Reliability of center of pressure excursion as a measure of postural control in bipedal stance of individuals with intellectual disability: A pilot study.

The high prevalence of postural instability in individuals with intellectual disability (ID) warrants the need for reliable and practical postural control assessments. Stabilometry is a postural control assessment that has been widely used for clinical populations. However, the scant systematic knowledge about the reliability of stabilometric protocols for adults with ID renders results questionable and limits its value for clinicians and researchers. The study's purpose was to develop a stabilometric protocol for adults with and without ID based on optimal combinations of shortest necessary trial durations and the least number of trial repetitions that guarantee sufficient reliability. Participants performed six trials of bipedal standing in 2 vision (eyes open vs eyes closed) x 2 surface (solid vs compliant) conditions on a force platform. Several parameters were calculated from the first 10-, 20-, and 30-s interval of every center-of-pressure (COP) trial data. For different trial durations, we identified the number of trials that yielded acceptable relative (intraclass correlation coefficient ≥ 0.70) and absolute (standard error of measurement < 20%) reliability using the Spearman-Brown prophecy formula. To determine the optimal combination of trial duration and number of repetition for each COP parameter, we implemented a two-step process: 1) identify the largest number of repetition for each of the three trial durations and then 2) select the trial duration with the lowest number of repetition. For both ID- and non-ID groups, we observed a trend whereby shorter trial durations required more repetitions and vice versa. The phase plane and ellipse area were the most and least reliable center-of-pressure parameter, respectively. To achieve acceptable reliability, four 30-s trials of each experimental condition appeared to be optimal for testing participants with and without ID alike. The results of this research can inform stabilometric test protocols of future postural control studies of adults with ID.

Introduction

Postural control is the ability to achieve, maintain and restore one’s center of mass (COM) within their base of support [1] and, as such, provides foundation for the successful and safe execution of sensorimotor tasks. This ability is the product of concerted activities of multiple body systems. Information about the body and the environment are continuously monitored by the central nervous system from visual, somatosensory and vestibular inputs. In response to postural instability, the central nervous system regulates the necessary voluntary and automatic adjustments in motor output to stabilize the body [2]. Therefore, the maintenance of postural control is contingent on intact neuromusculoskeletal functions and any disruption of the multiple systems that regulate and maintain postural control results in increased postural instability.

Intellectual disability (ID) is characterized by significant restrictions in both intellectual functioning (IQ ≤ 75) and adaptive behavior that have been evident before 18 years of age [3]. While intellectual impairment is the main feature of ID, motor impairments, specifically problems in postural control, are also commonly reported (e.g., [4, 5]). A systematic review by Enkelaar and colleagues [6] concluded that postural instability in individuals with ID is already evident in childhood and remains present during their entire lifespan. Furthermore, age-related decline in postural control manifests earlier in individuals with ID. As a result, adults with ID have a substantially higher incidence of falls and consequent hospitalization than the general population [7-9].

The prominence of postural instability among individuals with ID calls for its careful assessment to identify those who are at risk for falls. Moreover, reliable assessments are critical to monitor the effects of interventions designed to improve postural control. A common method for evaluating postural control is stabilometry, which is a recording of the center of pressure (COP) excursion during quiet upright stance using a force platform. COP excursion is used as a measure of postural control, as it has been demonstrated to approximate COM excursion (i.e., postural sway) under static conditions [10, 11]. However, the lack of standardization of the measurement procedure in stabilometry makes its use challenging and limits comparison between studies that have used different testing protocols [12]. This issue applies to clinical populations (including ID-individuals) as much as it does to the general population and has consequences on the reliability of results gained from stabilometry. Thus, our current study aimed to develop a reliable stabilometric protocol for evaluating postural control of adults with ID.

Ruhe, Fejer, and Walker’s [12] systematic review on the intrasession reliability of static bipedal stabilometry affirmed the lack of consistency in the methods used among postural control studies. They identified several variables, namely trial duration, number of trial repetitions, experimental conditions, and chosen COP parameters, that influence reliability of stabilometric results. Longer trial durations (60 s or more) and averaging more repetitions (3–5 trials) are generally recommended for increased reliability because of the inherently wide variability of COP measurements [13, 14]. Reliability is likewise affected by the experimental conditions, including visual and surface manipulations, implemented in stabilometric studies. Compared with eyes open (EO), eyes closed (EC) conditions generally yield higher reliability values in both healthy young [15] and older adults [16]. However, EC conditions require a longer trial duration because short trials, e.g., 10 s, do not provide sufficient time for adaptation to the destabilizing effect of the loss of visual input [14, 17]. As for experimental manipulations on surface conditions, standing on a compliant surface appears to be less reliable than standing on a rigid surface [12]. Nevertheless, the inclusion of a difficult condition (e.g., compliant surface) may be desirable because challenging conditions discriminate better between age- and pathology-related differences in postural control [18-21]. As to the difference between traditional COP parameters, measures of velocity and phase plane had the highest reliability while the ellipse area had the lowest [22, 23].

Given that reliabilities of COP measures vary depending on the participants’ characteristics like age and pathology [24], recommendations derived from the aforementioned studies may not be optimal or feasible for studies on a clinical population. For instance, the recommended 60-s trial may be too long for patients with postural instability to stand still. Studies on elderly individuals [23, 25] and patients with stroke [26] and Parkinson’s disease [27] reported adequate reliability of COP measures obtained from 30-s trials. These shorter trials could likewise benefit individuals with ID, as they may fatigue more easily [28] and have limited attention and motivation to complete tasks [29]. Although systematic knowledge about reliability and practicality of stabilometric protocols for adults with ID is scarce, several related studies have nonetheless been conducted (see S1 Table). These studies adopted different experimental conditions and widely varying testing specifications, ranging from 10–40 s of trial duration and 1–5 trial repetitions. There is further lack of consistency in sampling frequency of force plate data and COP-based stability parameters reported. Moreover, of these studies, only one paper published 26 years ago reported the reliability of COP measures in static bipedal stance in adults with ID. Suomi and Koceja [30] evaluated the postural control of 22 men with mild to moderate ID (Mage = 30.3, SDage = 5.5 years; MIQ = 57.8, SDIQ = 9.7) and a control group of 22 men and 22 women without ID. Participants performed three 15-s trials in EO and EC conditions. The authors found that intraclass correlation coefficient (ICC) values were higher in EC (ICC2,1 = 0.74–0.84) than in EO (ICC2,1 = 0.52–0.67) condition. Furthermore, ICC values for both conditions were lower for men with ID compared with the two control groups (men: ICC2,1 = 0.86–0.93; women: ICC2,1 = 0.67–0.90). The differences in reliability may contribute to the conflicting findings on the COP characteristics of individuals with ID in the literature. For instance, Suomi and Koceja [30] showed that the postural control of adults with ID is vision dominant (i.e., greater COP excursion increase from EO to EC conditions in the ID-group than in the non-ID-group) while Blomqvist et al. [4] and Dellavia et al. [31] provided data on the contrary.

