An investigation into the influence of biological sex,anthropometrics,footwear,and dual tasking on balance

1 Introduction

Balance can be defined as the ability of an individual to maintain their centre of mass (COM) over their base of support [1]. It is regulated by the interaction of afferent inputs from the visual, vestibular and somatosensory systems, the central nervous system (CNS), the efferent output system and the musculoskeletal system [2, 3]. Balance is essential as a prerequisite for performing all activities of daily living and functional tasks and impairments in balance result in elevated injury risk [4] and loss of independence [5]. The assessment and training of balance is a fundamental component of many rehabilitation programmes [6], however, to optimise balance assessment and rehabilitation, several fundamental decisions need to be reasoned by thephysiotherapist.

Many intrinsic factors may influence balance, including those influencing the input systems (visual vestibular and somatosensory), the processing systems (brain, CNS) or the output system peripheral nervous system (PNS, muscles etc.) [7, 8]. Whether physiotherapists should compare balance performance with unisex groups remains a subject of debate. This is because some previous studies have suggested that biological sex influences balance, with studies demonstrating balance is moderated by biological sex [9], whilst others have shown this not to be the case [10]. Some discrepancies in the literature may be explained by anthropometric factors, rather than biological sex, where between sex differences were removed, once balance performance was normalised to height [11, 12]. However, no studies have systematically explored a range of anthropometric measurements and their influence on static balance performance. Such information is essential for the identifying balance impairment and establishing rehabilitation targets.

Whether or not footwear should be used in the assessment and rehabilitation of balance is a factor for consideration by physiotherapists and individuals. Footwear has the potential to affect static balance [13] through the modification of somatosensory input [14] or the physical constraint of foot and ankle movement [13]. Despite this, there is relatively little in the existing literature regarding the effect of footwear on balance, with some authors reporting footwear positively affects balance [15] and others reporting no effect [16]. These studies were limited to single sex cohorts and single leg stance (SLS) only and no studies have systematically explored the effect of footwear on balance across a range of tasks and conditions and such information is an imperative for clinical decision making.

The balance literature suggests that adding a cognitive-task results in significant additional balance challenge. However, the evidence of the effect of dual tasks (DT) on static balance is conflicted, with some suggesting better static balance [17], whilst others suggest no effect [18] or an adverse effect on balance [19]. The lack of clarity in the literature over the real influence of DT on static balance has led to confusion about the optimal approach to static balance assessment and training within research and clinical practice contexts. As the understanding of adding an additional task is not well understood, physiotherapists cannot be clear in their decision making regarding its use in impairment identification or rehabilitation/training.

Thus, this study aims to comprehensively investigate the influence of biological sex, anthropometrics, the wearing or not of footwear and the relative effects of single and DT on static balance performance during quiet standing. Such knowledge is critical for understanding how such factors influence balance, informing assessment and rehabilitation decisions.

This will be achieved by:

Exploring differences between females and males in static balance performance.

This study will provide the foundation understanding to support decision making for static balance assessment and rehabilitation.

2 Methods

3 Results

This study aimed to clarify the influences of biological sex, anthropometrics, physical activity, footwear and DT on static balance performance during quiet standing, to provide evidential support for decision making in the assessment and rehabilitation of static balance.

Females outperformed males across all static balance performance tests, with some very large ES. Previous studies mirror those of the current study, with women performing better than men during static balance [12, 26]. Thus, the current study not only compliments the previous literature but demonstrates that this difference is present regardless of whether shoes are worn and whether single or DTs are performed during testing.

The reasons for the sex related differences during static balance performance are unknown. It is possible that anthropometric factors play a role, as it has been suggested that differences in balance between the sexes is merely an expression of the height of the individual [11, 27], since being taller results in a higher COM, however, differences were also found after normalisation for height. The current study established modest correlations with height but also weight in healthy adults during static balance during quiet standing. This correlation may help to explain the results, since males tend to have a relatively larger upper body, whilst females have greater mass concentrated in the lower body [28, 29]. However, additionally, waist and shoulder circumferential measures showed a modest effect on static balance performance, suggesting the greater the ‘size’ of the upper body including the waist, the poorer the balance performance across DLS tasks.

This larger upper body size is likely to raise the COM and potentially contributes to greater sway. Our correlational analysis seems to support a relationship between greater upper body size (albeit moderately) and poorer balance performance. This was particularly the case with DLS tasks. It is possible that such anthropometric relationships are evident during simpler balance tasks, as almost universally, SLS tasks were not significantly correlated with body size. Previous research has demonstrated that DLS moderately correlated with BMI [11] and the components of BMI [30], suggesting the significance of anthropometrics is evident in lower complexity tasks.

