Short communicationAll joint moments significantly contribute to trunk angular acceleration
Introduction
Dynamic balance during walking consists in large part of stabilization of the trunk. Trunk angular sway has been indicated as a reliable measure of balance stability (Allum et al., 2001, Allum et al., 2005, Adkin et al., 2005); however, there are inconsistencies in the literature as to how each of the lower extremity joint moments affect trunk angular acceleration.
Previous studies have argued that because the inertia of the trunk about the ankle joint is nearly eight times the inertia of the trunk about the hip joint, the ankle muscles would need nearly eight times the moment-of-force relative to provide the same angular acceleration of the head, arms, and trunk segment as the hip muscles (Winter, 1990; Winter et al., 1990, Winter, 1995). They concluded that the central nervous system (CNS) may recognize this and utilize the hip muscles as the primary actuator for trunk angular control with little involvement of the ankle muscles during gait (Winter et al., 1990). Alternatively, EMG studies have concluded that the role of distal musculature is as important as proximal hip musculature in maintaining balance using a movable platform to allow for perturbations (Tang et al., 1998). Similar observations, using joint power and muscle EMG patterns, show that reactive balance control is more likely a synchronized effort of the lower extremity joint moments to prevent collapse during perturbations (Ferber et al., 2002).
Although the contribution of each muscle (or joint moment) has been explored with regard to trunk propulsion and support and segmental power flow (e.g., Neptune et al., 2001; Kepple et al., 1996; Zajac et al., 2002, Zajac et al., 2003), similar relationships to trunk angular acceleration have been relatively unexplored. In the present study, we use a three-dimensional model and the resulting forward dynamic equations of motion to identify the relative contribution of each joint moment to the trunk angular acceleration and assess whether one joint more strongly accelerates the trunk than the others. We also analyze the contribution of sagittal plane moments on frontal plane trunk accelerations that are made possible through dynamic coupling (Zajac and Gordon, 1989), along with the contribution of frontal plane moments on sagittal plane accelerations, to assess whether caution is necessary when interpreting trunk angular acceleration with only sagittal plane analysis (MacKinnon and Winter, 1993).
Section snippets
Subject
A single subject whose joint moments and joint angular velocities are consistent with published data used in similar trunk control research (e.g., Winter, 1990; Fig. 1) was used for demonstration purposes. The subject walked at self-selected speed on an instrumented treadmill (Techmachine, Andrezieux Boutheon, France) for three trials of 30 s each.
Data collection
Kinematic data were collected using a 12 camera VICON system and sampled at 100 Hz to capture spatial positions of markers placed on the subject in a
Sagittal plane
The elements of the sensitivity matrix (angular acceleration per N m of moment produced) that affect the sagittal plane trunk angular acceleration are shown in Fig. 2. The sagittal plane trunk angular acceleration depends little on non-sagittal plane moments.
The sagittal plane trunk angular acceleration has a negative sensitivity to the moment generated at the ankle for most of the gait cycle (Fig. 2). Thus, an increase in plantar flexion moment (a more negative ankle moment about the x-axis)
Conclusion
The importance of a three-dimensional analysis should be considered when investigating trunk angular acceleration (Fig. 4). Although frontal plane moments do not have significant contributions to trunk angular accelerations in the sagittal plane, the sagittal plane moments, especially the ankle moment, have large contributions to trunk angular accelerations in the frontal plane.
All the moments are simultaneously controlled to result in a very stable angular acceleration at the trunk (Fig. 6,
Conflict of interest statement
We, the authors of the manuscript titled All joints significantly contribute to trunk stability to be submitted to the Journal of Biomechanics on 09/24/2009, declare that we have no proprietary, financial, professional, or other personal interest of any nature or kind in any product, service, and/or company that could be construed as influencing the position presented in, or the review of, the manuscript.
Acknowledgements
This work was supported by the National Institutes of Health (HD46820) and the Department of Veterans Affairs Rehabilitation Research and Development Service.
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