Elsevier

Clinical Biomechanics

Volume 15, Issue 5, June 2000, Pages 365-369
Clinical Biomechanics

Correlations between force plate measures for assessment of balance

https://doi.org/10.1016/S0268-0033(99)00096-0Get rights and content

Abstract

Objective. To compare different force plate measures for assessment of postural stability during normal standing. Five types of measures were analysed: (1) The standard deviation of the horizontal ground reaction force, (2) the standard deviation of the centre of pressure, (3) the mean velocity of the centre of pressure, (4) movement strategy measures, and (5) the standard deviation of the vertical ground reaction force.

Background. Force plate measures such as types 1–3 are frequently used, but they seem to have limited usefulness for assessment of balance on an individual level. Hence, there is a need to investigate alternative force plate measures (such as types 4 and 5).

Methods. Force plate measures for 20 stroke patients were (a) compared to each other, and (b) compared to a clinical balance test, called Berg’s balance test.

Results. There was generally a significant correlation (P < 0.01) between measures of types 1–3. The movement strategy measures were not significantly correlated to any of the other measures. The vertical force measure was only significantly correlated to the antero-posterior mean velocity of the centre of pressure. There was a significant correlation between (a) Berg’s balance test and the vertical force measure (rs=−0.76), and (b) the static part of Berg’s balance test and the antero-posterior mean velocity of the centre of pressure (rs=−0.6).

Conclusions. Measures of type 1, 2, 4 and 5 seem to quantify different aspects of human standing. The results also indicate that Berg’s balance test and the vertical force measure quantify a similar aspect of the postural stability.
Relevance

The results from this study may facilitate the understanding and interpretation of force plate measures.

Introduction

Postural stability and balance are defined as the ability to return the body close to the equilibrium point when exposed to a perturbation. Since the human body is not statically stable a control system is required to stabilise the body. This control system is called the balance system or the postural control system. It is desirable to assess the performance of the postural control system. This can for example be done by analysing how stable the total system (the body and the control system) is. Ageing and many pathologies, such as stroke, head injury, and cerebral palsy, often reduce the postural stability. When the postural control system is impaired, its improvement is an important goal of rehabilitation. The lack of objective and reliable methods for basic research and follow-up of treatments regarding postural stability is an important issue.

Many researchers have evaluated the postural stability by analysing the time-varying coordinates of the centre of pressure (CoP), which is measured with a force plate. The analyses are usually limited to statistical measures of the CoP, like for example the standard deviation, path length or mean velocity of the CoP [1], [2]. CoP-based amplitude measures have been shown to be highly correlated with centre of mass (CoM)-based amplitude measures [3]. The horizontal ground reaction force (F) is proportional to the horizontal CoM acceleration. Summary statistics of the horizontal force have therefore also been used to quantify the postural stability [1], [2]. However, these kind of measures seem to have limited usefulness since they cannot be used to reasonably determine whether a value for an individual falls within the range for a particular diagnostic group or not. It is therefore important to investigate alternative force plate measures for evaluation of postural stability.

A dominating opinion has been that the body sway, that takes place mainly in the horizontal plane, contains valid information about the postural stability. Measures of the CoP and the horizontal force components have been used to quantify the body sway, but the vertical force component has usually not been taken into consideration in this context. By studying the vertical force component, the influence of the body sway is drastically reduced. Instead the signal is affected by cardiac activity, however, it also contains information about the neuromuscular activity, which is necessary in order to maintain stability [4], [5].

An impairment of the balance system may reduce the stability, resulting in an increased body sway and/or an altered movement strategy [6]. During normal standing the medio-lateral sway is mainly controlled by using a load–unload strategy [2]. The corresponding movement strategy for controlling the antero-posterior sway is called the ankle strategy. In this case the body behaves like an inverted pendulum pivoting about the ankle joints [2], [6]. If the body is exposed to a perturbation, a hip strategy or a step strategy may be required in order not to fall [6]. A method where the antero-posterior movement strategy is quantified by estimating to what extent the subject behaves like an inverted pendulum was recently introduced and evaluated [7], [8]. The basic idea of the method is to compare the measured antero-posterior acceleration of the subject’s CoM with the acceleration of a single inverted pendulum’s CoM. If the two accelerations are identical the subject must have behaved like an inverted pendulum (used the ankle strategy).

If a measure is to be considered valid it must be correlated with the patients’ clinical status. In absence of a standard method to quantify postural stability the force plate measures can be compared with functional balance tests. In the present study, we used Berg’s balance test – a clinical test based on visual evaluation of different balance tasks [9]. The correlation coefficient between Berg’s balance test and statistical measures of the CoP has been reported to be moderate (−0.67 < rs < −0.46) [10].

The main objective of this study was to compare five different types of force plate measures: (1) The standard deviation of the horizontal ground reaction force components, (2) the standard deviation of the CoP, (3) the mean velocity of the CoP, (4) movement strategy measures, and (5) the standard deviation of the vertical ground reaction force. The force plate measures were (a) compared to each other, and (b) compared to Berg’s balance test. Our hypothesis was that all kind of measures should be reasonably correlated since they all are believed to reflect the performance of the postural control system.

Section snippets

Subjects

Force plate measurements were performed on post-cerebral vascular accident (CVA or stroke) patients. Twenty post-CVA patients admitted to the Uppsala University Hospital for rehabilitation were included in the study (12 males and 8 females, mean age: 50 years (range: 21–65 years), paretic side: 12 right and 8 left, mean time from stroke: 2 years and 4 months (range: 6 months–13 years). The inclusion criteria were the following: Hemiparetic patient between 18 and 65 years old, able to stand

Results

The raw data from the measurement with a static weight revealed that the noise was white. The standard deviation of the CoP, Fx and Fz was about 0.8 mm, 0.08 and 0.60 N, respectively. The resolution in the CoP, Fx and Fz was about 0.1 mm, 0.06 and 0.66 N, respectively. Unfortunately, the amplitude fluctuations in Fz corresponding to cardiac activity were only somewhat larger than the resolution of the signal, but for most patients the vertical force amplitudes were considerably larger.

One

Discussion

The horizontal force measures and the CoP measures were generally significantly correlated. This result is reasonable since both the CoP and the horizontal forces are related to the movements of the CoM [2]. The correlation between force based and CoP-based measures has been reported to be rather weak in a previous study [1]. The explanation to the disagreement between the studies might be that Goldie et al. [1] only used a test duration of 15 s, while a test duration of 30 s was used in the

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