Association of age with the threshold for detecting ankle inversion and eversion in upright stance

Age and Ageing,  Jan, 1995  by M.G. Gilsing,  C.G. Van den Bosch,  S-G. Lee,  J.A. Ashton-Miller,  N.B. Alexander,  A.B. Schultz,  W.A. Ericson

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Introduction

Considerable effort over the years has been directed at understanding how the central nervous system utilizes visual, vestibular and proprioceptive sensory information in human posture and movement. With age the weighting of these sensory inputs changes: for example, it is widely accepted that humans place increasing reliance on visual cues in order to maintain upright balance as they age. This age dependency has been demonstrated in bipedal stance [1, 2] and when resisting postural perturbations [3], but it is especially marked in unipedal stance [4, 5]. The fact that when vision is removed the ability of healthy elderly subjects to maintain their balance is significantly impaired suggests a deficit in one or more of the other sensory systems commonly thought to play a major role in maintaining balance: these systems include the vestibular system, lower extremity proprioception and the systems responsible for sensorimotor co-ordination [1]. Although afferent information on ankle angle is a source of feedback to the postural control system responsible for the transition from bipedal to unipedal stance and the regulation of unipedal stance [6], we could find only a brief abstract describing ankle proprioception in the frontal plane, no studies of the threshold for detecting inversion or eversion rotations, and no studies of the effect of ageing on these thresholds. The purpose of this paper is to provide that information.

The importance of proprioceptive input for the control of voluntary movement and for sensing and resisting postural perturbations has long been recognized [7, 8]. More recently, the role of proprioception in the co-ordination of multi-joint movements has become increasingly apparent [9]. One measure of proprioception is the threshold for detecting a passive directional change in the angle of a joint [10]. Another test consists of measuring the fidelity with which a given joint angle can be reproduced in the ipsilateral [11, 12] or contralateral [13] limb. Passive movements of joints have been used in most tests to try to characterize afferent information without involving the centrally-mediated 'sense of effort' [12]. Whether or not proprioception is enhanced in the presence of active muscle contraction(s) is controversial (see Discussion).

Proprioception has been tested in the neck and trunk [14-16], the shoulder [17], the elbow [18], the finger joints [19], the hip [20], the knee [10-12, 21], the ankle [22, 23] and the toe joints [19]. Few of these studies have included elderly groups and there is little agreement on whether there are systematic changes in proprioceptive threshold with age.

We focused on ankle proprioception in the frontal plane because of its importance for helping to prevent lateral falls in the single-limb-support phase of gait. We tested the null hypothesis ([H.sub.I]) that the threshold for sensing ankle inversion and eversion does not change with age. To accomplish this we measured thresholds in healthy young and elderly volunteers during upright stance using isovelocity step changes in ankle angle ranging from 0.1 [degrees] to 1.0 [degrees] using a servo-driven cradle. Secondary hypotheses tested were that ankle proprioception is unaffected by whether stance is bipedal or unipedal ([H.sub.II]), by sex ([H.sub.III]), whether the ankle is moved in inversion or eversion ([H.sub.IV]), or by different angular velocities of movement ([H.sub.V]).

Table I. Subject information (means and SD)

Table II. Sample bipedal standing results for one subject after
four blocks of 80 trials(*) illustrating how most trials were
concentrated near the detection threshold

Age differences in sensing the horizontal: The mean (SD) absolute angular error in rotating the cradle to the horizontal was 1.2 (0.9) [degrees] for the young and 1.6 (1.3) [degrees] for the old with the difference being found significant at the p [less than] 0.001 level.

Table IV. Lack of significant effect of the movement angular
velocity on mean (SD) bipedal proprioceptive acuity (degrees) in
five young male subjects

The differences in practice effect between the young and old would tend to have lessened any age differences found, not exaggerated them. The improvement of the older subjects in performance from the first bipedal trial block to the last (fourth) bipedal trial block was not unexpected, since elderly people often show a larger practice effect than young in involuntary tasks. This also suggests that the strength tests midway through the experiment did not adversely affect threshold measurements. The decrease in performance by the young group over the course of the test is best explained by a reduction in their concentration in the last bipedal trial block. In retrospect, we probably should have given feedback to subjects on their performance after each trial block to help sustain their interest and attention. It is widely recognized that age causes a deterioration in the ability to perform divided attention tasks [38]. Because of this the old may have had to concentrate more in these tests than the young. Might the age difference in [TH.sub.75] or [TH.sub.min] be explicable by earlier mental fatigue in elderly than young subjects? We rule this possibility out because such an effect would have shown up as a practice effect in the opposite direction to that found in the old group. Finally, we find it unlikely that the age differences could have been caused by the old taking greater advantage than the young of tactile pressure from the joystick or, in unipedal stance, the knee pad. Because ankle rotations were mostly less than a degree and centred about an axis collinear with the cradle they induced negligible tactile cues at the knee or finger. In fact the similarity of results obtained with (unipedal) and without (bipedal) the knee pad confirm it did not enhance proprioception in these tests.