Abstract
When standing, intrinsic ankle stiffness is smaller when measured using large perturbations, when sway size is large, and when background torque is low. However, there is a large variation in individual intrinsic ankle stiffness. Here we determine if individual variation has consequences for postural control. We examined the relationship between ankle stiffness, ankle torque and body sway across different individuals. Ankle stiffness was estimated in 19 standing participants by measuring torque responses to small, brief perturbations. Perturbation sizes of 0.2 & 0.9 degrees (both lasting 140 ms) measured short- and long-range stiffness respectively, while participants either stood quietly on a fixed platform or were imperceptibly tilted to reduce stability (0.1 Hz sinusoid; 0.2 & 0.4 deg). The spontaneous body sway component (natural random relatively rapid postural adjustments) and background ankle torque were averaged from sections immediately before perturbations. The results show that, first, intrinsic ankle stiffness is positively associated with ankle torque, and that this relationship is stronger for long-range stiffness. Second, intrinsic ankle stiffness is negatively associated with body sway, but, in contrast to the relationship with torque, this relationship is stronger for short-range stiffness. We conclude that high short-range intrinsic ankle stiffness is associated with reduced spontaneous sway, although the causal relationship between these two parameters is unknown. These results suggest that, in normal quiet standing where sway is very small, the most important determinant of intrinsic ankle stiffness may be stillness. In less stable conditions, intrinsic ankle stiffness may be more dependent on ankle torque.
Highlights
Posture is flexible and dependent on the coordination of all joints and muscles of the body, it can appear that the single point of the estimated whole body centre-of-mass (COM) is controlled as a means to achieve standing balance [1,2,3,4,5]
We investigated if the relationship between short- and long-range stiffness versus body sway was influenced by baseline ankle torque
Previous research has shown that people can have very different intrinsic ankle stiffness [12, 17,18,19,20, 22]
Summary
Posture is flexible and dependent on the coordination of all joints and muscles of the body, it can appear that the single point of the estimated whole body centre-of-mass (COM) is controlled as a means to achieve standing balance [1,2,3,4,5]. Previous studies of unperturbed upright standing have shown that during quiet stance the COM is constantly moving in a pseudo-random manner, producing an unpredictable and irregular sway of approximately 0.5 Hz [6], mostly in the antero-posterior direction [1, 4, 7,8,9,10]. Individual differences in intrinsic ankle stiffness versus body sway and ankle torque. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript
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