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| REVIEW ARTICLE |
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| Year : 2020 | Volume
: 10
| Issue : 1 | Page : 1-9 |
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Sex-based differences in postural stability: A systematic review
Connor James Dean1, Timothy C Sell2, Amanda M Robertson1
1 Michael W. Krzyzewski Human Performance Laboratory, Department of Orthopedic Surgery, Duke University, Durham, NC, USA
2 James R. Urbaniak Sports Science Institute, Department of Orthopedic Surgery; Department of Orthopedic Surgery, Duke University, Durham, NC, USA
| Date of Submission | 03-Feb-2020 |
| Date of Acceptance | 22-Sep-2020 |
| Date of Web Publication | 21-May-2021 |
Correspondence Address:
Mr. Connor James Dean
5011 Heddon Way, Greensboro, NC 27455
USA
 Source of Support: None, Conflict of Interest: None
DOI: 10.4103/DORJ.DORJ_2_20
Postural stability is a known risk factor for musculoskeletal injury although the impact of sex on postural stability is not well understood. This systematic review evaluated 24 studies that reported on postural stability. Findings regarding the impact of sex on postural stability were mixed, with some studies reporting male superiority, others reporting female superiority, and still others demonstrating no significant difference.
Keywords: Balance, injury prevention, lower extremity, postural stability, sex differences
How to cite this article:
Dean CJ, Sell TC, Robertson AM. Sex-based differences in postural stability: A systematic review. Duke Orthop J 2020;10:1-9 |
| Introduction | |  |
Musculoskeletal injuries in the United States cost Americans approximately $176 billion per year and account for 77% of all healthcare visits.[1] Of the 65.8 million visits to healthcare professionals per year for musculoskeletal injuries, 2.8 million are sports related.[2] Over 229,000 noncombat military personnel sustain a musculoskeletal injury annually accounting for 95% of their medically treated injuries.[3] The prevalence and cost of musculoskeletal injuries in the United States among active populations demonstrate a need to examine the causes of and potential solutions for this growing problem.
The common risk factors for musculoskeletal injury include, but are not limited to, age, body composition, health, physical fitness, anatomy, and skill level.[4],[5] Past systematic reviews have addressed many of these broad risk factors along with specific clinical correlates, such as ankle instability and cognitive impairment.[6],[7] Postural stability is also a risk factor for musculoskeletal injury and has been identified as a predictor of re-injury of the anterior cruciate ligament.[8],[9]
Postural stability can be classified as static or dynamic. Static postural stability is defined as “determining the movement of the body's center of pressure within the base of support during upright stance.”[10] Wikstrom et al. have defined (and measure) dynamic postural stability as the “assessment of an individual's ability to maintain balance while transitioning from a dynamic to a static state.”[11] Poor postural stability is a risk factor for lower extremity injury and disproportionately affects females over males.[12] Analysis of the injury rates for males and females participating in the same sport reveals that the types of musculoskeletal injuries, as well as the rates for injury, differ between sexes.[13] A review of literature has not yet been undertaken on the risk factors of postural stability and sex in a healthy adult population.
Given the significant number of injuries in active populations and the differences in injury rates between males and females, there is a requisite need to develop precise injury prevention programs to prevent musculoskeletal injury. Accounting for sex differences in the postural stability helps focus injury prevention programs on the areas where specific individuals are most at risk. The purpose of this review was to evaluate similarities and differences in static and dynamic postural stability in men and women. We hypothesized that women perform better during static postural stability tasks than men and that men outperform women on dynamic postural stability tasks. The results of this systematic review will provide evidence for the development of sex-based injury prevention programs.
| Methods | | |
This systematic review was conducted utilizing the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA).[14] The protocol was registered on PROSPERO (CRD42018082523).[15]
Search strategy
A medical research librarian with expertise in systematic review methodology performed a comprehensive search of Medline, EMBASE, CINAHL, and SPORTDiscus databases. The search terms applied were posture, postural stability, postural balance, stability, balance, body equilibrium, sex characteristics, gender difference, sex-based differences, sex factors, males, and females. The search was conducted on December 3, 2017. To be considered for this review, a study had to be written in English, published after 1965, and matched at least one of the keywords used to filter studies using the online search engines. Our search strategy for the electronic search of the literature is presented in [Table 1].
