The protocol will follow the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) statement [
21] and for this manuscript, the PRISMA Protocols (PRISMA-P) [
22]. When authors change the protocol, these are documented and published in the final work. The protocol for the systematic review has been registered in PROSPERO (
http://www.crd.york.ac.uk/PROSPERO/display_record.php?ID=CRD42018116597), registration number is CRD42018116597.
Eligibility criteria
The authors choose to follow the PICOS criteria (population, intervention, comparison, outcomes, study design) described in PRISMA [
21] (Table
1).
Table 1Inclusion and exclusion criteria according PICOS guidelines (population, intervention, comparison, outcomes, study design)
Population | Healthy adults older than 60 years of age. | Mental, psychological, physical, or motor disorders that may influence postural control. |
Intervention | Intervention or comparison with a dual-task condition in static postural balance. | Interventions or comparisons that present dynamic postural balance. |
Comparison | The comparison will be between single-task and dual-task condition. | |
Outcomes | Dual-task conditions to study static postural control. | |
Study design | All quantitative, qualitative, and mixed-methods designs. | Reviews, meta-analysis, abstracts, citations, scientific conference abstracts, opinion pieces, books, book reviews, statements, letters, editorials, non-peer reviewed journal articles, and commentaries in the review. |
The included population comprised adults of both genders who were older than 60 years of age. Exclusion criteria were mental (i.e. neurodegenerative disease, intellectual disability, psychosis or obsessive-compulsive disorders), psychological (i.e. personality disorder, somatic symptom disorder, dementia or delirium), physical (i.e. muscular dystrophy, hypotonia, injury to tendons or ligaments), or motor disorders (i.e. akinesia, bradykinesia or dystonia) that may influence postural stability and not due to the physiological decline of advancing age.
Any type of intervention will be included if the protocol consists of the following: (1) the evaluation of static postural stability while performing a secondary task condition, (2) participants performed the postural stability task with parallel feet, (3) the test was performed on a firm surface because of a greater reliability of the data [
23], and (4) with eyes open to avoid the effect on centre of pressure measures that the removed visual feedback causes [
24]. Instead, all kinds of interventions that consist of the performance of a dynamic primary task condition will be excluded.
The comparison will be considered between postural stability measured in a single static task and during the DT condition.
The outcomes will summarize the use of DT conditions to study postural stability when older adults are standing in a static upright posture. Regarding the typology of data, there are different centre of pressure measures such as planar deviation (mm), anteroposterior range or mediolateral range (mm), mean distance (mm), root mean square distance (mm), confidence circle area (mm
2), confidence ellipse area (mm
2), mean velocity (mm/s), mean frequency (Hz), phase plane parameter, fractal dimension, critical time interval, mean squared critical displacement, diffusion constant, but only a few of them (root mean square distance, mean velocity, mean frequency, phase plane parameter, and fractal dimension) present acceptable levels of relative and absolute reliability [
25]. The major limitation is the different levels of reliability of the centre of pressure measures [
26] and also the wide spectrum of units of measure. For this reason, only the distance (centre of pressure sway) and area of sway will be taken into consideration.
For the study design, all quantitative, qualitative, and mixed-methods designs will be included. Reviews, meta-analysis, abstracts, citations, scientific conference abstracts, opinion pieces, books, book reviews, statements, letters, editorials, non-peer reviewed journal articles, and commentaries in the review will not be include in the systematic review and meta-analysis.
Study record
Data management
Manuscript collection will be performed by two authors (LP and ET) working independently. Afterward, the manuscripts selection will involve a two-step process: a first selection using the function “find duplicates” included in EndNote (EndNote version X8; Thompson Reuters, New York, USA) for a computer-automated selection process; in a second moment, a manual review will be done to identify appropriate manuscripts. The manual selection will involve two investigators (LP and ET) who will work independently in a three-stage process. In the first stage, the titles of the selected manuscripts will be examined and screened against the eligibility criteria. In the second stage, the abstracts will be examined to identify eligible manuscripts. In the third stage, the full texts of the articles that appear to meet the inclusion criteria will be reviewed. In the case of disagreements between the investigators, a third investigator will consider the works independently and discuss the decision with the other investigators (AB and AP). The investigators will not be blinded to the manuscripts, study title, authors, or associated institutions during the screening process. A PRISMA flow diagram will be presented in the final output to summarize the search and screening processes.