In stabilometric studies comparing two groups (i.e., group of individuals with and without ID), it is necessary that the chosen procedure to assess postural control possesses similar reliabilities for either group. Shortened trial durations may make the testing feasible for individuals with ID. Likewise, including conditions that are more challenging may be better at detecting differences between individuals with and without ID. Reliability, however, should be as important a consideration as feasibility and discriminative power when deciding on assessments and procedures of its use. Thus, a good compromise of trial duration and repetition while testing postural control in varying degrees of difficulty is essential not only to achieve acceptable reliability for both groups but also to keep testing feasible for all participants, regardless of group membership. This pilot study’s primary purpose was to identify the shortest trial duration and the minimum number of repetitions that yield acceptable reliabilities of COP parameters in four (2 vision x 2 surface) conditions for adults with and without ID. We expected higher reliability to be obtained from longer trial durations and a greater number of trial repetitions. As a second goal, we aimed to determine effect sizes for the difference between the postural control of individuals with and without ID in order to inform sample size calculations of future studies.

Materials and methods

Participants

We recruited ten individuals into the study. Participants included five adults with a diagnosis of ID, who were identified through an assisted living facility for people with disabilities, and five age- and sex-matched controls without ID. All ID-participants have been diagnosed with ID by a physician. The following were the inclusion criteria: (1) age 18 to 40 years; (2) no reported neurologic, orthopedic, muscular, or cardiovascular symptoms or diagnosed medical condition; (3) no history of head or lower extremity injury in the past year; (4) ≥ 0.5 decimal visual acuity; and (5) abstinence from alcohol at least 24 hours prior to testing. We only included young adults to avoid introducing maturation and aging effects. Additionally, even though deterioration in postural control typically arises around the age of 60 [32], we chose a lower age limit of 40 years for two reasons. First, vestibular function has been shown to diminish after the age of 40 [33]. Second, based on the modelled lifespan trajectory of postural control ability of ID-individuals by Enkelaar et al. [6], ID-individuals are likely to experience age-related postural decline earlier than non-ID-individuals.

Study design and protocol were reviewed and approved by the UZ/KU Leuven Research Ethics Committee (B322201731833/S59931). Although the ability to make informed consent may be impaired in ID-individuals, Horner-Johnson and Bailey [34] documented that ID-participants were capable of giving their own consent to participate in low risk studies and offered suggestions to ensure that they understand every step of the consent process. Following these suggestions, we read the information and consent form aloud to them and asked after each section if they understood it, if they have questions about it, and if they agree to it. We also probed for understanding by having them express in their own words what the study is about, what voluntary means, what kind of risks they will be exposed to, and what is expected of them. A staff from their respective living facility was also present throughout the testing session. All participants provided their written informed consent prior to participation in the study, in accordance with the Helsinki Declaration.

Apparatus

We assessed postural control using a portable force platform (AccuSway, Advanced Mechanical Technology Inc. [AMTI], Watertown, MA, USA). Postural data were acquired and recorded using Balance Clinic software version 2.03.00 (AMTI) loaded on a Dell laptop. The acquisition sampling frequency was set at 100 Hz and was filtered using a fourth-order zero phase Butterworth low-pass filter with a cut-off frequency of 10 Hz [11, 12]. Four experimental conditions, with two vision by two surface conditions were included. Participants either stood directly on the force platform (solid surface) or on a 50 x 41 x 6 cm foam (Airex® Balance Pad, Airex AG, Sins, Switzerland) placed on top of the force platform (compliant surface). The foam pad has a density of 55 kg/cm3 and tensile strength of 260 kPa [35]. We tested the participants on both surface conditions with blindfolds (EC) or without (EO).

Experimental protocol

Before enrolling prospective participants into the study, we performed a phone screening where we asked participants and/or the staff nurse of the facility (in the case of ID-individuals) about demographic information, recent injuries and current health status, and regular physical activities. On the day of testing, we measured the participants’ height, weight, and foot length, and tested their visual acuity using a tumbling E chart. We also confirmed from self-report whether participants had consumed alcohol within 24 hours prior the test.

Each participant was tested in a single session, which lasted about 2 hours. We collected force plate data from two blocks of testing. Each block consisted of three consecutive 35-s trials in each condition, yielding 24 trials (3 trials x 4 conditions x 2 blocks). In the first block, the sequence was as follows: EO solid surface (EO-S), EC solid surface (EC-S), EO foam surface (EO-F), and EC foam surface (EC-F). This sequence was reversed in the second testing block. A 35-s trial duration was adopted after considering the available evidence that 30-s trials may be adequate for a clinical population, as well as to limit total testing duration. Test order was fixed and was identical for all participants. Participants sat for 1 minute on a chair to rest between trials. Between the two postural control blocks, we administered the performance subscale of the Wechsler Abbreviated Scale of Intelligence (WAIS) [36]. We tested the participants’ IQ to provide a measure to contrast intellectual function between the two groups and because we had no access to the ID-participants’ IQ scores (due to the facility’s privacy restrictions). Prior to the actual postural tests, we familiarized participants with the tasks during two 30-s warm-up trials for each condition.

During testing, participants stood barefoot at hip-width apart and with big toes pointing forward. After establishing proper foot positioning, we drew an outline of both feet to keep foot placement consistent across trials on both solid and compliant surfaces. Furthermore, intermalleolar distance was checked and maintained throughout all the trials. Before each trial, we instructed the participants to stand as still as possible with their arms to their sides. They were also instructed to look straight ahead at a 3-cm red circle located on the wall at eye level.

Data analysis

The initial and final 2.5 s of each of the 35-s COP time series were considered padding points to minimize amplitude distortion from data filtering and were excluded from further analysis [37]. We used four traditional COP parameters: mean COP velocity and displacement (anteroposterior [AP] and mediolateral [ML] directions), 95% confidence ellipse area, and phase plane. Phase plane is a stability parameter that incorporates the position and velocity of COP and was computed based on the combined stability parameter described by Riley, Benda, Gill-Body, and Krebs [38]. The three other COP excursion parameters, amplitude (average displacement from the mean COP), velocity (total length of the COP path per unit time), and ellipse area (area of the ellipse that captures 95% of the data points), were based on formulae used by Prieto, Myklebust, Hoffman, Lovett, and Myklebust [39]. We calculated these COP parameters from the remaining 30 s COP time series data, as well as from the first 10 s and 20 s of the trial in order to explore whether it is possible to shorten trial duration further without sacrificing reliability. To determine within-session learning and fatigue effects, we compared the COP parameters from the first three trials in each condition with the last three trials using a paired t-test. No statistically significant difference emerged (α-level at 95%). Thus, we decided to pool all six trials for subsequent analyses.