Inclusion and exclusion criteria
Once selected and the title screened for relevance to the research question, the inclusion and exclusion criteria [Table 2] were applied to the abstract and full-text screen. For studies to be included in this review, the subjects had to be between 13 and 40, had no previous medical conditions, had a member of the military or participant in sports activities, and had to maintain a healthy lifestyle. Each selected study had to measure the postural stability and report on both males and females. This population was chosen because those aged between 13 and 40 years without medical conditions are less likely to have developed age-related degenerative changes that would affect metrics of postural stability.
Study selection
Two authors performed the study collation, and a “yes” response from one of the two authors was sufficient for a study to pass through to the next level of screening. All papers were approved by at least one of the two reviewing authors (CJD and MAR). [Figure 1] presents the PRISMA flowchart for study selection. | Figure 1: Preferred Reporting Items for Systematic Reviews and Meta-Analyses flowchart for study selection
Click here to view |
Study quality
A modified Downs and Black scale[18] was used to measure the quality of the 21 included studies. The standard Downs and Black test uses 27 questions scored as “yes” (1), “no” (0), and “unable to determine” (0). The scores of the test are then summed and the higher scores affirm the higher quality. Not all questions in the standard Downs and Black test were relevant to this systematic review. A modified Downs and Black scale consisted of 14 questions that were useful in describing the study quality and the internal validity of the studies.[7] Only questions pertaining to relevant material were included. Only questions pertaining to relevant material were included. Question numbers 1, 3, 11, 21, 23, 2, 20, 15, 6, 7, 18, 4, 25, and 27 from the standard Downs and Black scale are included in this study. They are numbered 1-14 respectively in this review. The studies were assessed by two authors (CJD and MAR) to control for bias. Disagreements were discussed and mutually agreed on by the two reviewers.
Static and dynamic postural stability
To distinguish dynamic from static postural stability, a task was labeled “static” if the participant was not instructed to move in any horizontal direction from an initial base of support during the task, such as in Heitkamp et al.[19] Dynamic postural stability tasks were marked if participants transitioned from a dynamic to a static state.
Data extraction
Data extraction was carried out by one of the authors (CJD) following a systematic process of discerning variables associating with dynamic and static postural stability. This extraction was then reviewed by a second author (TCS). Variables extracted include number of subjects (by sex), average age of subjects (also by sex), the instrumentation utilized in the study, and the protocol executed during testing. The nature of the study of dynamic and static postural stability lends itself to a multitude of testing protocols. The specific measurements of dynamic and postural stability were extracted from the 21 studies; however, direct comparison of these variables across studies is limited. Due to the heterogeneity of both the protocols and the specific measurements to assess postural stability, the final variables extracted and reported are the conclusions of the studies as they relate to static and dynamic postural stability for males and females. Supplementary Online [Table 1] contains the complete table of 44 variables extracted from the results of collated studies reporting their quantitative and qualitative results.
| Results | | |
Search strategy and study quality appraisal
A total of 905 studies were identified. From this initial screen, 115 duplicates were removed leaving 790 unique studies for screening. A title screen for balance, stability, and sex factors was performed on the 790 studies; 109 were selected. The inclusion and exclusion criteria [Table 2] were applied beginning at the abstract screening. Abstracts for the 109 studies were then screened using our inclusion and exclusion criteria, with 80 papers being excluded. The final 29 studies went through a full-text review, and seven more were excluded resulting in 21 papers that make up this project [Figure 1].
Our summary of the Down and Black criteria for each of the included studies is presented in [Table 3]. Question 8 refers to the blinding of subjects to assigned groups. All studies collated did not blind subjects due to the nature of the balance protocols being tested, and thus, none met this criterion.
Data extraction
Below are the data extracted from the 21 studies including number and age of subjects, instrumentation, and protocol.
Number and age of subjects
The number of study participants in the 21 selected studies varied between a low of 16[29] to a high of 215.[24] Overall, there were 649 males and 572 females in the selected studies.
The subjects of all studies included in this systematic review fell well within the 13–40 years of age range set out in the inclusion and exclusion criteria. The average age of the subjects in reviewed studies ranged between 15.7[25] and 29[24] years.
