Postural control among older adults with fear of falling and chronic low back pain

Abstract

Objective

Altered Postural control could increase the risk of falling in older adults. Factors such as low back pain and fear of falling can be contributing factors to postural control instability. This study aimed to investigate the effect of chronic low back pain (CLBP) and fear of falling (FOF) on postural control of older adults.

Method

Forty-one older adults were included (27 LBP and 14 control). Among the participants, 22 people had high FOF, and 19 had low FOF based on Falls efficacy scale cut-off of ≥ 26. For postural control evaluation Center of pressure parameters (COP) of Standard deviation (Sd) of velocity, Sd of amplitude, path length and mean velocity in both Medial–lateral (ML) and Anterior–Posterior (AP) directions were measured. Mixed-model anova with two between group factor (Health status; with and without CLBP, and with high and low FES-I groups) and one within factor postural condition (four conditions with and without vision and Achill tendon vibration) was used.

Result

No significant interaction between groups (health status and FES-I) and group with condition (health status and condition or FES-I and condition) was observed for all COP parameters in both AP and ML direction. There was main effect of FES-I for all COP parameters in ML direction, with greater Sd of velocity, Sd of amplitude, path length and mean velocity in older adults with high FES-I compared to low FES-I in the ML direction.

Conclusion

High levels of FOF influenced static postural control in the ML direction. Therefore, paying attention to the lateral stability of older adults is of great importance.

Introduction

Falling is a significant health-related concern for older individuals (age > 60), as it can have a detrimental effect on their overall well-being, both physically and mentally [1, 2]. Recent research suggests that around 33% of the elderly population faces a noteworthy likelihood of experiencing falls [3, 4]. The occurrence of falls depends on extrinsic factors in the environment and intrinsic factors related to the physical capacities of the individual [5]. Intrinsic factors contributing to disturbing postural control include deterioration of the sensory-motor interaction systems, decline in visual, vestibular, and proprioceptive sensory systems [6], sarcopenia [7], and musculoskeletal pain [8] which can lead to postural instabilities in different contexts [9–11].

Among the older adults population, chronic low back pain (CLBP), is a common complication [12], which a recent meta-analysis of 35 studies indicated that between 21 and 75% of people 60 years of age or older experienced CLBP within the previous 12 months [13]. Recent studies indicate that CLBP negatively affects somatosensory processing, impaired lumbar proprioceptive acuity, diminished neuromuscular responses, and errors in trunk repositioning which may contribute to their postural instability and increase the risk of falling [14–16]. In addition, psychological factors, such as fear of pain and fear of falling (FOF) may adversely affect postural control [17, 18]. In this regard, a higher prevalence of FOF has been observed among older adults with chronic pain [19]. FOF can result in long-term physical inactivity, decreased self-confidence, and motor impairments among older adults, leading to significant negative impacts on their lives [20, 21].

Postural control is regulated through multisensory integration of proprioceptive, visual, and vestibular sensory inputs [22]. Any deterioration in the elements of this sensory mechanism or their interactions can disrupt postural control [23]. Older adults experience a natural decline in proprioceptive inputs, moreover with CLBP the absence of accurate lumbar sensory information, the dependency on ankle sensory inputs may increase [24]. Manipulating the sensory inputs can provide valuable information on postural control [25]. Hence, to address this issue, the proprioceptive information of muscles surrounding ankle joint could be disturbed by muscle–tendon vibration [26], or visual inputs can be disturbed. In addition, tendon vibration is a powerful somatosensory stimulus that triggers a kinaesthetic illusion of body movement in individuals standing upright. This phenomenon leads to a postural reaction referred to as vibratory-induced sense of falling [27]. Appling vibration, to the leg muscles during quiet standing, can induce postural instability, sometimes to the point of falling. This postural threat condition could evaluate postural responses during conditions with potentially sense of falls while standing in a static position [26].

Evaluating static postural control in older adults is important as it can aid in identifying potential balance issues and risk factors for falls [5, 28–30]. According to the inverted pendulum model of the body, Center of mass is regulate through movement of the center of pressure (COP) under the feet, while ankle plantar flexors/dorsiflexors torque controls the COP in the anterior–posterior (AP) direction [31], and hip adductor/abductor torque control the movements in the medial–lateral (ML) direction [32].

