Stability against backward balance loss: Age-related modifications following slip-like perturbations of multiple amplitudes.

Falls are one of the most serious problems in the elderly. Although previous studies clearly link the increased risk of falls with ageing, the mechanisms responsible for the modifications of reactive motor behaviours in response to external perturbations are not yet fully understood. This study investigated how the stability against backward balance loss is affected by aging and intensity of perturbations. The Margin of Stability (MoS) was estimated while eight young and eight elderly adults managed three slip-like perturbations of different intensities while walking at the same normalized speed. A compensatory step was necessary to regain stability. The forward swing phase of the trailing leg was rapidly interrupted and reversed in direction. Results have shown that ageing significantly affects the time required to select the most appropriate biomechanical response: even if the characteristic of the backward step was similar between groups, elderly subjects took more time to reverse the movement of their swinging limb, thus achieving a less efficient action to counteract the backward balance loss (lower MoS both during and at the end of the early compensatory reaction). In addition, young and elderly subjects scaled their reactions with respect to the perturbations intensity in a similar way by increasing the length of their backward step, thus revealing a context-dependent tuning of the biomechanical response that was not affected by aging. These behavioural features can be helpful in identifying the causes of increased fall risk among the elderly in order to define more suited intervention in fall prevention programs.