Instrumentation
From the 21 studies, eight distinct forms of instrumentation were used to measure the postural stability. Many of the same instruments were used across separate protocols. A force plate, the Biodex Stability System, and STAR/Y balance tests were all used in five studies (24% each). Nineteen of the studies used a single method of instrumentation to measure the postural stability, while the other two used two different instruments. [Table 4] summarizes the instrumentation used by the included studies.
Protocol
Twenty-two distinct protocols to assess the postural stability were identified in our review. Seventeen of the studies used a single protocol to measure postural stability, while the other four used at least two or more measurement protocols. The most common protocol across all studies was a Biodex single-leg balance assessment: three studies.[22],[31],[33] Nineteen of the 21 protocols had no duplicates, and often, the same instrumentation was used for multiple protocols across the various studies. For instance, three separate protocols were performed for the STAR excursion balance test. The standard protocol recorded the distances of leg extension in all directions including anterior, posteromedial, and posterolateral.[23],[33] A second protocol recorded sex differences on a STAR test in a fatigued condition.[35] The final protocol was a qualitative analysis by medical professionals as a predictor of risk of injury in lower extremities for males and females.[28] While there were few duplicate protocols across the included studies, protocols employed in different studies were very similar. For example, two protocols assessing static postural stability were different only in the amount of time the subject was to be standing, 60 s[20] or 30 s.[21] Time to stabilization was quantified using in two studies on landing either with double-legged[29] or single-legged landings.[11]
Postural stability
Of the 21 studies that met our inclusion criteria, three studies exclusively measured static postural stability and 18 measured dynamic postural stability [Table 5]. One study[38] measured both dynamic and static postural stability for males and females but presented only pooled results and was excluded from the systematic review. After extracting relevant data, the results of static postural stability tests were grouped into four unique categories: male (signifying that males had superior postural stability), female (signifying that females had superior postural stability), mixed (males and females were superior in different aspects of the same protocol), and no difference (no significant difference in performance between males and females) [Table 6].
Static
Due to the heterogeneity of data extracted in this study, only variables relevant to postural stability were tabulated in this study, while others are included in the aggregate data table [Table 5]. Three of the 21 studies measured static postural stability. The protocols and variables extracted were so different that a tabular comparison of data would be uninformative. Instead, we report the conclusions of the studies separate from variables of interest [Table 6]. These conclusions were validated through examination of the studies through a modified Downs and Black test.[18]
Dynamic
The same method of extracting data along with conclusions was used for studies with measures of dynamic postural stability [Table 5]. The categories defining those conclusions were identical between the static and dynamic trials. Nineteen of the 21 studies contained measures of dynamic postural stability and had their respective data extracted.
The 18 studies that assessed dynamic postural stability were mixed in their findings of any sex differences across all studies, with no pattern of sex-based superiority in certain postural stability tasks being evident. Balance control, measured by either STAR excursion balance tests or Y balance tests, was found to favor or slightly favor males in three out of six studies (n = 300).[25],[28],[33] In the other three studies, two (n = 287) found no sex differences[23],[24] and one (n = 40) favored females to have superior balance control after fatigue.[35] Females exhibited superior Biodex-quantified stability on an unlevel platform in four out of five studies (n = 224);[22],[26],[31],[34] the fourth study (n = 34) found no sex differences.[32] Both Biodex level of support tests (n = 86) were performed better by females who were able to reach greater angles of single-legged incline on an unstable platform.[26],[34] Of the remaining seven studies measuring dynamic postural stability, two found females to have superior dynamic stability[11],[36] (n = 118), one found males to have superior dynamic stability[16] (n = 24), and four found no difference (n = 158).[27],[29],[30],[37]
| Discussion | | |
This systematic review summarizes research on sex differences in postural stability in healthy subjects between 13 and 40 years of age. Examination of the relationship between sex and postural stability is central to the rationale for developing either sex-specific or generalized injury prevention protocols. Twenty-one studies met our inclusion criteria allowing us to evaluate the measurement methods for static and dynamic postural stability between men and women.
