Elsevier

Gait & Posture

Volume 36, Issue 3, July 2012, Pages 383-388
Gait & Posture

How an acute mastering of balance on a seesaw can improve the relationship between “static” and “dynamic” upright postural control

https://doi.org/10.1016/j.gaitpost.2012.03.018Get rights and content

Abstract

Postural control ability has been widely evaluated using undisturbed upright stance protocols. However, standing on a seesaw may offer additional insights due to changes in the available sensory information and the amplification of the motor command resulting from the translational and rotational movement of the device. These two tasks share close biomechanical and neurophysiological principles. To highlight their possible linkage, 32 young healthy adults participated in this study, which consisted of testing postural performance while standing on a firm surface or on a seesaw producing rolling or pitching movements. The results showed increased CP displacements along the seesaw's pitching or rolling axis and also along the perpendicular axis. However, comparing the two tasks can be difficult because of discrepancies in the ability to rapidly master the new constraints brought about by the seesaw. To highlight the role played by adaptation, 15 subjects of the whole sample participated in a complementary protocol consisting of a 20-min training session aimed at improving the mastery of the seesaw producing pitch motions. The relationship between the amplitudes of the CP displacements between “static” and “dynamic” tasks was investigated. Interestingly, whereas no statistically significant linear correlation was found before training, several significant correlations were found after pitch training for AP displacements. By emphasizing the key role played by short-term adaptation in standing performance, these results are likely to have potential implications regarding the conception of standardized tests aimed at evaluating postural ability in healthy or disabled subjects.

Highlights

► Standing on a seesaw modifies the available sensory information and amplifies the motor command with the translational-rotational movement. ► Both standing on a firm surface and standing on a seesaw share close biomechanical and neurophysiological principles. ► Comparing the two tasks can be difficult because of discrepancies in the ability to rapidly master the new seesaw constraints. ► Whereas no correlation was found before training, several significant linear correlations were found after pitch training. ► These results highlight the role played by short-term adaptation in standing performance.

Introduction

Still upright stance cannot be strictly maintained for neurophysiological and biomechanical reasons. Even though multiple factors might contribute, the main reason in our view originates from the non-vertically aligned segmental centers of mass requiring that shear forces be applied. Applying these forces can only be achieved by activating skeletal muscles whose tensions, unfortunately, cannot be held constant over time [1]. As a result, relative movements necessarily occur between the segments and the resulting angular momentum applied around the center-of-gravity (CG) is not null. This feature, along with the inability to stabilize the point of application of the resultant reaction force, the center-of-pressure (CP), prevents an upright subject from standing perfectly still.

This standing paradigm has retained the attention of scientists for more than a century as a means to highlight deficiencies in controlling this sensorimotor task. Even though standing still upright on a fixed force platform has become standard in evaluating postural performance, various weaknesses can be pointed out. The main one is the low constraint for the vestibular system. Data from the literature indeed report that the mean acceleration measured at the head level in a healthy individual standing upright is lower than the detection threshold of the semicircular canals of the vestibular system [2]. To solicit these sensory cues further, other paradigms, known as “dynamic” in contrast with the more “static” nature of undisturbed stance, have been proposed, including platform translation and/or rotation [3] and the seesaw [4], [5], [6]. The latter has retained our attention since it requires the standing subject to learn a new specific sensorimotor coordination. Indeed, when standing on a seesaw, the proprioceptive information from the ankle joints may not be in phase or correlated with the other sensory inputs, constraining the postural control system to generate motor responses based on feedforward control. In addition, the cylinder shape of the seesaw infers an amplification of the CP displacements due to the translation of the line of contact resulting from the platform rotation [7].

When comparing the “static” task, which has the subject standing on a firm surface, in most cases on a force platform, and the “dynamic” task, consisting in maintaining stance on a seesaw, several common points and differences can be emphasized. In addition to the availability or unavailability of the various sensory cues, the two tasks share a similar biomechanical objective, the control of the horizontal movements of the CG by CP displacements. Dissociating these two variables is indeed necessary to induce CG horizontal accelerations and therefore CG displacements. The first objective of the current study was therefore to determine the relationship between postural performance in both “static” and “dynamic” tasks by involving a sufficiently large sample of young healthy adults. Depending on how the feet are positioned on the seesaw, CP displacements and therefore body motions may be facilitated along the anteroposterior (AP) axis (pitch) or along the mediolateral (ML) axis (roll). As a result, the protocol was built around three conditions: standing on a firm surface (force platform) and on a seesaw favoring either pitch or roll body motions.

However, we anticipated that another variable might largely intervene in this comparison: the time spent in training before the test is performed. Indeed, although standing still upright on a firm surface is the result of learning over several decades, a much more reduced time is generally allowed before evaluating postural control on the seesaw. This point is nonetheless likely to play an incomparable role in this attempted comparison. Considering that standing is in fact the result of a learned coordination between CP and CG movements, one can therefore hypothesize that transposing this coordination while standing on a seesaw, at the onset, is largely inappropriate. By amplifying the CP displacements, the rolling of the seesaw increases the horizontal acceleration communicated to the CG and therefore the magnitude of the postural perturbation. Recalibrating this sensori-motor coordination is therefore required to limit the size of the perturbation, a process which necessitates the activation of structures involved on the voluntary control of movement (cerebellum, parietal and frontal areas). As hypothesized by Solopova et al. [8], based on transcranial magnetic stimulations data, “substantial functional reorganization occurs in the motor cortex under unstable support conditions”. This reorganization could have beneficial effects for the subject standing on a more stable support. To highlight this feature, the protocol was therefore completed by requiring some of the tested subjects to follow an additional short-term adaptation session whose duration was recently demonstrated to induce significant postural coordination modifications [9]. The second objective of this study was to assess whether a short-term adaptation on a seesaw would have immediate impact upon the “static” task performance and whether the relationship between postural behaviors encountered in both “static” and “dynamic” tasks could be improved. This result would emphasize the key role played by adaptation and would question the pertinence of using seesaws in postural evaluation protocols.

Section snippets

Methods

Thirty-two healthy young adults, 25 males and seven females, ranging in age from 21 to 26 years (body weight, 69.5 kg ± 11.1; height, 176.1 cm ± 7.0; mean ± standard deviation) with no known visual or balance pathology, gave their written informed consent and were included in this study. All the subjects were students in sports and physical education and participated in sports regularly.

All the subjects participated in three randomly performed conditions: (1) standing on a firm surface (REF), (2)

Results

A first analysis was performed for the whole group for the REF, ROL, and PIT conditions. As can be seen from the Friedman ANOVAs (Table 1, left column), statistically significant effects were found. Fig. 1 (gray bars) displays the results in terms of mean and standard deviations with the post hoc analysis. Through the variances, one can see that the “dynamic” PIT and ROL conditions produced larger CP displacements than the “static” REF condition. As expected, these effects depended on the axis

Discussion

The objectives of this study were to compare the postural effects induced by standing on a seesaw either by amplifying the CP displacements along the AP axis (PIT) or along the ML axis (ROL) and to highlight the possible role played by a short-term adaptation. Briefly, as expected, the results show that standing on a seesaw increases CP displacements principally for the axis along which the device is rolling. However, some increased CP movements were also observed along the orthogonal axis. The

Conflict of interest

None.

Acknowledgments

The authors wish to thank L. Northrup for editing the text and the anonymous reviewers for their helpful comments.

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