Reliability estimates of COP parameters obtained at the level of single trials were computed using ICC2,1, a 2-way random effects model with an agreement coefficient [40]. This model of ICC compares within-subject variability with between-subject variability, considering random effects over time. Five participants and six trials provide 80% power to differentiate ICC values between 0.70 and 0.95 with type 1 error set at 0.05 [41]. Given the pilot nature of the study, we did not apply α-value correction for multiple testing. Because the ICC only measures relative reliability, we also calculated the standard error of the measurement (SEM) as an estimate of absolute reliability [42]. We computed SEM, expressed in percentage, by dividing the product of the standard deviation (SD) and square root of (1 –ICC) by the mean and multiplying it by 100.

As a next step, we implemented the statistical method described by Lafond and colleagues [22]. For each trial duration (i.e., 10, 20, and 30 s), we used the Spearman-Brown prophecy formula (1) to identify the number of trials (k) that, when averaged, yielded an acceptable relative (ICC ≥ 0.70 [43]) and absolute (SEM ≤ 20 [23]) reliability estimate. We also computed the reliability coefficient of the average of increasing k repeated trials (R) with a single-measure reliability coefficient R, i.e., ICC2,1 using the same formula.

To determine the optimal trial duration and repetition for each COP parameter, we implemented a two-step process. First, for each of the three trial durations, we identified the largest k value from all four postural control conditions of both groups. Then, we selected the trial duration with the lowest k value. We chose to do it in this manner because, ideally, the chosen COP parameter should possess acceptable reliability for all the experimental conditions and for both groups.

Lastly, we performed independent t-tests between the two groups, using the COP parameters with acceptable reliability as the dependent variable. From the t-statistic and the sample sizes of the ID- (n1) and non-ID- (n2) group, we calculated Cohen’s d: Due to the study’s small sample size, we used Hedge’s g as the corrected effect size [44]. Using the calculated effect size, a post hoc sample size calculation was performed in G*Power version 3.1.9.4 for Windows [45]. All other statistical analyses were performed with Statistical Package for the Social Sciences (SPSS), PC Statistical Package version 25.0 for Windows (SPSS, Inc., Chicago, IL, USA).

Results

Description of the participants’ characteristics are summarized in Table 1. None of the ID-participants had Down syndrome. All participants were able to complete the testing protocol. During testing, we documented practical testing issues that are worth reporting. We found it challenging to standardize the foot position of the participants with ID when we resumed testing after every rest break. They had problems placing their feet within the foot tracings or kept moving their feet after it had already been correctly positioned. Difficulties standardizing foot placement were even greater for the standing trials on foam. During the 35-s duration of the actual standing trials, however, we did not encounter any problems. Considering these problems and the fact that all participants could easily remain standing for 35 s, fewer trial repetitions of longer trial durations appeared to be more convenient than more repetitions of shorter trial durations.

Table 1

Participant characteristics.

Characteristics¤	ID group (n = 5)¤	Non-ID group (n = 5)¤
M Age (years)¤	29.20 (7.92)¤	29.00 (7.62)¤
Sex (female / male)¤	1 / 4¤	1 / 4¤
M Foot length (cm)¤	26.50 (1.38)¤	26.44 (1.40)¤
M Performance IQ¤	75.80 (10.0)¤	115.60 (7.9)¤
M BMI (kg/m2)¤	25.98 (4.3)¤	22.25 (2.1)¤
M Weekly PA (hours)¤	2.90 (2.1)¤	3.05 (1.1)¤

Figures enclosed in parentheses are standard deviations; ID, intellectual disability; IQ, intelligence quotient; BMI, body mass index; PA, physical activity

The k repetitions necessary to obtain an ICC ≥ 0.70 and SEM ≤ 20 for every trial duration, as well as the corresponding ICC2, and SEM are listed in Table 2. In general, we observed a trend of decreasing k with longer trial durations. Thus, shorter trial durations had to be performed with more repetitions while longer trial durations required fewer repetitions to achieve the same level of reliability. This is true for both groups.

Table 2

Number of trials (k) that would yield an ICC2,k ≥ 0.70 and SEM% ≤ 20 and corresponding ICC2,k and SEM% for trial lengths of 10, 20, and 30 s.

