Introduction

The human body is not completely stationary, and the bipedal upright stance is unstable owing to a small base of support and high center of mass [1]. To maintain a stable standing position and to stand with as little sway as possible, the center of pressure (COP) must be continuously corrected. Disease (i.e., stroke) and injury (i.e., fracture) often impair the stability of the standing position [2–4]. Several studies have suggested that a decline in postural control leads to balance deficits and increases the risk of falling [5–7]; therefore, for several decades, many researchers have focused on the mechanisms underlying postural instability. Understanding the factors that can impact postural control is necessary for developing more effective rehabilitation strategies.

Body joints are actively involved in maintaining posture; furthermore, for the ability to stand with as little sway as possible, multiple joints of the body must be coordinated to increase stability [8,9]. However, when part of the body is artificially rendered inactive, it may not be able to perform smooth, efficient, and precisely coordinated movements. Several studies have examined the kinematic effects on the upper limb after physical inactivity imposed by casts, splints, bandages, cotton slings, and similar devices. Moisello et al. observed changes in interjoint coordination and drifting of the movement onset point during upper limb movement [10], while Scotto et al. reported that pointing movements on a personal computer delayed movement time while maintaining accuracy [11]. Other studies have shown an increase in the total duration of reaching movements and modified anticipatory postural adjustments of the elbow and shoulder [12,13]. Interestingly, these results were obtained in healthy adults and were induced by physical inactivity of only 10–24 h.

As demonstrated in previous studies, interjoint coordination may therefore change after a short period of physical inactivity, resulting in a different strategy to achieve a stable standing posture. However, to the best of our knowledge, although some studies have examined body sway in the upright position during joint restraint, none have performed such evaluations after physical inactivity [14,15]. In clinical practice, rehabilitation for stroke in the chronic phase of recovery includes constraint-induced movement therapy (CIMT), which involves restraint of the unaffected limb and forced use of the affected limb [16]. This technique is performed for a limited amount of time during the day, and there are reports that several weeks of intervention can improve balance [17,18]. Knowledge concerning the immediate effects of physical inactivity may not only clarify the acute adverse effects of inactivity, but also contribute to our understanding of the mechanisms by which CIMT results in recovery.

This study aimed to investigate the effects of 10 h of lower limb physical inactivity on postural sway in an upright stance after cast removal (when compared with postural sway before cast application) in healthy adults, using the minimum duration of restraint mentioned in a previous study of upper extremity kinematics as a reference [10]. We hypothesized that postural sway would be greater after cast removal than before cast application.

Methods

Results

There were no dropouts in this study. Fig 1 shows representative data from one participant, demonstrating the total path followed by the COP during the trials.

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Fig 1. Total path followed by the center of pressure during the trials in one participant.

EO: Eyes-open condition, EC: Eyes-closed condition.

https://doi.org/10.1371/journal.pone.0272969.g001

The changes in the COP before and after cast removal are summarized in Fig 2. There was a significant increase in the total trajectory length after cast removal when compared with that before cast application in both the EO and EC conditions (EO: p < 0.01, d = 0.53; EC: p = 0.02, d = 0.31). A similar increase in mean velocity was observed in both the EC and EO conditions (EO: p < 0.01, d = 0.09; EC: p < 0.01, d = 0.07). There was no significant increase in the COP RMS area after cast removal (EO: p = 0.68, d = 0.12; EC: p = 0.54, d = 0.15). The mean M-L COP had significantly shifted from the immobilized to the non-immobilized side after cast removal (EO: p < 0.01, d = 0.79; EC: p = 0.02, d = 0.49), while the mean A-P COP demonstrated no significant anterior or posterior displacement after cast removal (EO: p = 0.66, d = 0.11; EC: p = 0.71, d = 0.08).

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Fig 2. Changes in postural sway during quiet standing after 10 h lower limb immobilization.

White: Before, Gray: After 10 h. *p < 0.05. A-P: Anterior-posterior, COP: Center of pressure, EO: Eyes-open condition, EC: Eyes-closed condition, M-L: Medial-lateral, RMS: Root mean square.

https://doi.org/10.1371/journal.pone.0272969.g002

In the box-and-whisker plot, the center line represents the median value, while the top and bottom boxes represent the 75th and 25th percentiles, respectively. The whiskers indicate the minimum and maximum data points in the range.

