Article

Although balance during quiet standing is postulated to be influenced by multiple
factors, including ankle stiffness, it is unclear how different mechanisms
underlying increases in stiffness affect balance control. Accordingly, this study
examined the influence of muscle activation and passive ankle stiffness increases
on the magnitude and frequency of postural sway. Sixteen young adults
participated in six quiet stance conditions including: relaxed standing, four
muscle active conditions (10%, 20%, 30% and 40% maximum voluntary contraction
(MVC)), and one passive condition wearing an ankle foot orthotic (AFO). Kinetics
were collected from a force plate, while whole-body kinematics were collected
with a 12-sensor motion capture system. Bilateral electromyographic signals were
recorded from the tibialis anterior and medial gastrocnemius muscles. Quiet
stance sway amplitude (range and root mean square) and frequency (mean frequency
and velocity) in the sagittal plane were calculated from time-varying centre of
gravity (COG) and centre of pressure (COP) data. Compared to the relaxed standing
condition, metrics of sway amplitude were significantly increased (between 37.5
and 63.2%) at muscle activation levels of 30% and 40% MVC. Similarly, frequency
measures increased between 30.5 and 154.2% in the 20-40% MVC conditions. In
contrast, passive ankle stiffness, induced through the AFO, significantly
decreased sway amplitude (by 23-26%), decreased COG velocity by 13.8%, and
increased mean COP frequency by 24.9%. These results demonstrate that active
co-contraction of ankle musculature (common in Parkinson's Disease patients) may
have differential effects on quiet stance balance control compared to the use of
an ankle foot orthotic (common for those recovering from stroke).
CI - Crown Copyright (c) 2014. Published by Elsevier B.V. All rights reserved.

Langue : ANGLAIS