Elsevier

Gait & Posture

Volume 35, Issue 1, January 2012, Pages 11-17
Gait & Posture

Postural sway during quiet standing is related to physiological tremor and muscle volume in young and elderly adults

https://doi.org/10.1016/j.gaitpost.2011.03.028Get rights and content

Abstract

To examine the age-related deterioration in postural control, we investigated the association between postural sway during quiet standing and either amplitude of physiological tremor or muscle volume of the plantar flexors in 20 young and 20 elderly adults. They maintained a quiet standing position on a force platform for 60 s with their eyes open or closed. During quiet standing, physiological tremors detected using a piezoresistive accelerometer were recorded from the soleus muscle, and the center of pressure (COP) displacement and body acceleration in the antero-posterior direction were calculated using the ground reaction forces as an assessment of postural sway. Muscle volume was predicted from muscle thickness by an ultrasonographic image. The physiological tremor of the soleus muscle during quiet standing was significantly greater in elderly than in young adults, and a positive association between physiological tremor and the amplitude of postural sway was found for young and elderly adults combined. Furthermore, physiological tremor was positively correlated with the high-frequency component of COP sway during quiet standing. A significantly negative relation between the muscle volume of the plantar flexors and postural sway was found in both age groups. These results suggest that physiological tremor reflects high-frequency fluctuations in postural sway during quiet standing in young and elderly adults, and age-related increases in the postural sway amplitude in the antero-posterior direction may be related to a decrease in muscle volume of the plantar flexors for maintaining an upright posture.

Introduction

Age-related deterioration in postural control has been intensively investigated and well documented. Postural sway during quiet standing, frequently assessed using center of pressure (COP), is considered to be an effective measure to examine postural stability in the process of aging [1], [2]. Numerous papers have reported that postural sway, especially fast components of postural sway such as the velocity of COP (e.g., [1], [3]) and whole body acceleration [3] is larger in the elderly than in the young (e.g., [1], [3]). However, its mechanism has remained an open question.

During quiet standing, the plantar flexors are key agonist muscles, because the ankle torque controls the center of mass (COM) behavior [4]; plantar flexor activity plays a key role in exerting ankle joint torque as the COM is in front of the ankle joint center; hence, the activity of the plantar flexors are correlated with spontaneous body sway [5]. Therefore, the neural regulation of the plantar flexor muscles is essential in the control of postural sway.

In terms of the activity of plantar flexor muscles during quiet standing, it has been found that the motor unit synchronization recurring at around 10 Hz is remarkable in the soleus (SOL) muscle of healthy subjects [6]. The recurring muscle activity generates a physical oscillation in the muscle that can be measured by mechanomyography [7]. This physical oscillation of the SOL during quiet standing is considered to be the same phenomenon as ‘physiological tremor’ [8]. It has been demonstrated that physiological tremor can result in force fluctuations in isometric contraction of the lower limb [9]. Indeed, during quiet standing, physiological tremor produces fluctuations in the force exerted by the foot [6].

It has been frequently reported that physiological tremor increases with aging [10]. Therefore, the reported evidence that both physiological tremor and postural sway during quiet standing increase with aging leads us to hypothesize that the increased physiological tremor in the elderly may cause larger postural sway, especially in fast components, during quiet standing compared to the young. Thus, the first purpose of this study was to test this hypothesis by investigating the association between the amount of physiological tremor and postural sway during quiet standing in the young and elderly.

Although neural regulation of the plantar flexor muscles is essential in the control of postural sway as mentioned above, the muscle properties, such as muscle volume and strength, around the ankle joint can also affect postural stability as the muscle is the effector of the neural command. Since age-related changes in muscle properties, such as muscle volume, are known to occur [11], it is very likely that the age-related postural instability is caused by the age-related deterioration in muscle properties. Studies have shown that muscle strength is associated with the incidence of falls [12], [13], and that the muscle strength of the plantar flexors is correlated with the limit of stability in elderly people [14]. These findings suggest that maximum muscle strength can be closely related to dynamic balance measures such as the recovery of fall incidents and the limit of stability. However, the relationship between age-related deterioration of postural stability during quiet standing and decrement of muscle volume due to aging has not been explored at all. Since muscle volume is suggested to closely relate to muscle stiffness [15] and muscle stiffness plays a critical role in controlling quiet standing posture [16], [17], age-related decrease in muscle volume of the prime mover for maintaining an erect posture in quiet stance can be more directly related to postural instability in the elderly than its strength. Therefore, the second purpose of the present study was to test this hypothesis by examining the association between the muscle volume of the plantar flexors and postural sway during quiet standing in both the young and elderly.

This study aimed to investigate the conceivable neural and anatomical independent factors of age-related postural instability in the elderly. The correlation between each factor and postural sway was examined in both the young and elderly.

Section snippets

Subjects

Twenty young men (range: 23–35 years) and 20 elderly men (range: 65–75 years) volunteered for this experiment. They gave their written informed consent for the study after receiving a detailed explanation of the purposes, potential benefits, and risks associated with participation in the study. All subjects were healthy and had no history of any neurological disorders, and their vision was normal or adequately corrected. All procedures used in this study were in accordance with the Declaration

Results

Typical time series of the COP, ACCbody, and ACCSOL during quiet standing for 60 s in one young and elderly adult are shown in Fig. 1. It can be seen that the COP and ACCbody fluctuate while ACCSOL exhibits tonic activity in a global view. Comparing the data of young and elderly subjects, it seems likely that the trajectory of COP and ACCbody showed more complex fluctuation in the elderly than in the young-adult subjects. Only 1-s windows are presented in the bottom traces for an enlarged view

Discussion

The present study compared postural sway measures, such as the COP velocity and ACCbody, during quiet standing, physiological tremor, and muscle volume of the plantar flexors between young and elderly adults, and the correlation between postural sway and either physiological tremor of SOL or muscle volume of the plantar flexors. The main findings were that: (1) the physiological tremor of the SOL during quiet standing was greater in elderly than young adults, while the muscle volume of plantar

Conclusion

In the current study, we found that: (1) the physiological tremor of the SOL during quiet standing was greater in elderly than young adults, while the muscle volume of the plantar flexors was smaller in the elderly than the young, (2) the physiological tremor component was positively correlated to postural sway measures such as COP velocity and ACCbody for young and elderly adults combined, while muscle volume was negatively correlated, and (3) physiological tremor positively correlated with

Acknowledgments

This work was supported, in part, by a grant from the Meijiyasuda Life Foundation of Health and Welfare, and grant #15700396 from the Ministry of Education, Science and Culture of Japan (both to M.K.).
Conflict of interest

There is no conflict of interest.

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