The current study showed no correlation between total physical activity level and any static balance variables. This lack of relationship suggests that modifications to physical activity level will have little direct effect of balance ability. This is in agreement with previous authors, who did not find any correlation between sports performance level and static balance in athletes of any age [31].

To the best of the authors’ knowledge the current study is the first to investigate the effect of footwear on static balance performance in healthy adults under multiple positions and conditions. This information is essential for physiotherapists when deciding whether to assess and train static balance with or without footwear. To determine the optimal approach, it is first required to determine if balance performance differs across these conditions. The findings here demonstrate that overall, there were no differences in performance during static balance due to footwear. Two exceptions were found: (i) during DLSEC tasks wearing shoes worsened performance in both females and males; and (ii) wearing shoes resulted in better balance performance for males under the SLS condition. Pressure moving across the distribution of the plantar foot surface during static balance appears to be translated into movement around the COM, which could impact balance performance. Previous studies have demonstrated that plantar foot surface sensation is a critical contributor to the somatosensory part of the biofeedback loop involved in maintaining balance [32]. Arguably, this may be enhanced during the barefoot condition, as supported by findings in this study for the DLS condition but not the SLS condition. The difference may be explained by visual contribution. Potentially, the DLSEC condition may have resulted in a re-evaluation of the weighting of the balance input parameters to the biofeedback loop [33]. With vision present (SLS condition), reliance on plantar pressure feedback is unnecessary/less important, due to the dominance of the visual contribution. Without vision (DLSEC condition) it is possible that attention to the input from the plantar foot pressure is raised, resulting in the difference between both with and without footwear conditions. The current literature is surprisingly sparse regarding direct comparisons between the with and without footwear conditions; therefore, the current study offers a significant contribution to the current understanding. Previous studies have demonstrated mixed results regarding the effect of footwear on balance. Germano et al. [15] demonstrated similar findings for SLS, reporting COP excursion was greater in the barefoot condition. In contrast, Smith et al. [34] demonstrated that balance, whilst wearing shoes, was better for COP sway overall and in the anteroposterior direction during DLSEC. The reason for these conflicting findings is not clear; however, angular displacement of centre of gravity (COG) was used to determine balance performance, rather than perimeter length of the sway trace [34].

Significant differences were determined for DLSEC with footwear and without footwear conditions, demonstrating poorer static balance performance during DT. DT balance testing has increasingly been reported in the literature, since it affords greater discriminatory capacity than single task balance tests [35]. However, in this study, while performance was marginally worse with DT, no significant differences were determined except during the DLSEC task. This likely reflects the participants’ age, as the current study’s young participants have adequate capacity to complete a cognitive task with little ‘cost’ to the physical task. Previous literature, demonstrating a difference, focuses on older adults who have relatively less capacity to manage the competing demands of attending to both the cognitive task and the balance task [17]. Previous studies found no significant differences between single and DT during static balance performance, as measured by total COP displacement, ellipse area and COP velocity in the force plate in youths [18]. Therefore, it seems unlikely that the DT offers additional benefit over single task balance testing for young healthy individuals.

This study has made several contributions to understanding balance assessment and performance through the systematic exploration of biological sex, footwear, DT and correlational analysis, linking performance to baseline anthropometrics. This information is beneficial for physiotherapists when determining how best to test balance and prescribe balance exercises. Balance assessments should be completed with an understanding of female-male differences and comparing female or male specific reference scores is recommended. Wearing footwear seems to negatively affect females and males for DLSEC and males positively for SLS; however, the ES was small, demonstrating a minimal difference in actual values between the conditions. This suggests use of footwear has minimal clinical influence on balance performance. DT for routine balance testing in young healthy adults offers little, beyond single task balance testing in terms of the detection of balance deficits. In addition, this study offers insights that explain balance performance from which physiotherapists can integrate into their exercise justification. Clinicians might advise individuals to increase their physical activity but this is likely to have a minimal effect on balance performance, because of the lack of a relationship between physical activity and balance performance. A person’s height, weight, waist and shoulder circumferences did relate to balance performance; therefore, an awareness of the increased burden anthropometric characteristics may place on balance should be acknowledged.

5 Conclusion

In conclusion, females demonstrated better static balance across a range of tasks. The sex differences noted were present in both with and without footwear conditions, as well as in single and DTs. No large ES were observed between with footwear and without footwear conditions or between single and DTs for static balance. Regarding anthropometry, correlations were moderate between static balance performance and height, weight and upper body size. It is concluded that the taller, heavier and greater the size of the upper body, the poorer static balance performance will be. Such information can be utilised by physiotherapists to facilitate decision making when assessing performance and prescribing rehabilitation exercise.