By applying vibration to the ankle muscles or tendons and limiting visual sensory input, researchers can investigate how the body compensates for the altered sensory feedback, which could happen in real life situations, providing valuable insight into evaluating postural control [33]. Understand the underlying mechanism that can lead to fall is of great importance for therapeutic interventions. Although, several studies have focused on postural control alterations in older adults with CLBP or FOF, to our knowledge no study has evaluated the accompanied effect of CLBP and FOF on postural control during of older adults during different postural conditions with manipulated sensory input. This question raises that under deprived visual and ankle sensory input how CLBP and FOF interact with postural control strategy.

Method

Participants

This cross-sectional study included 41 older adults (≥ 60 years old), and was carried out at the laboratory of Rehabilitation Science Research Center from October 2021 to February 2022. The inclusion criteria for the CLBP group were pain in the lumbar region between the 12th rib and the gluteal fold lasting more than 3 months, or recurrent LBP for at least two episodes of pain, lasting two consecutive days in the last year [34]. The inclusion criteria for the control group were no experience of pain in the low back region in the previous year or back pain lasting more than one week that required medical intervention in the previous year [35]. Both groups were recruited from the same setting. The exclusion criteria for both groups were: Back surgery, history of falling during the past year, consuming drugs that impact balance, cognitive problems (mini-mental state examination < 21) [36], spinal deformities, neurological conditions, severe labyrinthitis, and long-term respiratory or cardiovascular problems. Lower limb pain with average pain intensity score of VAS ≥ 2/10 during the past 12 week. Furthermore no participant were involved in any kind of routine exercise plan or rehabilitation therapy for the LBP prior to the testing session [37]. All patients were referred by a specialist physician and examined by a physiotherapist to meet all inclusion criteria.

Ethical approval

All participants were informed and signed a consent form before commencing the study. Also, ethical approval for the study was granted by the Human Research Ethics Committee of the Iran University of Medical Sciences (IR.IUMS.REC.1400.282).

Outcome measures

Fear of falling

The Falls Efficacy Scale International (FES-I), is a self-reported instrument that was employed to evaluate the patient's level of fear about falling during sixteen activities of daily living, ranging from easy to difficult. The score ranges between 1 and 64, where 1 represents no concern at all and 4 represents great concern. More FOF corresponds with a higher score. The valid and reliable Persian version of this questionnaire was used in this study [38].

VAS

One frequent instrument for assessing pain is the Visual Analogue Scale (VAS). With only one horizontal line measuring 100 mm, it is a basic scale. The extremes of the pain are represented by marked. The centimeters that separate the left end from the designated point correlate to the score. The range of potential values is 0 (no pain) to 10 (highest pain possible) [39].

Vibrator

A vibrator device consisting of a direct current (DC) motor, equipped with an eccentric on the shaft was designed for this study. Moreover, the motor was embedded within a plastic tube, which had a diameter measuring 2 cm and a length measuring 5 cm. The vibrators induce vibration at a frequency of 60 Hz, with an amplitude of 0.5 mm [24]. The attachment of the vibrators to the participants' Achilles tendons at the level of the ankle joint was accomplished through the utilization of Velcro straps [40]. Achillis tendon vibration has been used, to evaluate postural control strategy in older adults [41]. which stimulate muscle spindles around the ankles, and induce a maximal illusion of elongation of that muscle and sense of joint movement [27]. This would evoke postural response in the backward direction [42]. To ensure that the participants were accustomed to the vibration stimuli, the vibrators were applied for three separate trials, each lasting 30 s, before the commencement of the main tests.

Experimental assessment with stabilimeter (Wii balance board)

The COP was measured using a Wii balance board (WBB) (Wii Balance Board™, Nintendo Co., Inc., Kyoto, Japan, serial number BC431808347, measuring 32 cm × 52 cm × 5.5 cm)) The reliability and validity of this device for evaluating COP sway as an indicator of postural stability has been reported to be acceptable [43]. This is an inexpensive tool for research and clinical evaluation. The WBB consists of a rigid platform with four uni-axial vertical force transducers located at the corners of the board, one transducer per foot. Each transducer is a load cell consisting of a cantilevered metal bar with a strain gauge that converts applied force to a voltage that is digitized and transmitted wirelessly by electronics in the WBB. The pressure transducers (piezoelectric balancing sensors), monitoring the forces in vertical (Z-axis) and horizontal—in the AP (Y-axis) and ML (X-axis)—directions. Data from the balance board were collected with a 100 Hz sample frequency [44]. The data was transmitted via a cable connected to a computer, and custom program written in Matlab extracted the COP data. The COP parameters were calculated using Matlab software. COP sway magnitude was determined as the Sd of velocity (mm/s), Sd of amplitude (mm), mean velocity (mm/s) and Path length of the COP in the AP and ML directions. The reliability of these COP parameters have been evaluated for static postural control assessment of older adults [45].

