Analysis of free moment and center of pressure frequency components during quiet standing using magnitude squared coherence

To date, no postural studies have investigated the specific relationship between linear (anteroposterior (AP) and mediolateral (ML)) postural sway and the free moment (FM) over the range of biomechanically important frequencies. The goal of the current paper is to study the relationship between FM and the AP/ML movements during quiet standing with respect to individual frequencies. Mean squared coherence, which measures the degree of the relationship between two signals as a function of frequency, is employed to address this question. The results showed that, in two conditions (eyes opened and eyes closed), at very low frequencies (<0.5 Hz), AP and FM were strongly correlated (>0.8) while there was a weak correlation between ML and FM (∼0.2). The situation reversed from (0.5 to 1.5 Hz), with AP/FM correlation decreasing, and ML/FM correlation peaking slightly below 1.0 Hz. Both conditions were only weakly correlated beyond 1.5 Hz. It is suggested that these observations arise from differences in ankle activation between the left and right sides, whereas at higher frequencies, high coherence between ML and FM is a hip control strategy.