Of the three studies measuring static postural stability, two (n = 66) found males to have superior static postural stability, and one (n = 30) found no difference. Błaszczyk et al.[20] found males to have superior static postural stability and Torres et al.[21] found no difference between males and females. Both ran similar center of pressure (COP) tests measuring changes in sway. Heitkamp et al.[19] found that males had fewer touchdowns in a one-legged balancing protocol. This is a small number of studies reviewed to reach significant conclusions. Further research is required to reach a sufficient conclusion on the question of sex and static postural stability.
Of the 18 studies that measured dynamic postural stability, four studies (n = 324) reported that men had superior dynamic postural stability, seven (n = 343) reported that females had better dynamic postural stability, one (n = 72) reported that men and women had mixed success at the postural stability task, and six (n = 406) found no difference in postural stability between men and women [Table 6]. The number of studies collated in this review that measures dynamic postural stability is sufficient to reach meaningful conclusions. However, the heterogeneity of testing protocols and the mixed results of the 18 studies make discerning a consistent relationship between sex and dynamic postural stability difficult. It may be the case that such a relationship does not exist. However, a consistent testing protocol of dynamic postural stability over a large subject group would be necessary to validate this conclusion.
The inconsistent relationship we found between sex and postural stability is likely due to the multitude of protocols and associated variables across our collated studies. There were 22 unique protocols from the 21 studies in this systematic review [Table 4]. The Biodex OSI test (that measures stability index) was the most common protocol, but it was only used in four studies. The STAR and Y balance tests were used in six studies, but these were administered under different circumstances and different variables were also reported (e.g., three distinct distances, averaged distances, significance of difference in distances between sex, and risk assessment postprotocol). Comparing postural stability across multiple studies is difficult given the absence of consensus on either the definition or the metric for testing postural stability. Before any concrete conclusions can be made on the impact of variables, like sex, on postural stability, there first needs to be an established, consistent protocol for its measure. Based on the range of protocols and measurements, it is obvious that postural stability is a complex variable, and simplification of that variable to the measurements of a single protocol is not likely to accurately capture an individual's true postural stability. Consistent application of reliable and valid measurements is pivotal to answering questions on the relationship between postural stability and sex.
Understanding differences in postural stability between men and women at risk for musculoskeletal injuries is central to the concept that injury prevention protocols should be tailored for each sex.[39] Separating by sex makes assessments of postural stability more reflective of an individual's relative risk for injury. The greater the precision of the measurement, the greater the predictive power of baseline and subsequent postural stability assessments. Clinical evaluation of the postural stability from a baseline can be used as a measure of an athlete's progression postinjury and supplement the athletic training protocols for return to activity.
The limitations of this systematic review are due to the search strategy, the low quality of the evidence, and finite databases from which we collated studies. Only English language studies were included in this review, and four databases were searched for relevant studies. The heterogeneity of methodologies made it difficult to compare results across all studies. This is due to both the complexity of postural stability itself and its many measurements. Height was not factored into data collection of many of the methodologies of the reviewed studies. A narrow age range limited the scope of this study along with restrictions to healthy, active subjects.
| Conclusions | | |
The relationship of dynamic and static postural stability to sex is complex. We found men to have better static postural stability in two of the three studies that reported static stability. This argued against our initial hypothesis; however, the low number of studies limits the generalizability of this finding. Dynamic postural stability was not found to be superior in either men or women. Patterns of results existed across protocols, but overall, there was no consistency regarding male or female superiority in tests of dynamic postural stability. This lack of agreement supports the implementation of generalized prevention programs over sex-specific programs. However, the relationship between sex and postural stability when applied to injury prevention protocols may need to be refined going forward.
Acknowledgments
The authors acknowledge Leila Ledbetter, MLIS, the Research and Education Librarian, at the Duke University Medical Center Library for her expertise and assistance in guiding us in searching the literature. The authors also acknowledge Donald Kirkendall, ELS, a contracted medical editor, for his assistance in the preparation of this manuscript.
Financial support and sponsorship
Grants supporting research at the Michael W. Krzyzewski Human Performance Laboratory contributed to this research.
Conflicts of interest
There are no conflicts of interest.
| Supplementary Online Table | | |
Supplementary Online Table 1: Global summary of the included studies
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[Figure 1]
[Table 1], [Table 2], [Table 3], [Table 4], [Table 5], [Table 6]
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