Selection process
Manuscripts written in English will be considered irrespective of the country of origin. The selection criteria will comprise two parts: acquisition of the manuscript and then comparison according to the inclusion and exclusion criteria.
Data collection process
The data will be collected according to the present standardized protocol process. The following information will be extracted from each included study and inserted into a Microsoft Excel (Microsoft Corp, Redmond, Washington) spreadsheet: lead author, year of publication, type of study (randomized, non-randomized, intervention, non-intervention), sample size, participants’ age (range and mean ± standard deviation), sex, cultural background, education and physical or sport activities practised, objective of the study, single task technique, DT technique, and influence of DT condition relative to the single task. The relevant information will be extracted from any section of the manuscript.
The data for the meta-analysis will be collected from the results section and tables of the manuscripts. Eventually, the graphs will be also used to extrapolate data using the program WebPlotDigitizer (version 4.2). If it will be impossible to collect the data from the manuscript, the corresponding author of the manuscript will be contacted twice before the exclusion of the study from the meta-analysis.
Risk of bias in individual studies
There are six domains for a potential risk of bias: selection bias, performance bias, detection bias, reporting bias, attrition bias, and unexplained bias [
27]. Unfortunately, neither the Cochrane Risk of Bias tools [
27] for randomized studies nor the ACROBAT-NRSI guidelines [
28] for non-randomized trials can be used as the authors of this systematic review expect to find, collect, and analyse both, randomized and non-randomized studies. Furthermore, intervention and non-intervention studies will be taken into consideration.
Regarding the reporting bias, two different investigators (LP and ET) will subjectively evaluate the results of the article took in examination and if there may be a lack of data, the main author of the article will be contacted to obtain information. In the absence of an answer, the article will be excluded. Since the present systematic review and meta-analysis will not evaluate the effect of an intervention, the dropout or the differences between the groups will not influence the results (attrition bias).
Regarding the meta-analysis, the risk of publication bias will be detected through a visual and a geometrical appreciation of the studies. Specifically, a funnel plot will be created to visually detect the existence of the file drawer problem, while the geometrical evaluation will be conducted through the Egger regression test [
29], assessing the asymmetry of the funnel plot.
Data synthesis
All the data will be collected and summarized descriptively with tables and graphs, and they will be analysed through a descriptive narrative synthesis to determine the influence of specific DT conditions on static postural stability.
To evaluate the influence of the DT conditions, a meta-analysis will be performed by an expert (AG) and an investigator (ET) will follow the process. The package metafor of the R software (version 3.5.1) will be used to calculate the mean effect size given by the subtraction of the DT from the single-task performance. For each study, all the variables will be treated as continuous and mean and standard deviation will be recorded, while for those studies revealing mean and standard deviations within plots the effect size will be calculated with WebPlotDigitizer (version 4.2).
The meta-analysis will be conducted in accordance with the Hedges and Olkin [
30] approach, an extension of Glass’ [
31] method, with a significance level of
α = .05. The heterogeneity across the studies will be assessed through the Cochrane’s Q and if the studies show high internal variability (thus,
p < 0.05), a subgroup analysis will be performed comparing the DT conditions grouped according to the classification used by Al-Yahya et al. [
16]: (1) manual task, (2) reaction time task, (3) discrimination and decision-making task, (4) mental tracking task, (5) verbal fluency task, (6) working memory groups, and (7) an additional category named “other” that will include different DT conditions that do not belong to the previously named groups.