Clinical validity of novel postural stabilization experimental indices based on hyperbolic transformation

Background: Postural stabilization is the function which allows an individual, after a transitional movement, to recover balance in a quiet erect posture. An experimental method has been proposed (Rabuffetti, 2011) and proved valid for the assessment of balance disorders in individuals with neurological diseases. It would seem that the two original indices were not fully independent since their concurrent distribution was confined by a hyperbolic boundary. Research question: A methodological advancement involving non-linear transformation techniques is required to overcome the limitations of the original approach. Methods: A hyperbolic transformation is applied to the original indices related to the mechanics of the stabilization (instability at beginning of stabilization and time rate of stabilization), thus defining two novel indices (Instability and Promptness). These novel indices may be related to different functional domains concerning, respectively, peripheral force capacity and central nervous motor control. The validity of these novel indices is quantified by their correlation with clinical scales in an already validated group of patients with Charcot-Marie-Tooth disease (N = 47) or Multiple Sclerosis (N = 20). Results: The novel indices generally improved validity compared to the original indices (+ 66% of indices show a statistically significant concurrent validity on a clinical scale). Moreover, Instability was more related to the Charcot-Marie-Tooth group (9 out of 12 valid correlations), and Promptness to the Multiple Sclerosis group (4 out of 5, when also considering statistical trends), in accordance to the, respectively, more peripheral and more central nature of the two neurological diseases. Significance: The novel postural stabilization indices support a clinical application for two reasons: 1) they have shown improved validity, compared to the original indices, in two groups of patients affected by neurological pathologies of different nature, 2) they are more closely related, compared to the original indices, to different functional domains.