Computation of balance parameters based on COP measures

The cop parameters were calculated according to the formula presented in Table 1:

Table 1.

COP y and COP x represent AP and ML direction, respectively

Parameter	Formula
Path length ML AP	${\text{Totex}}_{\mathit{ML}}={\sum }_{\text{n}=1}^{\text{n}=\text{N}-1}\sqrt{{\left({\text{Cop}}_{\text{y}},\left(\text{n},+,1\right),-,{\text{Cop}}_{\text{y}},(\text{n})\right)}^2}$ ${\text{Totex}}_{\mathit{ML}}={\sum }_{\text{n}=1}^{\text{n}=\text{N}-1}\sqrt{{\left({\text{Cop}}_{\text{x}},\left(\text{n},+,1\right),-,{\text{Cop}}_{\text{x}},(\text{n})\right)}^2}$
SD of amplitude ML AP	${\sigma }_{\mathit{ML}}=\sqrt{\frac{\sum (x_i-\overline{x}{)}^2}{N=1}}$ ${\sigma }_{\mathit{ML}}=\sqrt{\frac{\sum ({\text{y}}_{\text{i}}-\overline{\text{y}}{)}^2}{\text{N}=1}}$
SD of Velocity ML AP	${\sigma }_{\mathit{ML}}=\sqrt{\frac{\sum ({\text{v}}_{{\text{x}}_i}-\overline{\text{v}}{)}^2}{\text{N}=1}where{v}_{x_i}=\frac{x_{i+1}-x_i}{t_{i+1}-t_i}}$ ${\sigma }_{\mathit{ML}}=\sqrt{\frac{\sum ({\text{v}}_{{\text{y}}_i}-\overline{\text{v}}{)}^2}{\text{N}=1}where{v}_{y_i}=\frac{y_{i+1}-y_i}{t_{i+1}-t_i}}$
Mean Velocity ML AP	$\overline{v}=\frac{1}{T}{\sum }_1^T\sqrt{(x_{t+1}-x_t{)}^2}$ $\overline{v}=\frac{1}{T}{\sum }_1^T\sqrt{(y_{t+1}-y_t{)}^2}$

Mean velocity (mm/s): The mean velocity AP/ML is the average velocity of the COP in the AP/ML direction.

Path length (mm): The length of the total COP trajectory during the measurement.

Sd amplitude (mm): The standard deviation of amplitude refers to the variability in the position or displacement of the COP from its mean position.

Sd velocity (mm/s): The standard deviation of velocity refers to variability in the rate of movement (speed) of the COP over time.

Testing procedure

Demographic and characteristic variables were recorded before the study, in addition, participants were instructed to fill out the FES-I form before the postural control assessments. Older adults were classified into two groups of low and high FOF based on the cut-off score of ≥ 26 for the total score of FES-I [46]. The subject completed a sequence of four different postural task conditions in random order. Each condition was repeated three times, for 30-s, followed by a 60-s rest period. The average of three trials was used for final measurements. Subjects stood barefoot on the balance board, in an upright static position with arms resting at their sides. In eyes open condition individuals were asked to maintain their balance as steady as possible while visually focusing on a dot marked 1 m located directly in front of them. In the remaining trials, when a blindfold obscured their vision, the participants were instructed to maintain a straightforward stare. Each participant’s COP parameter was measured under four conditions including with and without vision and with and without Achilles tendon vibration.

Statistical analysis

To estimate the required sample size, a priori power analysis was conducted using G*Power version 3.1.9.2. A mixed ANOVA was entered with two independent groups (health status and FES-I level) and six repeated measures (Postural condition), power at 0.90, and an alpha level of 0.05. The power analysis yielded a sample size of 36 for a medium effect size (f = 0.20) with considering 15% attrition rate 41 older adults were included.