	Participants with intellectual disability¤									Participants without intellectual disability¤
Center of pressure parameters¤	10-s trial¤			20-s trial¤			30-s trial¤			10-s trial¤			20-s trial¤			30-s trial¤
	k¤	ICC¤	SEM¤	k¤	ICC¤	SEM¤	k¤	ICC¤	SEM¤	k¤	ICC¤	SEM¤	k¤	ICC¤	SEM¤	k¤	ICC¤	SEM¤
M amplitude ML¤	¤	¤	¤	¤	¤	¤	¤	¤	¤	¤	¤	¤	¤	¤	¤	¤	¤	¤
→EO-S¤	3¤	0.83¤	19.9¤	3¤	0.86¤	18.7¤	3¤	0.91¤	19.2¤	9¤	0.86¤	19.5¤	4¤	0.83¤	19.4¤	6¤	0.82¤	19.4¤
→EC-S¤	13¤	0.90¤	19.4¤	12¤	0.90¤	19.9¤	6¤	0.90¤	18.8¤	3¤	0.77¤	19.7¤	2¤	0.81¤	16.2¤	3¤	0.82¤	17.3¤
→EO-F¤	6¤	0.71¤	19.4¤	4¤	0.75¤	16.4¤	3¤	0.75¤	15.2¤	15¤	0.81¤	19.5¤	4¤	0.74¤	16.8¤	3¤	0.76¤	17.7¤
→EC-F¤	4¤	0.73¤	14.7¤	3¤	0.75¤	12.7¤	4¤	0.75¤	15.3¤	1¤	0.76¤	17.6¤	1¤	0.77¤	16.1¤	1¤	0.80¤	14.8¤
M amplitude AP¤	¤	¤	¤	¤	¤	¤	¤	¤	¤	¤	¤	¤	¤	¤	¤	¤	¤	¤
→EO-S¤	14¤	0.71¤	16.6¤	5¤	0.72¤	14.9¤	3¤	0.70¤	16.2¤	16¤	0.86¤	19.5¤	5¤	0.83¤	18.9¤	4¤	0.80¤	19.0¤
→EC-S*¤	—¤	—¤	—¤	15¤	0.70¤	16.5¤	12¤	0.72¤	14.4¤	—¤	—¤	—¤	—¤	—¤	—¤	—¤	—¤	—¤
→EO-F¤	2¤	0.75¤	16.2¤	2¤	0.75¤	16.2¤	1¤	0.76¤	14.9¤	20¤	0.84¤	19.9¤	9¤	0.72¤	19.2¤	5¤	0.74¤	19.7¤
→EC-F¤	5¤	0.71¤	17.3¤	2¤	0.74¤	16.2¤	2¤	0.78¤	14.7¤	3¤	0.73¤	18.0¤	2¤	0.80¤	13.7¤	3¤	0.75¤	14.7¤
M velocity ML¤	¤	¤	¤	¤	¤	¤	¤	¤	¤	¤	¤	¤	¤	¤	¤	¤	¤	¤
→EO-S¤	2¤	0.80¤	16.1¤	2¤	0.79¤	15.2¤	2¤	0.80¤	12.9¤	2¤	0.72¤	16.9¤	2¤	0.82¤	13.0¤	1¤	0.72¤	14.1¤
→EC-S¤	28¤	0.89¤	19.8¤	7¤	0.85¤	19.7¤	3¤	0.83¤	18.5¤	3¤	0.77¤	14.4¤	2¤	0.70¤	13.9¤	2¤	0.77¤	11.7¤
→EO-F¤	32¤	0.70¤	18.4¤	4¤	0.71¤	12.2¤	3¤	0.72¤	9.8¤	4¤	0.74¤	16.9¤	3¤	0.76¤	16.5¤	2¤	0.72¤	16.5¤
→EC-F¤	7¤	0.73¤	13.7¤	2¤	0.73¤	10.3¤	4¤	0.71¤	13.1¤	2¤	0.71¤	19.6¤	2¤	0.79¤	16.3¤	1¤	0.78¤	15.6¤
M velocity AP¤	¤	¤	¤	¤	¤	¤	¤	¤	¤	¤	¤	¤	¤	¤	¤	¤	¤	¤
→EO-S¤	2¤	0.82¤	11.6¤	2¤	0.82¤	10.9¤	1¤	0.79¤	12.2¤	1¤	0.83¤	14.4¤	1¤	0.80¤	14.6¤	1¤	0.82¤	13.2¤
→EC-S¤	7¤	0.81¤	19.3¤	3¤	0.79¤	18.1¤	2¤	0.84¤	15.9¤	6¤	0.73¤	17.1¤	2¤	0.78¤	14.7¤	1¤	0.73¤	14.9¤
→EO-F¤	2¤	0.73¤	13.2¤	3¤	0.76¤	12.0¤	2¤	0.71¤	13.4¤	3¤	0.77¤	16.1¤	1¤	0.70¤	17.4¤	1¤	0.74¤	15.0¤
→EC-F¤	3¤	0.76¤	11.3¤	1¤	0.74¤	10.3¤	1¤	0.78¤	9.7¤	2¤	0.75¤	16.0¤	2¤	0.79¤	14.7¤	1¤	0.75¤	15.1¤
Ellipse area¤	¤	¤	¤	¤	¤	¤	¤	¤	¤	¤	¤	¤	¤	¤	¤	¤	¤	¤
→EO-S¤	8¤	0.89¤	19.0¤	10¤	0.94¤	19.2¤	5¤	0.93¤	19.8¤	25¤	0.94¤	19.8¤	11¤	0.94¤	19.1¤	12¤	0.93¤	19.4¤
→EC-S¤	216¤	0.98¤	19.9¤	59¤	0.97¤	19.8¤	15¤	0.95¤	19.7¤	219¤	0.89¤	19.9¤	3¤	0.83¤	18.7¤	5¤	0.82¤	19.9¤
→EO-F¤	10¤	0.94¤	19.1¤	3¤	0.90¤	18.2¤	3¤	0.89¤	18.7¤	35¤	0.94¤	19.9¤	4¤	0.82¤	18.2¤	2¤	0.91¤	15.0¤
→EC-F¤	38¤	0.88¤	19.8¤	3¤	0.80¤	19.1¤	12¤	0.89¤	19.7¤	5¤	0.88¤	18.5¤	3¤	0.88¤	17.2¤	2¤	0.85¤	18.6¤
Phase plane¤	¤	¤	¤	¤	¤	¤	¤	¤	¤	¤	¤	¤	¤	¤	¤	¤	¤	¤
→EO-S¤	3¤	0.75¤	15.6¤	2¤	0.76¤	13.3¤	1¤	0.75¤	14.2¤	1¤	0.79¤	12.5¤	1¤	0.81¤	11.9¤	1¤	0.79¤	11.4¤
→EC-S¤	9¤	0.86¤	19.2¤	4¤	0.80¤	18.9¤	2¤	0.79¤	18.4¤	4¤	0.72¤	14.1¤	2¤	0.75¤	11.9¤	2¤	0.73¤	11.0¤
→EO-F¤	3¤	0.75¤	15.1¤	2¤	0.77¤	11.8¤	2¤	0.78¤	12.0¤	4¤	0.71¤	16.3¤	3¤	0.77¤	12.7¤	2¤	0.74¤	12.7¤
→EC-F¤	4¤	0.73¤	11.8¤	2¤	0.72¤	10.8¤	2¤	0.81¤	11.1¤	3¤	0.79¤	14.9¤	2¤	0.77¤	13.0¤	1¤	0.73¤	12.4¤

*k, ICC2, and SEM% cannot be calculated when ICC2,1 ≤ 0; ML, mediolateral; AP, anteroposterior; EO-S, eyes open, solid surface; EC-S, eyes closed, solid surface; EO-F, eyes open, foam surface; EC-F, eyes closed, foam surface

Ellipse area was the least reliable COP parameter as, even at the longest trial duration of 30 s, 15 repetitions were needed to meet our threshold for acceptable reliability. In contrast, the most consistently reliable COP parameter was the phase plane. Good reliability estimates were obtained with two 30-s trials. For mean ML and AP velocity, the most optimal combination was four 30-s trials, while six 30-s trials were necessary for mean amplitude in the ML and AP direction, excluding the mean AP amplitude in the EC-S condition. A value for k could not be calculated for mean AP amplitude in the EC-S condition because the ICC2,1 < 0 in all the trial durations for the non-ID-group. This can be indicative of poor reliability.

Table 3 compares the ID- and non-ID-group on COP parameters having acceptable reliability. Although we observed no significant differences (p < 0.05), phase plane in the EC-S condition demonstrated large effect sizes (Hedge’s g > 0.80) while phase plane in EC-F condition and mean AP velocity in the EC-S and EC-F conditions have effect sizes approaching .80. Using the most optimistic effect size of 0.90, a minimum sample size of 32 participants (16 per group) should have 80% power to detect differences in postural control between ID- and non-ID-group with α = 0.05.

