Age-associated changes in head jerk while walking reveal altered dynamic stability in older people.

Many older people have impaired dynamic stability, and up to one in three people over 65 fall each year. It is thought that older people walk more slowly to compensate for reduced capabilities. Here, we investigate whether head jerk, the first time derivative of acceleration, can further our understanding of age-associated changes in dynamic stability while walking. Gait parameters including cadence, step length, walking speed, harmonic ratios, step time variability, and jerk were measured in 43 young and 100 older people using accelerometers securely attached to the head and pelvis. Older people presented significantly (p ≤ 0.004) more mediolateral (ML) head jerk, but significantly less vertical (VT) head jerk. The dimensionless ratio, ML/VT jerk, demonstrated superior ability (89 % accuracy) in differentiating older from younger people. Principal component analysis indicated that ML/VT jerk was a distinct gait construct. ML/VT jerk was highly reliable, normally distributed, independent of stature or gender, and relatively unaffected by walking speed. In older people, reduced VT head jerk may indicate reduced gait vigour, and increased ML head jerk may indicate age-associated changes to dynamic stability. The smoother head movements evident in our younger group may be because they were more able to rely on automatic control and the dynamic (pendulum-like) stability of their systems.