Data analysis was conducted using SPSS software, version 26. Shapiro‐Wilks test was used to evaluate the normality of the data. Normality data distribution was evaluated before analysis, as was the sphericity and homogeneity of variance. The average of 3 recordings of postural variables was used for further analysis. Three-way mixed-design ANOVA was conducted for each COP parameter separately, with two between-group measures health status (with and without CLBP) and FES-I level (high and low FES-I score) and one within-group variable (4 postural task conditions). Post-hoc Bonferroni corrections for multiple comparisons, where appropriate, were computed to determine whether differences (P < 0.05) existed for each dependent variable between groups. A measure of effect size for the statistically significant effects was obtained using partial Eta squared values (η_p ²).

Result

Data distribution was normal according to the normality test. Study participants comprised older adults with a mean age of 64.8 (4.3) years and BMI 26.3(3.2). Participants included were mainly women (63.5%). No significant difference was observed between age, gender, BMI between individuals with and without CLBP with high and low fear. Detailed characteristics of the whole sample and women/men are shown in Table 2..

Table 2.

The mean (Sd) and percent of the study variables.

		N (%)
The mean (Sd) and percent of the demographic characteristic
Sex	Male	15 (36.5%)
	Female	26 (63.5%)
Health status	CLBP	27(65.8%)
	NCLBP	14(34.2%)
FES-I	High FES-I	22(53.6%)
	Low FES-I	19(46.4%)
		Mean (Sd)
Age (year)	CLBP
	High FES-I	66.67(4.01)
	Low FES-I	62.17(2.78)
	NCLBP
	High FES-I	66.00(4.69)
	Low FES-I	64.57(5.22)
Age	Total participants	64.8(4.3)
	CLBP
Height (cm)	High FES-I	165.27(5.59)
	Low FES-I	166.92(5.55)
	NCLBP
	High FES-I	168.71(7.08)
	Low FES-I	168.86(7.84)
Weight (kg)	CLBP
	High FES-I	74.33(10.34)
	Low FES-I	75.25(7.783)
	NCLBP
	High FES-I	68.71(7.84)
	Low FES-I	64.57(5.25)
VAS (CLBP)	High FES-I	6.27(0.79)
	Low FES-I	5.42(1.08)
FES-I	CLBP
	High FES-I	37.80(5.784)
	Low FES-I	21.42(3.118)
	NCLBP
	High FES-I	28.29(1.380)
	Low FES-I	18.29(1.113)

Abbreviations: FES-I falls efficacy scale international, CLBP chronic low back pain, NCLBP non-chronic low back pain, VAS visual analogue scale

Interaction effects

Table 3 summarizes the results of the mixed ANOVAs for each COP parameters. No Significant interaction was obtained between health status and condition and FES-I and condition and health status and FES-I for all COP variables (Table 4). Table 3 shows the mean and Sd of the COP parameters for different testing conditions in the groups.

Table 3.

Mean (Sd) of COP parameters n the medial–lateral and anterior–posterior direction

