Comparison of methods for determining parameters related to ankle joint quasi-stiffness during quiet standing

Ankle joint quasi-stiffness is a measure representing the control of quiet standing. Here we investigated three methods for determining parameters associated with ankle joint quasi-stiffness. Eleven healthy, young males were asked to stand quietly with eyes open (EO) and eyes closed (EC). Ankle joint quasi-stiffness was computed using two methods, one proposed by Winter et al. (Winter DA, et al. J Neurophysiol 80: 1211-1221, 1998) ([Formula: see text]) and another by Loram and Lakie (Loram ID, et al. J Physiol 540: 1111-1124, 2002) ([Formula: see text]). Both values were normalized using participants' body sizes. The short-term scaling exponent in stabilogram diffusion analysis ([Formula: see text]), which has been linked to quasi-stiffness, was applied. A two-way repeated measures ANOVA revealed a significant main effect of methodology (p < 0.0001, partial η² = 0.42), but no significant effect of eye condition (p = 0.1947, partial η² = 0.06), nor interaction effect (p = 0.4742, partial η² = 0.02). Intraclass Correlation Coefficient (ICC(3,1)) analysis indicates good reliability between [Formula: see text] and [Formula: see text] with an ICC value of 0.858 (p < 0.001) in the EO condition, and moderate reliability with an ICC value of 0.718 (p = 0.004) in the EC condition. [Formula: see text] and [Formula: see text] exhibited a very strong negative correlation in the EO ([Formula: see text] = -0.909) condition and a moderately strong negative correlation in the EC ([Formula: see text] = -0.782) condition. [Formula: see text] and [Formula: see text] exhibited moderately strong negative correlation in both the EO ([Formula: see text] = -0.664) and EC ([Formula: see text] = -0.555) conditions. The good and moderate reliability between [Formula: see text] and [Formula: see text] suggests that both measures capture similar stiffness attributes relating to their neuro-mechanical components, despite a significant methodological difference. Additionally, relatively high correlations of [Formula: see text] with [Formula: see text] and with [Formula: see text] suggest that the stochastic characteristics of centre of pressure in the short period indeed reflect the overall quasi-stiffness at the ankle joint.