Dynamic stability and variability of perturbed walking in young adults

Abstract

Falls are the third major cause of inadvertent injury in young Australian adults aged between 18-35 years. The inability of an individual to respond to external perturbations due to walking on an inclined surface, or internal perturbations such as dual-task walking, are known to be associated with significantly higher risk of balance loss. Significant factors known to increase risk of balance loss during walking include performing an additional task requiring high motor-cognitive, sensory or cognitive load (internal perturbations), and walking on uneven surfaces such as sloped terrain (external perturbations). At present, however, dynamic balance of the entire body and the risk of balance loss during walking under such perturbations is not well understood. The objective of this study was to investigate dynamic stability and variability of the human body during walking, and assess the influence of external, motor-cognitive, sensory and cognitive perturbations on dynamic balance, including surface inclination, use of a cell phone, auditory and visual stimulation, and mental calculation.

Nineteen healthy young adult males were recruited. Three-dimensional joint kinematics were obtained using an optical motion capturing system as subjects walked at their self-selected speed on an instrumented treadmill. Dual-tasking was simulated by subjecting participants to motor-cognitive, visual, cognitive and auditory perturbations during walking including cell phone usage (talking, texting and reading), watching a video clip, listening to music, and performing numeric calculations mentally. External perturbations were also applied through alteration of surface inclination. Variability analysis was performed on spatiotemporal gait parameters using Detrended Fluctuation Analysis (DFA) and Standard Deviation. Dynamic stability was subsequently estimated for the entire body as well as the head, trunk and lower extremity joints using linear and nonlinear measures including Margin of Stability (MoS), Lyapunov Exponent (LyE) and Maximum Floquet Multipliers (MaxFM). A novel method was devised to assess stability using Margin of Stability at heel contact (HC) and minimum foot clearance (MFC), gait events associated with backward and forward balance loss, respectively. Slip and trip propensity estimated using Required Coefficient of Friction (RCoF) and MFC height, respectively. Finally, the most destabilizing additional task while walking was determined using deviation of MoS and trip propensity values during dual-task trials from the corresponding values during baseline walking.

The results showed that dual-tasking during walking adversely affects balance in a direction specific-manner. Specifically, cell phone texting and reading while walking reduces balance in the mediolateral direction, while cell phone talking increases the risk of tripping in the anteroposterior direction. Upslope terrain increased the risk of balance loss in the anteroposterior and vertical directions and did not affect gait balance in the mediolateral direction, while walking down was associated with greater stability in the anteroposterior direction. Cognitive and sensory perturbations affected gait balance mostly in the anteroposterior and vertical directions rather than the mediolateral direction. Analysis of trip propensity showed that motor-cognitive dual-tasking due to cell phone usage while walking, cognitive and sensory perturbations due to performing additional auditory and visual tasks while walking are associated with greater risk of tripping, as measured by a lower MFC height. Particularly, talking while walking, and cognitive and sensory dual-tasking while walking may ultimately lead to an increase in risk of tripping in young adults. However, the risk of tripping in young individuals is not sensitive to external perturbations caused by sloped terrains. Participants mostly changed their step length and step time during walking under perturbations. Among the various measures used to determine the most destabilizing secondary task while walking, MFC height was more sensitive to the applied perturbations. Talking while walking was associated with the largest deviation from baseline condition.

The findings of this investigation confirmed that head stabilization during ambulation has higher priority compared to other segments, and individuals try to adopt different strategies to attenuate perturbations from the lower body to the head. The current data highlighted the importance of arm swing in balance maintenance during walking under perturbations, and demonstrated that individuals try to compensate restricted arm swing during walking by modulating step width. With respect to gait adaptations, the results of this research support the idea that individual’s response to applied perturbations through dual-tasking while walking depend on the magnitude of the applied perturbation. The evidence from this study suggests that talking while walking is the most challenging secondary task during locomotion among the applied perturbations in this study, and additional sensory tasks are the least challenging one.

These findings have significant implications for development of a gait training protocol for more frail people to successfully address common perturbations arising from daily living activities during everyday life. These data also suggest that MFC height analysis and local stability analysis of the lower body should be performed to gain better understanding on the effect of additional concurrent task while walking on the risk of tripping and gait stability, respectively. The analysis of MoS presented extends knowledge of step-to-step balance changes during walking at different gait events associated with common fall patterns occurring at HC and MFC. Further work needs to be done to analyse the influence of similar attention-demanding secondary tasks or ‘distractions’ in more vulnerable populations, including the elderly, fallers, and individuals with sensory or motor impairments that affect locomotor control.