adults are greatest in time-critical situations where muscle strength is required, such as restoring balance during a sudden stop or near fall.

"Stopping suddenly is a complex and time-critical task that requires rapid visual processing, rapid strategy planning and rapid motor execution, during which whole body balance must be maintained," Schultz said. "Older women's longer response time in these situations may help explain why the rate of falls and serious injuries from falls is approximately 1.5 to 2 times higher in older females than in older males."

Schultz added that the experimental data do not support the popular belief that thought processes slow down with age. "Muscles appear to receive the neural signal to stop at roughly the same time in both young and old adults," he said, "but there may be a difference in the speed and strength of muscle contractions. Muscles in older adults seem to contract more slowly than in younger adults, with older females having lower strengths than older males. Additional muscle physiology studies will be needed before we can know for sure."

The research program is funded by the National Institute on Aging, the Vennema Endowment and the Older Americans Independence Center, which is part of the Geriatrics Center. Other researchers participating in the project include James A. Ashton-Miller, a research scientist in mechanical engineering and applied mechanics and the Institute of Gerontology; and Neil B. Alexander, assistant professor of internal medicine and assistant research scientist in the Institute of Gerontology.