This study used a cross-sectional study design. The sample size of this study was calculated using G * Power (version 3.0.10) [16]. It was based on a two-way repeated-measures ANOVA with an f = 0.30 effect size derived from pre-experimental results. A minimum of 14 participants were needed to achieve 80% statistical power (α = 0.05). To account for potential drop-out and technical errors during the experiment, the study involved 18 FAI patients, which consisted of 8 males and 10 females.

This study only included participants who met the following criteria: (a) they reported experiencing ankle instability, loss of control, or leg weakness during certain activities; (b) their score on the Cumberland Ankle Instability Tool (CAIT) was 24 or lower [17, 18]. The CAIT can discriminate between subjects with and without functional ankle instability. The CAIT questionnaire contains 9 questions to evaluate the subjective perception of the ankle joint during different types of daily activities, such as walking, running, going up and down stairs, and jumping. The total score was 0 ~ 30, and 24 or below suggested an unstable ankle score; (c) they had suffered at least one severe ankle sprain with more than one day of limited mobility, as well as another sprain after the initial injury, with no recent acute sprain within the past three months; (d) they were between the ages of 18 and 25 yrs; and (e) their results from the anterior drawer test and talar tilt test were negative.

The following individuals were excluded from the study: (a) those who had previously undergone surgery or suffered a lower limb fracture; (b) those who had observable skeletal abnormalities in their foot or ankle; (c) those exhibiting mechanical instability or acute pathological symptoms in their lower extremities; and (d) those who had used or received interventional foot orthosis treatment. The participants’ characteristics are displayed in Table 1.

Table 1 Physical characteristics of the participants with FAI

The study was granted ethical approval by the Ethics Committee of Honghui Hospital Affiliated to Medical College of Xi’an Jiaotong University (NO. 202,207,008). All subjects in the study participated voluntarily and signed an informed consent form for the protocol.

Materials and instrumentations

To improve ankle support for participants and eliminate the potential influence of varying types of footwear on the results, this study utilized standardized footwear during testing. It has been demonstrated that athletic shoes with wide soles and sturdy wrapping surfaces enhance the stability of the human body [19]. With this in mind, we used standardized footwear (361° CO. Ltd., Xiamen, China) in the same design for both men and women (Fig. 1a) as the test shoes. The sole is broad, and the upper surface is made of lightweight woven material, providing strong wrapping around the ankle. The test socks were regular knitted cotton socks.

Fig. 1

a The running shoes and (b) the elastic ankle braces in this study

In this study, participants wore elastic cross-band ankle braces (Kante Haiyue, Yangzhou, China) (Fig. 1b). These braces are made of nylon and Spandex, featuring a winding cross-band pattern, and are equipped with Velcro for secure attachment at the end of the band. The brace tension can be easily adjusted, and the heel region is open.

Three types of insoles were evaluated: smooth insoles (control), insoles with 1-mm protrusions, and insoles with 2-mm protrusions. The three types of insoles had similar characteristics except for the varying heights of the protrusions. They were made from EVA material (with a shore value of C50, as displayed in Table 2). For the customization of the custom-made insoles, full-foot scanning was carried out using a 3D foot scanner (Upod-s, Carotec Ltd. Guangzhou, China) in a neutral, non-weight-bearing position. The insole design software Insole CAD (Version 5.4.0, Vismach Technology Ltd. Wuhan, China) was utilized to design full foot arch support orthopedic insoles according to each participant’s foot shape. Therefore, these customized insoles could fit well onto the plantar surface of the participants’ feet to provide appropriate arch and heel support. A 3D carving machine was then used to carve the insoles. Kinetic data during landing tasks were collected using the Kistler 3D force plate (9287B, Switzerland) with a sampling frequency of 1000 Hz.

Table 2 Characteristics of the insoles in this studyProcedures

During the experiment, the researchers provided the participants with a comprehensive explanation of all experimental protocols and safety measures. The participants wore experimental shoes and familiarized themselves with the test procedures. To ensure safety, all participants underwent three single-leg landing trials to confirm that they could complete the tasks safely. The test conditions consisted of six combinations (3 types of insoles × 2 types of ankle braces). The researchers randomized the test orders beforehand and then informed the participant of his/her specific test orders on the day. The participants were requested to perform three trials for each test condition during the experiment.

