Effect of ageing and vision on limb load asymmetry during quiet stance

Abstract

Although the identification and characterization of limb load asymmetries during quiet standing has not received much research attention, they may greatly extend our understanding of the upright stance stability control. It seems that the limb load asymmetry factor may serve as a veridical measure of postural stability and thus it can be used for early diagnostic of the age-related decline in balance control. The effects of ageing and of vision on limb load asymmetry (LLA) during quiet stance were studied in 43 healthy subjects (22 elderly, mean age 72.3±4.0 yr, and 21 young, mean age 23.9±4.8 yr). Postural sway and body weight distribution were recorded while the subject was standing on two adjacent force platforms during two 120 s trials: one trial was performed with the eyes open (EO), while the other trial was with the eyes closed (EC). The results indicate that LLA was greater in the old adults when compared with the young control subjects. The LLA values were correlated with the postural sway magnitudes especially in the anteroposterior direction. Eyes closure which destabilized posture resulted in a significant increase of body weight distribution asymmetry in the elderly but not in the young persons. The limb load difference between EO and EC conditions showed a significantly greater effect of vision on LLA in the elderly compared to the young subjects. The observed differences in the LLA may be attributed to the decline of postural stability control in the elderly. Ageing results in the progressive decline of postural control and usually the nervous system requires more time to complete a balance recovery action. To compensate for such a deficiency, different compensatory strategies are developed. One of them, as evidenced in our study, is preparatory limb unload strategy (a stance asymmetry strategy) which could significantly shorten reaction time in balance recovery.

Introduction

Approximately, 30% of the elderly population sustain a fall each year, constituting an important health problem (O'Loughlin et al., 1993). Falls have different major consequences, such as hip fracture, and longer-term consequences, ranging from restriction of activity to institutionalization (O'Loughlin et al., 1993; Tinetti and Williams, 1997). Postural instability is strongly associated with falls, and is the best single predictor of falls (Kellog International Working Group, 1987). Therefore, much of the research done so far has been focused on postural sway and its characteristics during quiet stance. Significant increase of sway was observed in old adults (for review see Blaszczyk et al., 1993c).

By definition, a person irrevocably loses balance during quiet stance when his/her center of mass falls outside limits of stability (Blaszczyk et al., 1994). In posturography these limits are defined by preferred or optimal center of pressure (COP) position within base of support and by the size of the safety border (Blaszczyk et al., 1994). The ageing is associated with a well-documented decline in the integrity of many physiological systems that participate in control of postural stability (Overstall, 1980; Woollacott et al., 1986, Woollacott et al., 1988; Horak et al., 1989; Manchester et al., 1989). Age-related neurodegenerative changes in the neuromuscular control and decreased resolution of the sensory inputs result in the sensory signals that are contaminated with a greater noise and physiological delays to compare with young healthy subjects (Welford, 1981; Blaszczyk et al., 1993c, Blaszczyk et al., 1994). As documented in our previous studies (Blaszczyk et al., 1994) from the biomechanical point of view, the postural stability in the aged persons is altered for two reasons. First of all, they cannot estimate the optimal COP position as precisely as young persons. Secondly, their postural stability drops sharply due to evidently larger COP oscillations at the borders of stability. The effect of decreased signal-to-noise ratio at the border of stability has been well documented during voluntary COP displacement while leaning (Blaszczyk et al., 1993a, Blaszczyk et al., 1994). To maintain equilibrium when reduced and impaired sensory information to be available an extended margin of safety is required. It implies that the additional time and attentional resources need to be allocated to the postural task (Inglis et al., 1994; Horak et al., 1989; Teasdale et al., 1993).

Due to the limited speed of neural and muscular processes, however, in the case of a balance perturbation, a recovery program must be initiated and completed in very limited time. To solve the timing problem, a set of fixed balance recovery strategies is used by the nervous system. At least three different types of balance recovery programs have been distinguished in the literature: ankle- hip- and step-strategy (Nashner, 1976). Selection of the strategy depends among others on the magnitude of disturbance. Against small unexpected perturbations most humans stabilize the posture using torque produced in the ankle joints (ankle strategy). With greater disturbances balance is recovered dynamically, e.g., taking a step (step strategy).

The age-related deficits result in compensatory modification of the control. For example the standard recovery strategies that are used in normal subjects are modified and adapted to a new physiological context. Such a functional adaptation to somatosensory and vestibular loss has been well documented (Horak et al., 1989, Horak et al., 1990; Timman and Horak, 1998; Allum and Honegger, 1998). These authors showed that sensory deficits altered the type of postural response selected under a given set of conditions e.g., vestibular loss resulted in a lack of hip strategy, whereas this strategy was increased in the case of somatosensory loss.

Generally, a certain margin of safety, which would provide sufficient time to complete a recovery program, must be maintained in postural control (Blaszczyk et al., 1994). From this perspective an optimally controlled postural system should provide an equal margin of stability in every direction. This implies that a fully symmetrical system would theoretically provide maximum stability (Maki et al., 1987; Blaszczyk et al., 1994). In postural control, however, at least two kinds of asymmetries are evidenced: anatomical and functional. Thus, the global symmetry mentioned above must also take into account these constraints. For these reasons normal position of the center of gravity in the elderly is shifted forward to protect the backward stability border (Blaszczyk et al., 1994). The forward stability border is well controlled both due to directional sensitivity of all sensory inputs participating in balance control and to directional asymmetry of the motor (in this case mainly locomotive) system.

Besides the perturbation magnitude, the selection of a balance recovery program depends on many factors such as physiological state of the organism, the availability of sensory information as well as environmental context. The relative strength of the perturbation and the selection of an appropriate corrective strategy depend on the quality of postural control, which declines dramatically with age (Horak et al., 1989). In the case of greater balance disturbances, the only potential strategy to recover equilibrium is to take a step or grab onto a stable object since the ankle or hip strategies might not be successful any more. In contrast to the ankle and hip strategies which depend on stance symmetry, the effectiveness of the step strategy may be augmented by limb load asymmetry that helps to make a step (Blaszczyk et al., 1994). Thus, one could expect a tendency for asymmetrical limb load distribution with age. These studies were also inspired by a consistent observation of limb load/unload phenomenon in human while loosing balance, which was noticed during our previous experiments (Blaszczyk et al., 1994). Namely, the onset of fall in anteroposterior direction was always preceded by an attempt to unload one limb. Example of such a postural correction is shown in Fig. 1. In this case, a young subject suffering from vestibular areflexia, while standing with eyes closed lost equilibrium in the backward direction. To test the hypothesis on the existence and on the role of load/unload preparatory strategy in balance control in the old adult the following experiment was done.