Table 3

Means and standard deviations of center of pressure parameters of participants with and without intellectual disability (ID), as well as values for t-statistic, significance (p), and effect size (Hedge’s g).

	with ID¤		without ID¤
	M¤	SD¤	M¤	SD¤	t¤	p¤	Hedge’s g¤
M velocity ML (mm/s)¤	¤	¤	¤	¤	¤	¤	¤
→EO-S¤	4.6¤	1.2¤	4.8¤	1.2¤	-0.29¤	0.78¤	0.17¤
→EC-S¤	5.5¤	2.1¤	5.0¤	1.1¤	0.40¤	0.70¤	0.23¤
→EO-F¤	9.9¤	1.4¤	9.5¤	2.4¤	0.30¤	0.77¤	0.16¤
→EC-F¤	16.1¤	2.8¤	15.3¤	4.9¤	0.30¤	0.77¤	0.17¤
M velocity AP (mm/s)¤	¤	¤	¤	¤	¤	¤	¤
→EO-S¤	8.6¤	2.3¤	7.3¤	2.3¤	0.96¤	0.37¤	0.55¤
→EC-S¤	11.7¤	4.3¤	8.8¤	2.3¤	1.3¤	0.22¤	0.76¤
→EO-F¤	16.8¤	3.4¤	14.8¤	4.1¤	0.85¤	0.42¤	0.49¤
→EC-F¤	27.7¤	5.5¤	22.4¤	6.5¤	1.4¤	0.21¤	0.78¤
Phase plane (unitless)¤	¤	¤	¤	¤	¤	¤	¤
→EO-S¤	10.3¤	2.8¤	8.5¤	2.0¤	1.1¤	0.30¤	0.64¤
→EC-S¤	13.4¤	4.8¤	9.8¤	1.8¤	1.6¤	0.15¤	0.90¤
→EO-F¤	20.8¤	4.7¤	18.9¤	4.0¤	0.69¤	0.51¤	0.40¤
→EC-F¤	32.8¤	7.6¤	26.9¤	6.1¤	1.4¤	0.21¤	0.77¤

ML = mediolateral; AP = anteroposterior; EO-S = eyes open, solid surface; EC-S = eyes closed, solid surface; EO-F = eyes open, foam surface; EC-F = eyes closed, foam surface

Discussion

The purpose of this pilot study was to identify the optimal combination of trial durations and number of trials to meet threshold criteria for COP-parameter reliabilities of ICC ≥ 0.70 and SEM ≤ 20. For the two most reliable COP parameters (i.e., mean velocity and phase plane), four 30-s trials turned out to be the optimal combination to meet our criteria. This combination applies to both ID- and non-ID-group and across all postural control conditions. Shorter trial durations (e.g., 10 and 20 s) were less optimal. Any testing time savings gained from shorter trial durations would be more than outweighed by the number of trial repetitions necessary to achieve similar reliabilities as with our optimal combination.

From our results, each COP parameter required different numbers of repetitions reflecting the parameters’ reliabilities. For mean COP amplitude in the AP and ML directions to be reliable in all conditions, 30-s trials had to be repeated six times. These findings are at odds with the lone reliability study on COP measures in men with ID by Suomi and Koceja [30]. In their study, three 15-s trials were sufficient for the COP amplitude in the AP and ML directions to obtain the same reliability criteria that we set for this pilot study (note that only EO-S and EC-S conditions were tested in their study). Several explanations could account for this incongruent finding. Suomi and Koceja reported standard deviations of the COP amplitude, as opposed to mean COP amplitude that we described in our study. However, it is unlikely the sole reason for the discrepancy because existing studies (e.g., [23, 46]) have calculated very similar reliability coefficients between the mean and standard deviation of COP amplitude. Another possible explanation is related to the difference in sampling and cut-off frequency of the raw COP data. This pilot study used a sampling rate of 100 Hz with a 10 Hz cut-off while Suomi and Koceja’s study used a 50 Hz sampling frequency and a cut-off frequency of 5 Hz. In Ruhe and colleagues’ [12] systematic review on the reliability of stabilometry, they determined that cut-off and, to a lesser extent, sampling frequency significantly affects the reliability of COP data. Further, they recommended a sampling frequency of 100 Hz with a 10 Hz cut-off for traditional COP parameters like amplitude, velocity and ellipse area. We also found that mean AP amplitude in the EC-S condition had negative ICC values. While this suggests that this specific COP measure is not reliable, existing literature (e.g., [12]) does not support this. An alternative interpretation would be to consider this negative ICC as a spurious result arising from the low sample size of our study. We entertain this possibility considering that it significantly differed from the ICC values calculated in the other postural control conditions. Furthermore, previous studies on young adults with and without ID have reported higher reliability coefficients for AP amplitude in EC-S conditions [15, 30, 47].

Among the COP parameters we evaluated, ellipse area had the worst reliability for both the ID- and non-ID-group. Previous studies on healthy young and older adults [48] and young adults with musculoskeletal disorders [23] also found poor ICC values for ellipse area. The three 30-s trial protocol implemented in these studies was insufficient to yield ICCs ≥ 0.70 in EO-S and EC-S conditions. The inadequacy of three repetitions to achieve acceptable reliability on both EO-F and EC-F was also reported in a study on young gymnasts even when the trial duration was 120 s [49]. Our pilot study’s findings taken together with existing literature provide evidence against the use of ellipse area as a measure of postural control, especially for short trial durations.

In contrast, mean COP velocity and phase plane possessed the highest reliability, which agrees with multiple studies on clinical and non-clinical population without ID [12, 23, 25, 50]. Furthermore, we found that four 30-s trials yield reliable data on COP velocity and phase plane. Two studies involving healthy young adults produced conflicting results on the reliability of mean COP velocity in EC-S conditions. For two 30-s trials, Takala and colleagues [47] reported a reliability coefficient below our threshold (ICC = 0.46–0.54) while Pinsault and Vuillerme [51] obtained ICCs > 0.70. Our findings are in agreement with Pinsault and Vuillerme’s results, as we also found that two 30-s trials were adequate to obtain reliable data on mean COP velocity for our non-ID group. Note that we used the same form of reliability coefficient (i.e., ICC2,1) as did Pinsault and Vuillerme in their study. Takala et al. [47] did not explicate their version of ICC and it is possible that related differences can account for discrepancies in the findings. In our study, four trials were necessary for the ID-group to obtain the same level of reliability as the non-ID-group. We found no studies reporting reliability of mean COP velocity obtained from four 30-s trials in participants with ID. The greater number of repetitions needed for the ID-group may be related to the higher performance variability typically observed within this population [52, 53]. The problem of large performance variability of COP parameters causing lower ICC values in non-ID clinical populations has been documented by Harringe and colleagues [49].

For phase plane, two repetitions of 30-s trial were enough to reach acceptable reliabilities for the ID- and non-ID-group. Raymakers, Samson, and Verhaar [54] also demonstrated that two trials were adequate for reliable phase plane data in adults without ID. However, they used 50-s trials in the EO-S condition only and used the coefficient of variation as a reliability coefficient, which limits comparability between their study and ours. A study on young adults with anterior cruciate ligament injury by Hadian et al. [50] reported ICC values of the phase plane parameter in the EO-S, EC-S, and EC-F conditions. They showed that three 30-s trials were required for ICC ≥ 0.70 and SEM ≤ 20. This is one repetition more than what our findings suggested but the difference in study population may explain the dissimilarity.