Group
Condition	Parameter	Low FES-I/NCLBP	Low FES-I/ CLBP	High FES-I/ NCLBP	High FES-I/ CLBP
Eyes Open No vibration	Sd Amplitude (ML) Sd Amplitude (AP)	0.13(0.04) 0.32(0.15)	0.17(0.05) 0.33 (0.10)	0.24(0.09) 0.31 (0.09)	0.24(0.14) 0.37(0.10)
	Sd velocity (ML) Sd velocity (AP)	0.28 (0.07) 0.62(0.20)	0.31 (0.08) 0.59(0.11)	0.43 (0.19) 0.72(0.23)	0.40 (0.20) 0.71(0.18)
	Path length (ML) Path length (AP)	5.73(1.99) 13.42(4.41)	6.17(1.98) 12.94(2.41)	8.64(3.95) 16.11(5.50)	7.98(3.53) 15.79(4.09)
	Mean velocity (ML) Mean velocity (AP)	0.19(0.06) 0.44(0.14)	0.20(0.06) 0.44(0.07)	0.29(0.12) 0.58(0.16)	0.26(0.11) 0.52(0.13)
Eyes Open, Achilles Vibration	Sd Amplitude (ML) Sd Amplitude (AP)	0.17(0.05) 0.49(0.21)	0.21(0.09) 0.57(0.25)	0.26(0.13) 0.47(0.16)	0.25(0.09) 0.49(0.08)
	Sd velocity (ML) Sd velocity (AP)	0.38(0.04) 0.83(0.10)	0.43(0.15) 0.95(0.28)	0.53(0.15) 0.95(0.26)	0.51(0.18) 1.03(0.29)
	Path length (ML) Path length (AP)	8.51(1.18) 17.48(1.94)	8.98(3.30) 19.39(6.71)	11.30(3.36) 20.93(5.92)	10.77(3.85) 22.42(6.49)
	Mean velocity (ML) Mean velocity (AP)	0.28(0.03) 0.58(0.06)	0.33(0.11) 0.70(0.21)	0.38(0.10) 0.71(0.18)	0.35(0.12) 0.74(0.21)
Eyes Closed, No Vibration	Sd Amplitude (ML) Sd Amplitude (AP)	0.15(0.07) 0.45(0.14)	0.19(0.05) 0.45(0.11)	0.22(0.13) 0.38(0.12)	0.22(0.09) 0.41(0.12)
	Sd velocity (ML) Sd velocity (AP)	0.32(0.09) 0.84(0.24)	0.34(0.07) 0.95(0.22)	0.39(0.07) 0.94(0.27)	0.45(0.20) 1.00(0.35)
	Path length (ML) Path length (AP)	6.57(2.06) 19.79(6.00)	7.60(2.17) 21.41(5.52)	8.63(3.79) 19.84(6.55)	8.81(3.41) 22.20(7.36)
	Mean velocity (ML) Mean velocity (AP)	0.23(0.07) 0.65(0.20)	0.27(0.06) 0.72(0.16)	0.30(0.11) 0.73(0.27)	0.29(0.11) 0.74(0.24)
Eyes Closed, Achilles Vibration	Sd Amplitude (ML) Sd Amplitude (AP)	0.20(0.08) 0.58(0.16)	0.22(0.09) 0.66(0.18)	0.25(0.14) 0.57(0.15)	0.28(0.11) 0.61(0.18)
	Sd velocity (ML) Sd velocity (AP)	0.51(0.09) 1.30(0.21)	0.62(0.36) 1.66(0.58)	0.60(0.18) 1.41(0.31)	0.65(0.23) 1.72(0.51)
	Path length (ML) Path length (AP)	0.94(0.16) 28.22(4.94)	1.21(0.42) 37.31(13.18)	1.22(0.28) 31.76(7.28)	1.31(0.40) 39.46(12.06)
	Mean velocity (ML) Mean velocity (AP)	0.37(0.08) 0.94(0.16)	0.41(0.20) 1.21(0.42)	0.48(0.12) 1.22(0.28)	0.47(0.15) 1.31(0.40)

Abbreviations: Sd Standard deviation, AP Anterior–Posterior, ML medial–lateral, FES-I falls efficacy scale international, CLBP chronic low back pain, Sd Amplitude (mm), Sd velocity(mm/s), Mean velocity (mm/s), Path length (mm)

Table 4.

Main effects and interactions between independent factors (health status and FES-I) and postural control conditions for COP parameters

		Condition			Condition * Health status			Condition * FES			FES-I			Health status
		F	Sig	ηp2	F	Sig	ηp2	F	Sig	ηp2	F	Sig	ηp2	F	Sig	ηp2
Sd Amplitude (mm)	ML	3.19	0.02*	0.08	0.03	0.99	0.00	0.57	0.63	0.01	6.01	0.01*	0.14	0.62	0.43	0.01
	AP	29.13	0.00*	0.44	0.23	0.87	0.00	0.46	0.70	0.01	0.71	0.40	0.01	1.39	0.24	0.03
Sd Velocity (mm/s)	ML	18.48	0.00*	0.32	0.55	0.64	0.01	0.24	0.86	0.00	5.09	0.03*	0.12	0.59	0.44	0.01
	AP	63.00	0.00*	0.63	2.58	0.05	0.06	0.03	0.99	0.00	1.71	0.19	0.04	0.04	0.83	0.00
Path length (mm)	ML	36.02	0.00*	0.49	0.61	0.60	0.01	0.24	0.86	0.00	4.43	0.04*	0.10	0.23	0.63	0.00
	AP	70.39	0.00*	0.65	3.66	0.01	0.09	0.41	0.74	0.01	1.88	0.17	0.04	2.99	0.09	0.07
Mean velocity (mm/s)	ML	36.04	0.00*	0.49	0.10	0.95	0.00	0.40	0.74	0.01	5.16	0.02*	0.12	0.06	0.79	0.00
	AP	76.38	0.00*	0.67	1.87	0.13	0.04	0.88	0.45	0.02	3.67	0.06	0.09	1.36	0.25	0.03

