Influence of knee position on the postural stability index registered by the Biodex Stability System

The importance of knee position for bodily stability is described by some authors, however Biodex Stability System (BSS) trials have not been used to assess the reliability and effects of different knee positions. The purposes of this study were to test the reliability of BSS indices using two knee positions in the measurement protocol (either permitting slight knee flexion or maintaining them locked in total extension) and to compare the BSS indices between these two knee positions. The measurements were taken of the Overall Stability Index (OSI), Anterior–Posterior Stability Index (APSI) and Medial–Lateral Stability Index (MLSI) during a 30 s protocol which gradually and automatically increased in difficulty among 21 healthy female subjects (22.8 ± 1.0 years old). The subjects performed four trials which, without visual feedback, permitted knee flexion as well as four trials which did not, in a randomized order. The first two trials in each set were used for familiarization only. Permitting slight flexion yielded better reliability results (OSI Intra-class Correlation Coefficient [ICC] = 0.93, APSI ICC = 0.90, MLSI ICC = 0.89) than maintaining the knee in total extension (OSI ICC = 0.88, APSI ICC = 0.87, MLSI ICC = 0.79), with good agreement in the Bland and Altman test. Moreover, permitted knee flexion in BSS presented better balance stability values for OSI (P = 0.001) and APSI (P = 0.024), however the MLSI did not present significant difference between positions (P = 0.345).