Based on our findings, reliable COP measures could be obtained with trial durations even shorter than 30 s albeit at the expense of increased numbers of trial repetition. However, increasing the number of repetitions to compensate for shorter trials has its own shortcomings. First, the increased number of repetitions for shorter trials may lead to a longer active testing time (i.e., time when the force plate is recording the COP excursion) than longer trials and fewer repetitions. For instance, our results showed that mean COP velocity needed four, seven and 32 repetitions of 30-, 20- and 10-s trial durations, respectively. Computing for active testing time (4 x 30 = 120 s; 7 x 20 = 140 s; 32 x 10 = 320 s), we found that we could save the most time with the longest trial duration. This applies, as long as the longest trial duration is feasible for the participants. Second, more between-trial periods also added to passive testing time (i.e., time before and after trials). For our participants with ID, the passive testing time was often longer because standardizing their foot position and reminding them of the instruction (e.g., stand still, keep quiet, and maintain forward gaze) needed multiple repetitions. Lastly, shorter trial durations do not provide adequate time for the postural system to adapt to the postural control challenge, especially for more difficult standing conditions [17], and it is more likely to miss lower frequency component of the COP signal [14]. Therefore, a trial duration of 30 s appears adequate to yield reliable data of certain COP parameters. This is supported by studies on healthy young adults [17, 23], as well as on adults with ID [30] and other clinical population [25-27].

In sum, the lack of consistency in stabilometric test protocols and the lack of systematic reliability studies targeting assessments of postural control in individuals with ID may explain conflicting results in the literature. To our knowledge, our study is the first to use an appropriate statistical approach [22] to determine the optimal configuration of trial duration and repetition that yields acceptable relative and absolute intrasession reliability in adults with ID. It was found that four 30-s trials are adequate to achieve acceptable reliability of COP parameters, particularly mean COP velocity and phase plane. We have performed this analysis simultaneously on both clinical and non-clinical population (i.e. ID- and non-ID-group) to ensure that the trial duration and repetition combination provides reliable COP parameters in both groups. As to our second goal, the effect sizes from our current study informed us than no fewer than 32 participants (16 participants x 2 groups) are needed to have sufficient statistical power for the detection of postural control differences between adults with and without ID.

Our results provide an important first-step in the development of a stabilometric testing protocols that are reliable and feasible for future studies. As with the general population, establishing a standard procedure for stabilometry in the ID population could strengthen its potential to not only identify individuals with postural instability but also evaluate accurately the effectiveness of postural control interventions. Furthermore, unreliable outcome measures are unlikely to discriminate those with and without postural control issues and are likely to give incorrect information on effectiveness of intervention. Only through the standardization of stabilometry and identifying reliable COP parameters could we establish stabilometric reference data that are comparable between studies. Ultimately, this would help to complete our understanding of the postural control problems experienced by individuals with ID.

It is important to acknowledge that this pilot study’s findings must be interpreted with caution due to several limitations. The most important one is the small sample size, as well as representativeness of our sample. Our ID-participants were generally young, healthy, active, and high functioning individuals. Thus, applicability of our findings to older adults with more severe ID and comorbid health problems may be limited. In particular, individuals with more severe ID, who often have worse cognitive and motor skills and are more likely to have comorbid health conditions [3, 55], could have limited ability to complete stabilometric tests even for shorter durations and with fewer repetitions. Feasibility of stabilometry may be a greater concern than reliability in severe impairments [56]. The heterogeneity of sex within the groups may have introduced the increased variance in the COP measures. Lastly, we also limited our analysis to traditional linear COP parameters and did not include nonlinear parameters (e.g., sample entropy, fractal dimension, etc.). Keeping these shortcomings in mind, we consider the reported data preliminary and recommend a confirmatory reliability study with a larger sample size. Nevertheless, the findings of this pilot study could guide decisions on testing protocols for stabilometric studies among individuals with mild ID. This is particularly helpful given the limited applicability of the recommended optimal trial duration (i.e., 60 s) for reliable COP parameters used in stabilometry for adults with ID.

Summary of stabilometric studies in static bipedal stance on a sample that includes adolescents and adults with intellectual disability (ID).

(DOCX)

Click here for additional data file.

Trial data of the calculated COP parameters per trial duration, trial repetition number, and experimental condition.

(XLSX)

Click here for additional data file.

23 Jul 2020

PONE-D-20-19187

Reliability of center of pressure excursion as a measure of postural control in bipedal stance of individuals with intellectual disability: A pilot study

PLOS ONE

Dear Dr. Pineda,

Thank you for submitting your manuscript to PLOS ONE. After careful consideration, we feel that it has merit but does not fully meet PLOS ONE’s publication criteria as it currently stands. Therefore, we invite you to submit a revised version of the manuscript that addresses the points raised during the review process.

The comments from the two reviewers are straightforward and may be easily addressed. Please take a close look to the comments and address each comment in a response letter.

Please submit your revised manuscript by Sep 06 2020 11:59PM. If you will need more time than this to complete your revisions, please reply to this message or contact the journal office at plosone@plos.org. When you're ready to submit your revision, log on to https://www.editorialmanager.com/pone/ and select the 'Submissions Needing Revision' folder to locate your manuscript file.

Please include the following items when submitting your revised manuscript:

A rebuttal letter that responds to each point raised by the academic editor and reviewer(s). You should upload this letter as a separate file labeled 'Response to Reviewers'.

A marked-up copy of your manuscript that highlights changes made to the original version. You should upload this as a separate file labeled 'Revised Manuscript with Track Changes'.

An unmarked version of your revised paper without tracked changes. You should upload this as a separate file labeled 'Manuscript'.

If you would like to make changes to your financial disclosure, please include your updated statement in your cover letter. Guidelines for resubmitting your figure files are available below the reviewer comments at the end of this letter.

If applicable, we recommend that you deposit your laboratory protocols in protocols.io to enhance the reproducibility of your results. Protocols.io assigns your protocol its own identifier (DOI) so that it can be cited independently in the future. For instructions see: http://journals.plos.org/plosone/s/submission-guidelines#loc-laboratory-protocols

We look forward to receiving your revised manuscript.

Kind regards,

Robert Didden

Academic Editor

PLOS ONE

Journal requirements:

When submitting your revision, we need you to address these additional requirements.