Abbreviations: Sd Standard deviation, ML medial–lateral, AP anterior–posterior, FES-I falls efficacy scale international

^*Significant P value < 0.05

Main effect

A significant main effect of FES-I group was found in the ML direction for all COP parameters. Sd of velocity (F = 5.09, P < 0.001, ηp2 = 0.12), Sd of amplitude (F = 6.01, P < 0.001, ηp2 = 0.14), path length (F = 4.43, P < 0.001, ηp2 = 0.10) and mean velocity (F = 5.16, P < 0.001, ηp2 = 0.12), (Table 4). This can be interpreted that irrespective of postural test conditions and health status, the high FES-I group had higher path length, mean velocity, and Sd of velocity and Sd of amplitude in the ML direction compared to Low FES-I group (Fig. 1).

Fig. 1.

A Sd of velocity, B Sd of amplitude, C Path length, D Mean velocity. Abbreviations: Sd Standard deviation, FES-I falls efficacy scale international. Main effects of group and postural conditions were significant at P <0.05. Error bars are standard error of mean ±1

The main effect of the condition was significant for all COP parameters, for Sd velocity (F _ML = 18.48, P < 0.001, ηp2 = 0.32) (F _AP = 63.00, P =  < 0.001, ηp2 = 0.63), Sd amplitude (F _ML = 3.19, P =  < 0.001, ηp2ML = 0.08) (FAP = 29.13, P =  < 0.001, ηp2AP = 0.44), path length (F ML = 36.02, P =  < 0.001, ηp2ML = 0.49) (FAP = 70.39, P =  < 0.001, ηp2AP = 0.65), and mean velocity (F ML = 36.04, P =  < 0.001, ηp2ML = 0.49) (FAP = 76.38, P =  < 0.001, ηp2AP = 0.67) (Table 4). Results of the multiple comparisons indicated that COP measures increased when eyes closed compared to open. More over the COP measures were greater in the conditions with Achill vibration compared to without vibration. The highest COP sway was observed during the most difficult testing condition with eyes closed and Achilles tendon vibration compared to other conditions (Table 4). This means that mean velocity, path length and Sd of amplitude and Sd of velocity of all participants increased from eye open to eyes closed and from without vibration to Achilles tendon vibration conditions in both ML and AP direction (Fig. 1).

Discussion

The findings of this research revealed larger COP parameters in the ML direction of older adults with higher FOF, regardless of having CLBP or the postural testing condition. Researchers have attributed increased COP sway velocity, amplitude, and variability as an indicators of postural instability in older adults [45, 47]. These results suggest that both older adults with and without CLBP that had high FOF showed postural instability.

In addition, CLBP and FOF showed no interaction for COP parameters, we expected different postural control strategy in CLBP individuals with high FOF compared to other groups of older adults. This finding could be due to the different mechanisms that FOF and CLBP primarily affects balance. FOF impacts postural control through psychological and behavioral mechanisms, such as increased anxiety, cautiousness behavior and changes in attentional focus [48].On the other hand, CLBP may influences balance through altered neuromuscular activity and motor control of trunk muscles [49]. These different pathways may not significantly interact. Their combined effect on COP parameters may not always show a clear interaction due to these complex factors. The influence of FOF and CLBP on postural control strategy may vary greatly among individuals depending on factors like severity of pain, overall health condition and psychological state, and the specific postural task being tested. Since static standing is not a high threatful standing condition, this may make it difficult to observe a consistent interactive effect. More research on postural control of older adults is required to fully understand their relationship in different contexts.

Another notable finding of our study was the increased postural instability specifically in the ML direction among older adults with high FOF. It has been suggested that difficulties in effectively controlling postural stability in the lateral plane of motion are relevant to the issue of falling in older adults [28]. Previous research has indicated that COP displacement in the ML direction serves as a better indicator for identifying fall risk in older adults [28]. FOF can contribute to reduced confidence and coordination during movement. This may lead to exaggerated movements or excessive corrections during postural adjustments [50], resulting in greater displacement of the COP in both medial and lateral directions. The greater postural sway has been attributed to a higher risk of falling in older adults [47].

