Lower safety factor for old adults during walking at preferred velocity.

Older adults are more prone to falls during walking than young adults, although they walk more slowly and demonstrate higher stability state. This paradox of higher stability state but less safe locomotion let us hypothesize that older people may move closer to their dynamic stability limits. In order to investigate this hypothesis, the present study examined the safety factor of dynamic stability in old and young individuals when walking at their preferred velocity. Twelve older and 12 young male participants walked at their (a) walk-to-run transition velocity (WRV, i.e., maximum capacity) and (b) preferred walking velocity (PWV, i.e., actual applied load). Whole body kinematic data and ground reaction forces were captured. Dynamic stability was assessed using the "margin of stability (MoS)" as a criterion for the stability state of the body (extrapolated center of mass concept). The safety factor was calculated as the ratio between MoS at WRV and MoS at PWV. We found that, although older participants walked slower and provided a higher stability state compared to young ones, they showed a significantly reduced safety factor during preferred walking. This confirmed our hypothesis. Old adults do not walk slowly enough in relation to their maximum walking velocity, resulting to a lower safety factor during normal locomotion. Apparently, the age-related muscle degeneration affects WRV more than PWV. The resulting lower safety factor for the older participants may partly explain the increased risk of falls in their daily life, in spite of slower locomotion.