1. Please ensure that your manuscript meets PLOS ONE's style requirements, including those for file naming. The PLOS ONE style templates can be found at

https://journals.plos.org/plosone/s/file?id=wjVg/PLOSOne_formatting_sample_main_body.pdf and

https://journals.plos.org/plosone/s/file?id=ba62/PLOSOne_formatting_sample_title_authors_affiliations.pdf

2. Please describe in your methods section how capacity to consent was determined for the participants in this study.

[Note: HTML markup is below. Please do not edit.]

Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

1. Is the manuscript technically sound, and do the data support the conclusions?

The manuscript must describe a technically sound piece of scientific research with data that supports the conclusions. Experiments must have been conducted rigorously, with appropriate controls, replication, and sample sizes. The conclusions must be drawn appropriately based on the data presented.

Reviewer #1: Yes

Reviewer #2: Yes

**********

2. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #1: Yes

Reviewer #2: Yes

**********

3. Have the authors made all data underlying the findings in their manuscript fully available?

The PLOS Data policy requires authors to make all data underlying the findings described in their manuscript fully available without restriction, with rare exception (please refer to the Data Availability Statement in the manuscript PDF file). The data should be provided as part of the manuscript or its supporting information, or deposited to a public repository. For example, in addition to summary statistics, the data points behind means, medians and variance measures should be available. If there are restrictions on publicly sharing data—e.g. participant privacy or use of data from a third party—those must be specified.

Reviewer #1: Yes

Reviewer #2: Yes

**********

4. Is the manuscript presented in an intelligible fashion and written in standard English?

PLOS ONE does not copyedit accepted manuscripts, so the language in submitted articles must be clear, correct, and unambiguous. Any typographical or grammatical errors should be corrected at revision, so please note any specific errors here.

Reviewer #1: Yes

Reviewer #2: Yes

**********

5. Review Comments to the Author

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #1: The authors of the article entitled “Reliability of center of pressure excursion as a measure of postural control in bipedal stance of individuals with intellectual disability: A pilot study” aimed to test reliability of different stabilometric protocols for individuals with intellectual disability (ID) based on optimal combinations of shortest necessary trial durations and the least number of trial repetitions that guarantee sufficient reliability. The authors found that to achieve acceptable reliability, four 30-s trials of each experimental condition appeared to be optimal for testing participants both individuals with and without ID.

The study topic is interesting and may contribute to improve postural control investigation accuracy in individuals with ID. In general, the study is well designed. However, the authors have to respond to some comments regarding the manuscript. Also, there are some sentences that require rewriting and some statements that need to be addressed and clarified.

Abstract:

Authors are encouraged to more describe the method in the abstract. If there was a word limit condition you can shorten the Background of the Abstract.

Introduction

I think that the use of tables to summarize literature is not appropriate in original article.

Line 64: Because this also has consequences on the reliability of results gained from stabilometry, our current study aimed to develop a reliable stabilometric protocol for evaluating postural control of adults with ID. This sentence needs to be reworded.

Line 73: This applies to individuals with ID who may fatigue more easily [15] and have limited attention and motivation to complete tasks [16]. This sentence needs to be reworded.

Line 92: please replace with by including

Method

Postural control may be influenced by sex. So, including both male and female could be the origin of high standard deviation.

Postural control may be influenced also by the foot size. Please provide information.

Furthermore, the groups were not BMI matched that could influence results.

For individuals with ID, authors must obtain written informed consent from parents or legal garden.

Having history of injury in the past 12 month requiring medical attention must be one of exclusion criteria in postural control exploration.

Line 144: Please replace was by were

Line 147: We had four experimental conditions, with two vision by two surface conditions. Please rephrase this sentence; use the passive form

Line 151-153: This detail must be placed in the “Participant” section. Furthermore, if I understood correctly, you realized the IQ assessment using the Wechsler Abbreviated Scale of Intelligence. So, you must present this in the experimental design as a part of your study.

Please specify what participants do during the one minute of rest (standing; seated).

Please specify if all participants have the same foot positioning on the platform and the foam surface.

Please specify the characteristics of foam surface

Results

Line 236: Please consider “Table 4 compares the ID- and the non-ID-group on COP parameters having acceptable reliability.

Tables

All tables must be at the same form with the table 3

Reviewer #2: Compliments to the authors for paying attention to this important topic. Balance issues are a large problem in people with ID, as well as suitable measuring instruments. This study into measures of postural control is of relevance for future studies into this topic.

Points below have to be addressed to allow the reader to better understand the choices made in the study, the procedures and some details.

Introduction

Overall, the introduction is quite long, and I think it is possible to reduce the length a bit and make it more to the point. For example, the parts about the parameters that influence reliability are informative, but because of the length deviate the attention from the relevance of this study for people with ID. I think making this part a bit more to the point will help in strengthening the introduction.

Some additional points for the introduction:

- Page 3, line 62-64: It is not clear from the text that the lack of standardization in measurement procedures is an issue in the general population or specifically in people with ID. By reading the rest of the introduction I assume both, but it is important to specify this, throughout the text.

- Page 3, line 73: based on what findings is a trial length of 30s specified for clinical populations?

- Table 1 can be omitted from the introduction. If this was a review providing a complete overview of the studies this would be relevant, however this overview is not complete as stated by the authors. It is sufficient to describe the differences seen in test protocols in people with ID (Sampling frequency, trial length, experimental conditions, outcome parameters) in the text.

Methods

- Page 8, line 129: Why was age 18-30 years a inclusion criteria? Why this focus on relatively young adults?

- Page 9, table 2: Table 2 should be presented and described in the result section. The IQ scores of the ID group are quite high, especially because an IQ of 70 (max 75) is indicative of a significant limitation in intellectual functioning. Does this group really represent a group with ID? How was the ID classified, and people with ID selected?

- Page 9, line 139: please add the reference number of the ethical committee to this protocol.

- Page 9, line 152-153: did you did an assessment with performance subscale of the Wechsler Scale, or was this score reported from for example the participant files?

- Page 9, apparatus and tests: Also report how the other data that was collected; age, sex, BMI, weekly PA hours. All collected data and outcome measures must be described. It will also be useful to describe the outcome measures of the stabilometry with an explanation of them. An average reader, who is not fully aware of these measurements will need an explanation of these outcome measures.

- Why were these specifications chosen for the measurement protocol (35s trials, 100 Hz sampling frequency)? From the introduction we have learned that a lot of different specifications are used, so please report why these were specifically chosen? Especially, why was the 35s trial chosen, since 60s trails were found to be more reliable? Because one of the aims is to assess which duration would be most suitable and reliable, I would expect that longer trials would have been included as well.

- Page 11, line 220: specify for what comparisons t-test were performed.

Results

- The total test duration was quite long, 2hrs. Did people with ID need more rest than the 1 min between trials?

