Dynamic Stability of the Upper Body During Walking

Abstract

The general purpose of this study was to examine factors that may influence acceleration characteristics of the upper body during walking, thereby clarifying the means by which the postural system facilitates dynamic stability of the upper body during walking. Upper body accelerations were measured during a range of straight-line walking tasks. Time domain, frequency domain, signal regularity and coupling analyses were used to 1) provide new insight into gait-related upper body accelerations during walking in normal healthy adults, and 2) determine how the postural system accommodates to perturbations that challenge upper body stability during walking. The specific perturbations to the postural system that were examined in the present study were the normal ageing process, changes in walking speed, and fatigue of the cervical and lumbar erector spinae. In general, the patterns of accelerations measured at the level of the head were an attenuated version of those at the lower trunk during normal walking. Power spectral analysis revealed that both the head and lower trunk in the anterior-posterior (AP) and vertical directions (VT) directions were characterised by a single peak frequency corresponding the step frequency during normal walking. However, the most notable of all attenuation profiles was the difference between accelerations of the head and lower trunk in the mediolateral (ML) direction. ML trunk accelerations were characterised by multiple low amplitude frequency peaks, which were attenuated to a single peak at the head corresponding to stride frequency. The coupling between acceleration directions was greater for the head than the lower trunk, suggesting that the postural system promotes a coordination strategy which enhances global stability of the head. Subdividing the upper body into neck and trunk segments facilitated a more comprehensive description how the gait-related oscillations are prevented from impacting on the motion of the head. Overall, acceleration amplitude, power content, and regularity were predominantly regulated by the trunk segment, especially for the AP and ML directions. This suggests that the trunk segment plays a critical role in modulating the amplitude and structure of gait-related oscillations prior to reaching the neck segment and thus the head. It was envisaged that examining factors that may challenge the individual (the normal ageing process), and the walking task (changes in walking speed, and induced fatigue of the upper body), would provide new insight into the extent to which the postural system prioritises head stability during walking. Regardless of the challenges imposed on the postural system due to the ageing process, upper body movement was organised in a manner which assisted in maintaining a degree of head stability comparable to those observed under normal walking conditions. Similarly, the importance that the postural system places on maintaining head stability was evident in the attenuation characteristics of the trunk and neck segments when walking speed was manipulated, and neuromuscular fatigue induced. Manipulating walking speed highlighted the critical role that the trunk segment has in regulating upper body accelerations arising from gait-related events. Aside from a minor contribution from the neck segment in the direction of travel at preferred and fast walking speeds, regulation of accelerations occurred due to the dynamics of the trunk segment. Inducing neuromuscular fatigue of the cervical and lumbar erector spinae groups (CES and LES) revealed compensatory movement strategies of the upper body, with a view of enhancing head stability. For several variables quantifying attenuation, fatiguing one muscle group, such as the CES, resulted in changes in the dynamics of another level of the upper body, such as the trunk segment. The trunk segment was particularly dominant in regulating upper body accelerations under fatigued conditions, further reinforcing the importance to control accelerations prior to reaching the neck and head. Overall, the results of this study suggest that optimal head stability is reliant on the trunk segment providing a stable base of support for the neck and head. By regulating accelerations via the trunk segment, the postural system is effectively regulating the orientation of the neck and head, and the inclusive sensory organs. It was evident that the postural system prioritises, and in general, maintains head stability during walking despite challenges imposed on upper body stability.