One-leg stance in healthy young and elderly adults: a measure of postural steadiness?

Abstract

Objective. To investigate postural steadiness during 30 s of one-leg stance in healthy young and elderly adults, by analysing the pattern of the ground reaction force variability.

Design. A laboratory set-up was used to analyse the variability of the ground reaction forces in relation to time as a measure of postural steadiness.

Background. The one-leg stance test is a measure considered to assess postural steadiness in a static position by a temporal measurement. The common notion is that a better postural steadiness, i.e. less force variability, allows for longer time standing on one leg. However, there is lack of evidence how postural steadiness during one-leg stance changes over time.

Methods. Twenty-eight healthy elderly and 28 healthy young adults were tested by means of force plates assessing ground reaction forces while performing one-leg stance.

Results. During one-leg stance, two phases could be identified in both groups: First a dynamic phase, a rapid decrease of force variability, and thereafter a static phase, maintaining a certain level of force variability. During the first 5 s of one-leg stance the force variability decreased significantly more in the young group resulting in a lower force variability level during the static phase than in the elderly.

Conclusions. The difficulties in maintaining the static position in elderly seems dependent on the reduced initial decrease in force variability and/or musculoskeletal components. We suggest that the first 5 s are crucial when assessing balance during one-leg stance.
Relevance

Our findings contribute to the knowledge base of one-leg stance performance in young and elderly adults.

Introduction

One-leg stance (OLS) is a frequently used clinical tool for assessment of balance in persons with various balance disorders (Berg et al., 1989; Bohannon and Leary, 1995; Frzovic et al., 2000; Tinetti, 1986). Control of body posture and balance is a complex process in which multiple subsystems and environmental factors interact to maintain balance (Woollacott and Shumwaycook, 1990). The ability to stand on one leg is used alone or as an item in clinical balance tests assessing postural steadiness in elderly (Berg et al., 1989; Bohannon and Leary, 1995; Tinetti, 1986). The difficulty of keeping the centre of mass above the centre of pressure is reflected in the variability of the ground reaction forces (GRF) (Horak and Macpherson, 1995; Patla et al., 1990). In this study, postural unsteadiness is expressed as both the level and change of GRF variability. The task of standing on one leg requires an initial voluntary action of moving the centre of mass over the forthcoming stance leg, followed by the task of maintaining postural orientation in space. This requires control of weight support, vertical alignment of the different body segments and equilibrium (Horak and Macpherson, 1995; Rogers and Pai, 1990).

The clinical test of OLS assesses postural steadiness in a static position by a quantitative measurement i.e. the number of seconds a person can maintain the OLS position, thus implying that better postural steadiness would allow for longer standing on one leg. However, established balance scales require different OLS times for maximal score (Berg et al., 1989; Bohannon and Leary, 1995; Tinetti, 1986). For the highest score on the Berg Balance Scale (Berg et al., 1989), a subject is supposed to stand unsupported for at least 10 s on one leg, while in Bohannon's ordinal balance scale (Bohannon and Leary, 1995) 30 s are required, and in Tinetti's Balance Subscale (Tinetti, 1986) a subject has an alleged normal balance if he/she is able to stand on one leg without support for 5 s.

The ability to switch from two-to one-leg standing is required for many everyday motor tasks such as turning, climbing stairs, walking and dressing. However, the clinical OLS test focuses on assessing a static OLS position. Although static standing is useful during everyday activities, 30 s of OLS poorly reflects everyday motor tasks, especially for the elderly, and its usefulness is therefore questionable. Also, the reason for measuring a certain time window of standing on one leg needs further investigation.

In a laboratory approach, force plates are frequently used as tools for quantifying OLS parameters (Frändin et al., 1995; Goldie et al., 1989; Hanke and Rogers, 1992; Tropp and Odenrick, 1988). Some researchers have investigated the steadiness during OLS by monitoring the centre of pressure, without yielding much insight in terms of predicting instability (Frändin et al., 1995; Horak and Macpherson, 1995). Measurement of GRF during OLS has been shown to be valid and reliable in this regard (Goldie et al., 1989; Hanke and Rogers, 1992). Goldie et al. (1989) have shown in healthy young individuals that the variability of the GRF signals was more sensitive in discriminating the changes in steadiness during OLS than the variability of the centre of pressure. However, there is lack of evidence regarding how postural steadiness during OLS changes over time.

The aim of this study was to investigate postural steadiness during 30 s of OLS in young and elderly adults by means of experimental force plate measures. Since the weight shift itself causes most of the postural adjustments (Rogers and Pai, 1990), we chose to explore the vertical and medial/lateral (M/L) force variability focusing on the first second after lift-off and the different time windows in the clinical balance tests (5, 10 and 30 s) during OLS. Because the magnitude of the force impulse (i.e. the integration of force over a time interval) is important for a correct weight shift to OLS (Pai et al., 1994; Rogers and Pai, 1990), we also investigated the vertical and lateral force impulses prior to the weight shift and their relationship to the initial force variability.