Elsevier

Gait & Posture

Volume 33, Issue 3, March 2011, Pages 412-417
Gait & Posture

Phase synchronisation of the three leg joints in quiet human stance

https://doi.org/10.1016/j.gaitpost.2010.12.014Get rights and content

Abstract

Quiet human stance is a dynamic multi-segment phenomenon. In literature, coupled ankle and hip actions are in the focus and examinations are usually restricted to frequency contributions below 4 Hz. Very few studies point to the knee playing an active role, and just one study gives evidence of higher frequency contributions. In order to investigate the dynamic coupling of all three leg joints in more depth, we revisited an experimental data set on quiet human stance. Since phase synchronisation is a strong indicator of non-linear coupling behind, we used the phase synchronisation index (PSI) to quantify the degree of leg joint coupling as a function of frequency. One main result is that we did not find any synchronisation between ankle and hip across the whole frequency range examined up to 8 Hz. In contrast, there is significant synchronisation between ankle and knee at a couple of frequencies between 1.25 Hz and 8 Hz when looking at the kinematics. Their joint torques rather synchronise below 2 Hz. There is also synchronisation between knee and hip kinematics above 6 Hz, however, only significant at one frequency bin in our data set. From this, we would infer that the multiple mechanical degrees of freedom contributing to quiet human stance should be chosen according to, thus map, physiology. Thereby, the knee is indispensable and bi-articular muscles play a central role in organising quiet human stance. Examining the non-stationarity of phase synchronisations will probably advance the understanding of self-organisation of quiet human stance.

Introduction

A change of paradigm has occurred in the postural research community. The simplification of a single inverted pendulum (SIP) model is not appropriate for quiet human stance. A couple of recent publications [1], [2], [3], [4], [5], [6], [7], [8], [9], [10] agreed in that quiet stance is a dynamic multi-segment phenomenon. Two of these studies [1], [4] concentrated on a quantitative description of ankle and hip joint coupling and distinguished kinematic in-phase from anti-phase relations of both joints. Few studies [2], [3], [4], [7], [8], [11], [12] ascertained contributions of the knee joint to quiet human stance. For example, high correlations of all three leg joints [2] have been found recently. In particular, these authors gave evidence that both kinematic and torque correlations between ankle and knee are prominent. On the one hand, all those studies analysing ankle and hip movement focused on fluctuations at frequencies below 4 Hz. On the other hand, there is evidence that the kinematics of ankle and knee are correlated in the frequency range 4–8 Hz [2]. Even though quiet human stance is a multi-segment phenomenon the state of all degrees of freedom involved do not necessarily evolve independently. Observing any phase synchronisation would be a strong pointer to non-linear coupling behind [13], [14], [15]. Concurrently, the dimensionality of a multi-degree-of-freedom system would be reduced for the lifetime of synchronisation. Within this study we aim at investigating whether phase synchronisations can be observed between leg joints during quiet human stance. In that, we use the “phase synchronisation index” (PSI) [16], [17], [18], [19] which enables to analyse phase synchronisation across a range of frequencies. Especially, going beyond the focus of previous studies on ankle and hip strategies, the degree of an involvement of the knee should be quantified.

Section snippets

Experimental data acquisition and digital filtering

Here, we revisited a data set of 60 trials from 10 subjects who had stood quietly on firm ground. Details about experiments and methods (e.g., inverse dynamics) can be found elsewhere [2], [3].

Digital filtering of measured input data is a crucial processing step in inverse dynamics. Inevitably, this comes along with the manipulation of the frequency content. Our data were sampled at fs = 115.5 Hz during a trial length of Ttrial = 4.43 s. In order to calculate a filtered sample value we used a

Results

Regarding phase synchronisation both of joint angular velocities (Fig. 3) and of joint torques (Fig. 4), we only find high numbers of significant PSIs when the knee is involved, i.e., between immediately adjacent joints (ankle–knee, knee–hip). In contrast, the hip does not synchronise to the ankle at any frequency, neither kinematically nor on muscular level. The joint angular velocities of knee and ankle synchronise in a broad frequency range 1.25–8 Hz. There are significant PSIs at 2–3 Hz and

Quiet human stance relies on knee function

The knee joint is synchronised to both the ankle and the hip joint, whereas there is no systematic long range synchronisation between ankle and hip joints. This applies to joint kinematics as well as joint torques. Very general, our result is in striking contradiction to literature in which coupling between ankle and hip joints was asserted (e.g., in [1], [9]). As in this research field the predominantly assumed model for human stance is currently either a single (SIP) or a double inverted

Acknowledgements

MG and SG were supported by “Deutsche Forschungsgemeinschaft” (DFG) grants during data acquisition (MG: MU1766/1-1) and manuscript preparation (MG: MU1766/1-3; SG: SE1042/2). Furthermore, the work was supported by the DFG grant WI1166/9-1. MG sends many, many thanks to Sascha at Café Lieders in Jena: a marvellous combination of coffee and music helped to pull this through at the end.
Conflict of interest statement

None of the authors is or was exposed to any conflicts of interest related to the

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