Discussion

The present study investigated changes in postural sway during quiet standing after 10 h of lower limb physical inactivity. The COP measurement, performed while quiet standing, using a force plate provides a highly reproducibility assessment of postural balance, unaffected by circadian rhythms [24]. Therefore, it is unlikely that the postural sway is simply due to time variation. To the best of our knowledge, the current study is the first to quantify postural stability after physical inactivity. Most notably, an increase in COP movements was observed after cast removal when compared with those before cast application, and the COP had shifted from the fixed to the nonfixed side. As these significant differences were observed in healthy adults, our findings suggest that changes in COP movements during quiet standing were caused by physical inactivity rather than disease or injury.

The study that experimentally examined the effects of transient blockage of somatosensory information, the COP was altered by dermal injection of a local anesthetic into the sole of the foot [25]. The COP movements increased with increasing severity of sensory loss during standing after ischemic blockade [26]. Thus, previous studies have shown that somatosensory modalities in the lower extremities contribute to postural adjustment. In the present study, as we included healthy participants without disease, we attributed these changes to the deprivation of somatosensory and motor information caused by disuse. On the contrary, certain interpretations suggest that increased standing postural sway reflects an exploratory role by the central nervous system [27]. Thus, the increase in COP sway due to nonuse may have been an exploratory strategy of several sensory sources to keep the body in equilibrium.

In addition, several neurophysiological studies have examined the effect of limb nonuse on sensorimotor representation in healthy persons [18–32]. Facchini et al. reported a decrease in cortical excitability after 3–4 days of two-hand finger immobilization using transcranial magnetic stimulation [28]. Interestingly, several studies have demonstrated that arm immobilization for only 8–12 h induces a decrease in both somatosensory and motor evoked potentials and reduces the excitability of the somatosensory motor cortex [29–31]. These previous studies suggested that short periods of limb nonuse may trigger synaptic depression in the somatosensory motor cortex. Furthermore, another study has shown that the neural activity of peripheral nerves in the somatosensory pathway changes after 10 h of cast immobilization [32]. Therefore, increase in COP movements during quiet standing may be caused by modifications in limb representation in the nervous system due to physical inactivity. Reversible changes in the nervous system may inhibit the feedback or feed-forward processes necessary for posture maintenance [33].

The second main observation was that the postural sway shifted from the fixed side to the nonfixed side after cast removal. Researchers have observed that, among those with unilateral lower limb disorders, COP sway in the M-L axis is greater among patients with stroke and those who have undergone total knee arthroplasty [34,35]. Therefore, the results of this study suggest that after 10 h of cast immobilization, healthy participants may have maintained their standing posture by adopting a compensatory strategy consisting of lateral control, similar to that adopted in disease states. Conversely, the A-P axis was not affected, which may be why the COP RMS area did not expand.

Abdullahira et al. have argued that the effects of lower limb CIMT may be overestimated and that gait may be negatively affected by the asymmetry caused by unilateral limb restraint [36]. Our results may support this claim.

The present study had some limitations. First, the participants were allowed to use the non-immobilized leg for daily activities and were not restricted from full activity during the period of immobilization. Recent studies suggest that muscle fatigue increases postural asymmetry during quiet standing [37]. Thus, it is possible that the condition of the nonfixed limb affected COP sway. Second, we did not collect subjective stability outcomes in this study. Third, the effect sizes observed in this study were not large. Further studies are required to verify our findings while accounting for these limitations.

Conclusions

The current findings indicated that after 10 h of lower limb physical inactivity, healthy adults exhibited an increase in COP movements and a shift of the COP from the fixed to the nonfixed side during quiet standing. These results indicate that short-term disuse may cause acute changes in COP movements during quiet standing. Future studies should investigate the fundamental mechanisms underlying static postural control after inactivity.

Supporting information

Acknowledgments

The authors wish to thank the laboratory students for their support on this work.