The dynamic stability of the participants was assessed using a single-leg landing task protocol [14, 20] (Fig. 2). The testing platform was positioned 20 cm above the ground and 5 cm away from the edge of the force plate. Participants were directed to stand on the raised platform with their arms on the waist and facing forward. Upon receiving the start signal, the participants dropped toward the center of the force plate. Participants were instructed to drop from the raised platform without an initial vertical velocity. This was visually checked to confirm that the participants did not jump up before leaving the raised platform. After landing, only one limb (the affected limb) supported their weight, while the nonaffected limb was lifted up in the air. Participants were instructed to maintain an upright posture with their eyes open and gaze forward, hands at their waist, and the nonaffected thigh and calf vertically aligned. We required the participants to maintain a single-leg stance for 15 s after landing because this duration has been used in previous studies [15], and it allows us to assess both the initial impact and postural recovery phases of landing. To minimize the effect of fatigue, participants were provided with a 2-minute interval between each landing trial. The researcher closely monitored the procedure to ensure the participant’s safety.

Fig. 2

Single-leg landing test without initial velocity

To ensure the validity of the test, the test was repeated if any of the following occurred during: (a) Hands were not naturally positioned at the waist; (b) The nonaffected limb (nonsupporting side) made contact with the ground; (c) The affected limb (supporting side) failed to maintain balance, resulting in the nonaffected limb touching the ground within 15 s of landing; (d) Initial velocity was present at the start of landing (by visual observation); or (e) landing with the nonaffected limb on the ground. In such cases, it was necessary to repeat the test to obtain valid data.

Participants were instructed to subjectively evaluate both stability and comfort of their current condition using two separate 100-mm visual analog scales (VAS) following the landing stability test. The left endpoint of the scale represented “extremely poor”, while the right endpoint represented “extremely good”. Participants were instructed to rate the stability and comfort of various shoe insoles and ankle brace conditions by marking a vertical line at the appropriate position on the scale. The distance from the left endpoint to the marked line represented the score of the respective condition.

Data processing

Several indices were calculated from the ground reaction force data to indicate dynamic postural stability using a custom-written program. Data during the initial 3 s after landing were analyzed. The indices calculated included the anterior-posterior stability index (APSI), mediolateral stability index (MLSI), and vertical stability index (VSI), which reflect the stability in the anterior-posterior, mediolateral and vertical directions, respectively. The dynamic postural stability index (DPSI) is an extensive evaluation computed by amalgamating the ground reaction force in all three directions, reflecting the comprehensive stability of the human body [21]. Higher APSI, MLSI, VSI and DPSI values correspond to decreased dynamic stability, whereas lower values indicate enhanced stability [22].

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(1)

$$\text\text\text\text=\sqrt^}}}Body\text$$

(2)

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(3)

$$\text\text\text\text=\sqrt^+\sum ^+\sum ^}}Body\,Weight$$

(4)

Statistical analysis

In this study, SPSS (version 21.0, SPSS, NY, USA) was utilized for data analysis. For hypothesis testing, a two-way repeated measures analysis of variance (ANOVA) was conducted to investigate the effects of textured insoles and ankle braces on the dependent variables. If the interaction between the two factors (presence/absence of ankle brace and textured insoles) was significant, a simple effect analysis was used to compare different insoles with and without wearing the ankle brace. Otherwise, the main effect analysis was performed to analyze the impact of the two factors on the dynamic stability of FAI patients [23]. A Tukey correction was used to adjust the significance level of post hoc tests, with a significance level of alpha set at 0.05. The effect size was calculated using partial eta-squared (η2p), which indicates the proportion of variance explained by each factor or interaction. where η2p ≥ 0.01 is a small effect, η2p ≥ 0.06 is a medium effect and η2p ≥ 0.14 is a large effect [24].