Effect of age on anticipatory postural adjustments in unilateral arm movement

Abstract

Aging is frequently accompanied by a deterioration in postural control. Accordingly, the elderly adopt postural strategies in order to maintain balance. The purpose of this study was to compare anticipatory postural adjustments in (healthy) 10 young and 10 elderly subjects using electromyography (EMG) and biomechanical parameters. While standing on a force platform, subjects performed voluntary, arm-raising movements under five conditions: self-paced at three different velocities, self-paced with load and an externally triggered, both at maximal velocity. The force platform provided information on vertical torque (T_z) and center of pressure anteroposterior displacements (COP). EMG activity was recorded from the biceps femoris, quadriceps, tibialis anterior and soleus muscles. Voluntary movements were associated with an early COP backward shift and an anticipatory T_z. At low velocity, elderly subjects did not show any impairment in stability. At maximal velocity, T_z was delayed in all conditions in the elderly group, whereas COP latency was reduced only in the self-paced condition without load. Despite this decrease in anticipation, the movement was performed at the same velocity as in younger subjects. The elderly adopted various muscle strategies in order to perform the same movement with less stability. In the self-paced condition, elderly subjects used a hip strategy, whereas young subjects used an ankle strategy. In the triggered condition, the strategy corresponded to increased activation of certain thigh muscles, rather than a sequence modification. Hence, local muscle strategies were used to counteract the overall delay in postural preparation revealed by biomechanical parameters.

Introduction

Execution of a voluntary movement (such as arm movement performed when standing) is preceded by a sequence of postural modifications which are reflected by recruitment of the postural (leg and trunk) muscles associated with accelerations of the various body segments [1], [2], [3]. These anticipatory postural adjustments (APAs) are movement-specific, and thus, appear to be centrally organized prior to movement execution [4]: they tend to minimize the potential disturbance of the forthcoming movement and allow equilibrium to be maintained during execution.

The postural control system has been shown to decline with age. Elderly subjects often experience a deterioration in the integrity of many of the physiological systems which participate in the control of postural stability [5], [6], [7], [8], [9]. As the quality of sensory information decreases, postural stability control becomes increasingly difficult and requires a wider safety margin [10]. Many authors have used reaction time conditions to study the postural strategies adopted by the elderly when they perform upper limb movements [11], [12]. In these studies, postural adjustments were analyzed through various parameters, such as center of pressure (COP) displacement or postural and voluntary muscle activities. During pull-and-push arm movements, the elderly show deficiencies or slowness in achieving a safe degree of postural stability prior to initiation of voluntary movement [10]. By studying the same movement paradigm in response to a signal during sitting or in the standing position, Stelmach et al. [13] showed that elderly subjects reacted more slowly in the standing condition and concluded that the slowness was associated with a stabilization deficiency. Inglin and Woollacott [11] observed a delayed latency in elderly subjects for both postural and voluntary muscles, with a greater time interval between activation of these two muscle groups. In contrast, other authors studying postural adjustments in elderly subjects (unilateral shoulder flexion at different velocities) have noted that delays in postural muscle activation resulted in decreased latency between postural and voluntary muscle activation [12], [14]. The authors observed age-related differences (notably delayed activation) in the timing of two postural muscles and concluded that the elderly may not be as proficient in controlling movement under very fast or very slow conditions [12], [14]. For an arm-raising movement, a wider range of values for COP excursion speed was observed for elderly subjects, compared with young subjects [15].

In view of the discordance between these various results, we thus decided to study the control of posture associated with a voluntary, unilateral arm flexion performed by young and elderly subjects in the standing position. We investigated APAs evidenced by biomechanical parameters (such as COP movements and vertical torque (T_z)) and by EMG data during different movement paradigms, in order to compare age-related differences in postural adjustment. In a previous study, we had demonstrated that vertical torque (T_z) is a useful parameter for quantifying APA associated with unilateral movement [16], [17].

The goal of this research is to provide additional information on the organization of postural control in elderly subjects performing arm raising, a common everyday movement. A number of questions were addressed: are APAs altered under all types of movement conditions? And if so, which strategies are used to maintain posture and equilibrium?