Discussion

- How representative are these results for the group of people with ID. It is mentioned that results may not be applicable to people with more severe ID, however they may also not be applicable to older adults for the same reasons (worse motor control and skills, more cognitive problems), and perhaps those with moderate levels of ID, behavioral problems etc. The study sample seems to be quite a high functioning group of people with ID (IQ around 75 and quite active) which limits the representativeness to other groups of people with ID, especially since this sample only included 5 participants with ID. This limitation should be made more clear in the discussion.

**********

6. PLOS authors have the option to publish the peer review history of their article (what does this mean?). If published, this will include your full peer review and any attached files.

If you choose “no”, your identity will remain anonymous but your review may still be made public.

Do you want your identity to be public for this peer review? For information about this choice, including consent withdrawal, please see our Privacy Policy.

Reviewer #1: Yes: Borji Rihab

Reviewer #2: No

[NOTE: If reviewer comments were submitted as an attachment file, they will be attached to this email and accessible via the submission site. Please log into your account, locate the manuscript record, and check for the action link "View Attachments". If this link does not appear, there are no attachment files.]

While revising your submission, please upload your figure files to the Preflight Analysis and Conversion Engine (PACE) digital diagnostic tool, https://pacev2.apexcovantage.com/. PACE helps ensure that figures meet PLOS requirements. To use PACE, you must first register as a user. Registration is free. Then, login and navigate to the UPLOAD tab, where you will find detailed instructions on how to use the tool. If you encounter any issues or have any questions when using PACE, please email PLOS at figures@plos.org. Please note that Supporting Information files do not need this step.

28 Aug 2020

First of all, we would like to express our gratitude to the reviewers who have taken the time to review our manuscript for possible publication in PLOS ONE. We have carefully reviewed your comments and responded to them with the aim of improving our manuscript. To aid the reviewers, we prepared a matrix to organize our responses to the comments given by the reviewers. The full matrix can be perused in the attached response letter to the reviewers.

Both reviewers have provided great feedback to further sharpen our manuscript. In particular, we wanted to highlight in this response box some of those feedback. First, the Reviewer 1 (and the editor) has raised an excellent point about determining our ID-participants' ability to provide informed consent. We have discussed our decision not to require legal representation from our ID-participants with our institutional ethics review board and we have come to agree that it also important to allow our participants to exercise their personal autonomy. In the attached response letter to the reviewers (and the manuscript itself), we have detailed our strategy to ensure that our participants understand the details related to their participation of the study. Second, we have removed Table 1, which summarized the methodological differences between stabilometric studies in ID-samples. While agree with Reviewers 1 and 2 that it can be omitted, we still believe that it may be of value to readers who may be keen on reading more about the specifics of these study (even though it may not necessarily be a complete summary). Lastly, we have also updated the limitation section of our discussion in order to reflect limitations that we may have inadvertently glossed over.

Submitted filename: Response_to_Reviewers.docx

Click here for additional data file.

1 Oct 2020

Reliability of center of pressure excursion as a measure of postural control in bipedal stance of individuals with intellectual disability: A pilot study

PONE-D-20-19187R1

Dear Dr. Pineda,

We’re pleased to inform you that your manuscript has been judged scientifically suitable for publication and will be formally accepted for publication once it meets all outstanding technical requirements.

Within one week, you’ll receive an e-mail detailing the required amendments. When these have been addressed, you’ll receive a formal acceptance letter and your manuscript will be scheduled for publication.

An invoice for payment will follow shortly after the formal acceptance. To ensure an efficient process, please log into Editorial Manager at http://www.editorialmanager.com/pone/, click the 'Update My Information' link at the top of the page, and double check that your user information is up-to-date. If you have any billing related questions, please contact our Author Billing department directly at authorbilling@plos.org.

If your institution or institutions have a press office, please notify them about your upcoming paper to help maximize its impact. If they’ll be preparing press materials, please inform our press team as soon as possible -- no later than 48 hours after receiving the formal acceptance. Your manuscript will remain under strict press embargo until 2 pm Eastern Time on the date of publication. For more information, please contact onepress@plos.org.

Kind regards,

Robert Didden

Academic Editor

PLOS ONE

Additional Editor Comments (optional):

Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

1. If the authors have adequately addressed your comments raised in a previous round of review and you feel that this manuscript is now acceptable for publication, you may indicate that here to bypass the “Comments to the Author” section, enter your conflict of interest statement in the “Confidential to Editor” section, and submit your "Accept" recommendation.

Reviewer #1: All comments have been addressed

**********

2. Is the manuscript technically sound, and do the data support the conclusions?

The manuscript must describe a technically sound piece of scientific research with data that supports the conclusions. Experiments must have been conducted rigorously, with appropriate controls, replication, and sample sizes. The conclusions must be drawn appropriately based on the data presented.

Reviewer #1: Yes

**********

3. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #1: Yes

**********

4. Have the authors made all data underlying the findings in their manuscript fully available?

The PLOS Data policy requires authors to make all data underlying the findings described in their manuscript fully available without restriction, with rare exception (please refer to the Data Availability Statement in the manuscript PDF file). The data should be provided as part of the manuscript or its supporting information, or deposited to a public repository. For example, in addition to summary statistics, the data points behind means, medians and variance measures should be available. If there are restrictions on publicly sharing data—e.g. participant privacy or use of data from a third party—those must be specified.

Reviewer #1: Yes

**********

5. Is the manuscript presented in an intelligible fashion and written in standard English?

PLOS ONE does not copyedit accepted manuscripts, so the language in submitted articles must be clear, correct, and unambiguous. Any typographical or grammatical errors should be corrected at revision, so please note any specific errors here.

Reviewer #1: No

**********

6. Review Comments to the Author

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #1: The paper was significantly improved. The authors have adequately addressed all comments. I recommed publication.

**********

7. PLOS authors have the option to publish the peer review history of their article (what does this mean?). If published, this will include your full peer review and any attached files.

If you choose “no”, your identity will remain anonymous but your review may still be made public.

Do you want your identity to be public for this peer review? For information about this choice, including consent withdrawal, please see our Privacy Policy.

Reviewer #1: Yes: Borji Rihab

12 Oct 2020

PONE-D-20-19187R1

Reliability of center of pressure excursion as a measure of postural control in bipedal stance of individuals with intellectual disability:A pilot study

Dear Dr. Pineda:

I'm pleased to inform you that your manuscript has been deemed suitable for publication in PLOS ONE. Congratulations! Your manuscript is now with our production department.

If your institution or institutions have a press office, please let them know about your upcoming paper now to help maximize its impact. If they'll be preparing press materials, please inform our press team within the next 48 hours. Your manuscript will remain under strict press embargo until 2 pm Eastern Time on the date of publication. For more information please contact onepress@plos.org.

If we can help with anything else, please email us at plosone@plos.org.

Thank you for submitting your work to PLOS ONE and supporting open access.

Kind regards,

PLOS ONE Editorial Office Staff

on behalf of

Professor Robert Didden

Academic Editor

PLOS ONE