Effects of window size on ankle joint stiffness calculation during quiet standing: how the rule changes the result

Measuring ankle joint stiffness (AJS) during quiet standing QS using an inverted pendulum model typically involves a single calculation covering the entire period of QS. This study compared AJS using the same 20.0s set of QS postural sway data but employing seven different calculation windows (0.25s, 0.5s, 1.0s, 2.0s, 5.0s, 10.0s and 20.0s). AJS was calculated for both anterio-posterior AP and medio-lateral ML directions of sway. Postural sway data from 19 subjects were used to calculate mean±SD and time-normalized AJS over the same 20s period of QS. Statistical power of this study was 0.99. The AJS had ICCs ranging from 0.47 to 0.85 with coefficient of variations ranging from 11.1% to 31.8%. There were significant differences in AJS between window sizes (P<0.0001) for both directions of sway. Specifically, AJS calculated by 1.0s windows was significantly larger (P<0.01) than others, except 0.5s, while the AJS of the largest two windows 10.0s and 20.0s were significantly smaller (P<0.01) than all others in both directions of sway. In conclusion, it is recommended that 1.0s windows be used to calculate AJS and that stiffness analyzed as a continuous signal offers a more complete picture of how AJS behaves during QS.