Effect of increasing difficulty in standing balance tasks with visual feedback on postural sway and EMG: Complexity and performance David Barbado Murillo a , Rafael Sabido Solana a,⇑ , Francisco J. Vera-Garcia a , Narcis Gusi Fuertes b , Francisco J. Moreno a a Sport Research Centre, Miguel Hernández University of Elche, Elche, Spain b Sport Science Faculty, University of Extremadura, Cáceres, Spain article info Article history: Available online 2 June 2012 PsycINFO classification: 2330 Keywords: Postural sway EMG Fuzzy Entropy Task difficulty Adaptability Visual feedback abstract Studies about the relationship between complexity and perfor- mance in upright standing balance have yielded mixed results and interpretations. The aim of the present study was to assess how the increasing difficulty in standing balance task affects per- formance and the complexity of postural sway and neuromuscular activation. Thirty-two young healthy participants were asked to stand still on a stability platform with visual feedback in three lev- els of difficulty. EMG signals from gastrocnemius medialis, tibialis anterior, rectus femoris and biceps femoris were measured with surface electromyography. As task difficulty increased, the ampli- tude of postural sway also increased. In the antero-posterior axis, Fuzzy Entropy (complexity) of postural sway decreased from the stable condition to the medium instability condition, and increased again at the highest instability condition. Fuzzy Entropy in the medio-lateral axis was higher in the stable condition; however, no differences were observed between the two instability condi- tions. Lower values of Fuzzy Entropy in postural sway during stable condition correlated with greater percent increases in postural sway in medio-lateral and antero-posterior axis from the standing still condition to the highest instability condition. In addition, mean and coefficient of variation of EMG increased and Fuzzy Entropy of EMG decreased when the difficulty in standing balance tasks increased. These results suggest that the higher postural sway complexity in stable condition, the greater capacity of the 0167-9457/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.humov.2012.01.002 ⇑ Corresponding author. Address: Sport Research Centre of Elche, Miguel Hernández University, Avda. de la Universidad s/n, 03202 Elche, Spain. Tel.: +34 965 22 24 37; fax: +34 965 22 24 56. E-mail address: rsabido@umh.es (R. Sabido Solana). Human Movement Science 31 (2012) 1224–1237 Contents lists available at SciVerse ScienceDirect Human Movement Science journal homepage: www.elsevier.com/locate/humov