The observation of increased postural sway in the ML direction suggests that there may be direction-specific variations in the utilization of different sensory inputs for maintaining postural stability [51]. According to the inverted pendulum model, when the feet are positioned parallel to each other, postural sway in the AP direction is primarily influenced by somatosensory inputs from the ankles, resulting in a dominant ankle strategy. On the other hand, ML sway is controlled by the hip strategy [52]. The muscle tone of the hip abductors/adductors control COP sway in the ML direction and it is related to the selected base of support by the person [32]. The COP lateral displacement occurs with constant unloading of one limb and instantaneous loading of the others. In wider base of support the same percentage change in the loading of each limb would cause higher velocity and larger amplitude of COP displacement in the ML direction [32]. In the present study the instruction provided for patients was to stand in their comfortable foot position, therefore older adults with high FOF might have stood in a wider base of support compared to low FOF group. Previous studies have indicated that a decline in hip abductor muscle strength and endurance is associated with increased ML postural instability [53]. Nevertheless, it is of utmost significance to effectively address the issues about the regulation of ML stability as episodes of falling in the lateral direction are frequently observed among elderly individuals and are correlated with an increased likelihood of sustaining a hip fracture, as opposed to falls occurring in other directions [54, 55]. Consequently, the regulation of ML stability may be considered a crucial domain for implementation. Gadelha et al. showed older adults with severe sarcopenic presented higher FOF and they had greater COP sway, in the ML direction [56]. However, we did not evaluate the muscle strength or morphology in the present study. high FOF can lead to decline in physical activity and increase the rate of muscle atrophy and reduce joint flexibility, negatively impacting overall neuromotor function. These findings highlight the importance of addressing optimal lateral stability in older adults with high FOF [57].

Additionally, our study demonstrated that as the difficulty of the postural tasks increased (e.g., applying vibration, removing vision), there was an increase in COP sway in all individuals. This finding aligns with previous studies [24, 58, 59]. During conditions involving tendon vibration, which induces a sense of illusory movement, higher COP oscillations were observed. In such conditions, older adults may experience a heightened sense of postural perturbation and instability. As individuals perceive postural instability, muscular tension, and stiffness may increase, leading to greater effort required to maintain balance and increased COP displacement [33]. The results revealed no interaction between groups and testing conditions, this means that the postural control behavior of older adults with and without CLBP and FOF were not significantly different during the absence of vision or accurate ankle sensory information. Moreover, increased postural sway was observed in all individuals with challenging situations (eyes closed, Achilles tendon vibration), when dealing with inaccurate sensory inputs may be considered as an optimal response to gather accurate sensory information for maintaining standing balance [60]. Furthermore, the NCLBP in our study consisted older adults without any history of moderate or mild pain solely in the body region such as the neck and lower limb [61]. While, chronic pain in all parts of the body is prevalent among older adults and can impact their postural control [19, 62].

Limitation

First, the cross-sectional nature of the data collection means that causal relationships cannot be established; instead, we can only make inferences about associations FOF and postural control changes between parameters. Second, most adults in the control had at least pain and inflammation at least in another location of the body, this might have had a confounding effect on postural control evaluations. Third, only young older adults were included in the study which limits the generalizability of the study.

Conclusion

Older adults with FOF demonstrated increased postural sway in the ML direction, which reveals lateral instability during quiet static standing, regardless of postural task condition and having CLBP.

Abbreviations

COP

Center of Pressure

FOF

Fear of Falling

CLBP

Chronic Low Back Pain

VAS

Visual Analogue Scale

FES-I

Falls Efficacy Scale International

ABC

Activities-specific Balance Confidence Scale

DC

Direct current

WBB

Wii Balance Board

AP

Anterior–posterior

ML

Medial–lateral

Sd

Standard deviation

η_p²

Eta squared values

Authors' contributions

M.S: Conceptualization, Methodology, Writing- Original draft S.S : Supervision, Conceptualization, Application of statistical and Writing - Review & Editing: Y.S: study design, Software, Validation, Data Curation S.SA: Conceptualization, Methodology and Writing - Review & Editing.

Funding

No funding was received for conducting this study.

Data availability

The datasets used and/or analyzed during the current study are available from the corresponding author upon reasonable request.

Declarations

Ethics approval and consent to participate

Initially, the aim of the study was explained to the participants. They were insured that their information would remain confidential. They were also informed that participation is totally voluntary, and they have the right to withdraw from the study whenever they wish.

Informed consent was obtained from each participant prior to study participation. The study was approved by the Ethics Committee at the Iran University of Medical Sciences (IR.IUMS.REC.1400.282) and was carried out in accordance with the principles of the Declaration of Helsinki.

Consent for publication

Not applicable.

Competing interests

The authors declare no competing interests.