How postural behaviour in undisturbed upright stance can be used to assess the physical characteristics of various models of ankle orthoses

Objectives. To assess various ankle orthoses presenting different physical characteristics. Design. The postural performance in bi-pedal stance of 14 healthy adults was assessed during four different ankle orthoses conditions. Background. Resultant ankle stiffness is an important component in undisturbed upright postural regulation. Methods. The centre of pressure displacements issued from the force platform were used to compute the horizontal motions of the centre of gravity, the controlled variable, and those of the difference between the centre of pressure and the vertical projection of the centre of gravity from which ankle stiffness can be estimated. Their respective contribution was assessed through a frequency analysis and a fractional Brownian motion modelling to determine their spatio-temporal linkage and their degree of control. Results. Ligacast, the more rigid model, induces the larger decrease, principally in the medio-lateral direction, in both centre of pressure minus centre of gravity and centre of gravity amplitudes. Dynastab reduces only the centre of pressure minus centre of gravity motions in both medio-lateral and antero-posterior directions. Lastly, no specific effect was observed for the less rigid Ligaflex for each motion and each direction. These effects are mainly explained by a reduction in the distance covered until the corrective process is initiated, the corresponding time interval being unchanged. In addition, depending on the model, the corrective processes appear more or less controlled. Conclusions. Modulating the ankle joint stiffness by using various models of orthoses induces some contrasted behaviours, hence suggesting that postural control does not result solely from resultant ankle stiffness.