Long latency ankle responses to dynamic perturbation in older fallers and non-fallers

The purpose of this study was to examine the effects of falling history and aging on the latency and magnitude of long latency responses in the lateral gastrocnemius (LG) and tibialis anterior (TA) muscles of older adults.

Single observation study.

Emory University School of Medicine, Center for Rehabilitation Medicine, serving the greater Atlanta, Georgia, area.

There were 62 community-dwelling adults aged 60 or older, 32 with a history of two or more unexplained falls in the past year and 30 with no history of falls in the past year.

The electromyographic activity of the gastrocnemius and tibialis anterior muscles was recorded bilaterally during repeated 10 degree dorsiflexion perturbations to standing subjects at the acceleration of gravity and a velocity of approximately 100 deg/sec. The latency and magnitude of the long latency EMG responses (LLR) of both muscles were determined and compared between individuals by falls history and by decades of age.

No significant differences between falls status groups existed for mean LLR latencies or magnitudes of either LG or TA. Attenuation of the LLR throughout the trials was significantly different between groups. Non-fallers demonstrated attenuation of the LG magnitude over the first five stretches. Fallers showed maladaptation of the LG response, with the latency becoming shorter over 25 trials. More than half of the subjects in both groups had one or more instances of temporal reversal of the response latencies, with the TA response occurring earlier than the LG response during the first five stretches. No differences existed in the LLR between age decades within faller or non-faller groups. A timed measure of mobility (Up & Go) showed a significant difference between falls status, but not between age decades in either group.

Lack of differences between falls status and age decade groups over all trials suggests that LLRs remain intact between older fallers and non-fallers and between older age groups. The mechanism of perturbation is a distinguishing feature of this study and probably contributes to our findings. The appearance of an atypical response strategy in both groups, where the TA response often occurs before the LG response, has raised the question of whether there is always a stereotypical pattern of LLRs during postural perturbations outside the experimental setting.