Anticipation mechanism and influence of fatigue in body sway control

The aim of this work was to identify the anticipatory mechanism and effects of fatigue of gastrocnemius muscle activity on the body sway control during quiet stance in upright position. Stabilometric and electromyographic (EMG) signals were synchronously collected from 23 subjects. The gastrocnemius fatigue was induced by a sustained plantar flexed posture until muscle failure. The acquisition lasted for 120 s before and after the induced fatigue. The stabilograms were subsampled to 5 Hz and the EMG was preprocessed to remove the 60 Hz mains noise and as its harmonics, by a tailor made digital Butterworth filter, order 20, band-pass with 10 and 300 Hz cut-of frequencies. After mean removal, the root mean square values of the EMG were calculated for each 200 ms window. The normalized cross correlation function was estimated between signals. Anticipation was verified before and after fatigue (p < 0.0001), and increased with fatigue (p < 0.02). Anticipation was identified as playing an important role in body sway regulation. The fatigue influence on the sway latency was not well established yet. A decrease on the rate of force production and the inertial moment of the body might contribute more to the increased latency than the posture control alterations itself.