Frequencies in Center of Pressure Time Series above 1Hz during Quiet Upright Stance Reflect the Use of a Hip Strategy

Abstract

Upright stance is primarily maintained through the use of ankle and hip strategies, involving respective rotations about the two joints. Choice of and coordination between these two strategies is regulated by the central nervous systems (CNS). Postural control mechanisms used to maintain upright stance are often assessed (or inferred) using a spectral decomposition of center of pressure (COP) time series. Existing evidence, however, is contradictory in some respects. Specifically, there is conflicting evidence whether COP frequencies above 1.0 Hz reflect CNS function or use of a hip strategy. In this study, fatigue was induced to the lumbar extensors and shoulder flexors through intermittent sub-maximal (60% MVC) isotonic exercises, and used to ‘perturb’ postural control. Use of hip strategies was assessed by entropy methods, which indicates the level of ‘complexity’. Coordination between ankle and hip strategies was analyzed using a coherence method. Both power spectrum and Tsallis and entropies showed that the medio-lateral (ML) hip strategy was impaired by lumbar extensor fatigue. Coherence (i.e. coordination) between ankle and hip motions was reduced following fatigue in both muscles groups. Integrated with earlier evidence, these results indicate that, during quiet upright stance, COP frequencies above 1.0 Hz correspond to the hip strategy rather than reflecting CNS function in postural control.