Bilateral symmetry of vertical time to stabilization in postural sway after double-leg landing in elite athletes with unilateral chronic ankle sprain

Participants

A total of 30 professional male athletes were assigned to three groups of soccer (n = 10, ages [mean ± SD] = 23.9 ± 2.33) and basketball (n = 10, ages [mean ± SD] = 24.1 ± 2.3) with unilateral CAI and healthy control (n = 10, ages [mean ± SD] = 24 ± 2.66). The sample size was calculated using G*Power at an effect size of 0.5, a test power of 0.8, and an α value of 0.05. This study was carried out according to the Declaration of Helsinki and approved by the local Ethics Committee of Bu Ali Sina University (code IR.BASU.REC.1399.007). Before experimental procedures began, all the participants reviewed and voluntarily signed an informed written consent form. This study was performed in the Sports Rehabilitation Laboratory of Bu Ali Sina University (Hamedan-Iran) from September 10, 2020 to December 20, 2020.

In our study, professional athletes working in Premier League soccer and basketball teams were selected on the basis of the inclusion and exclusion criteria recommended by the International Ankle Consortium [20]. Specifically, the inclusion criteria for soccer and basketball groups were a history of ankle sprains requiring two rounds of medical treatment or more, a feeling of fear over instability in ankle function, collapse while performing physical activities, gaining a Cumberland Ankle Instability Tool (CAIT) score ≤ 24, and a confirmed severity in the functional instability of the ankle, as determined via anterior drawer and talar tilt tests performed by an experienced physician [6]. Finally, the athletes recruited for participation should have had CAI on right foot only, with the affected leg as the dominant one. The dominant leg was identified by the athletes or determined through a test that compelled the dominant leg to hit a soccer ball [6]. The exclusion criteria were a history of lower limb surgery and a history of fractures and acute damage to the musculoskeletal structure or joints of the lower limbs (e.g., sprains and fractures) in the past 3 months leading to at least 1 day of physical activity loss [20].

The participants in control group (5 professional soccer athletes and 5 basketball players) were healthy with no history of ankle or knee injury in the past 12 months, as well as no history of surgery or fractures of the lower extremities; chronic diseases, such as patellofemoral pain syndrome [21]. The right leg was dominant in control group athlete (Fig. 1).

Fig. 1figure 1

Flowchart of the study design

Experimental procedure

The tests were performed at the Sports Rehabilitation Performance Laboratory. At the first session, the participants signed an informed written consent form and completed questionnaires related to their histories of injury and CAIT scores. Initially a 15 minutes warm-up exercises including 5 minute running on the treadmill at the self-selected speed and stretching exercise focusing on the lower extremity were performed [6, 16], after which the participants were asked to perform a double-leg landing movement on a platform from a step that was 30 cm high and positioned 15 cm from the center of a plantar pressure platform. They were instructed landing without jump while regaining postural stability in the shortest possible time and maintaining the posture with the least possible oscillations for 10 seconds. No instructions were given to the participants on how to land; they were directed only to place their hands on their hips. All landings were completed in a double-legged manner and barefoot (Fig. 2). To familiarize themselves with the correct movement, the participants were given the opportunity to practice the target task three times before the tests were commenced. Finally, three successful trials with a rest time of 30 seconds were recorded between each landing for each participant [11, 12].

Fig. 2figure 2

Phases of double-leg landing task from the step. a Standing on the step, b Preparing for landing, c Landing begins, d Flying in the air, e Landing

Instrumentation

A plantar pressure platform (FDM-S, Zebris Medical GmbH, Germany) composed of 2560 high-sensitivity sensors and having an acquisition frequency of 120 Hz was used to record the vTTS and CoP trajectories during the double-leg, landing task.

Data processing

The CoP oscillation parameters included the following items, which were measured 3 and 5 seconds after the completion of the target task:

CoP sway velocity (CoP SV) (displacement velocity of the CoP during trial).

CoP displacement in the mediolateral (ML) (Minor axis) and anteroposterior (AP) (Major axis) directions. (Fig. 3).

Fig. 3figure 3

Win FDM-S software, the image shows the average load distribution under the feet. The color scale on the left quantifies the load distribution. a The horizontal pink line is the connecting line of the three CoPs (Center of pressure). The middle cross is the center of pressure of the whole body. The ellipse around includes 95% of the CoPs. Minor axis (CoP displacement in mediolateral direction), Major axis (CoP displacement in anteroposterior direction)

The vTTS data was obtained in each test using WinFDM-S software (09.02.01 version). The trial data were then exported from the software, and the vTTS in each test was obtained using MATLAB (version R2010b, MathWorks, Natick, MA) following the procedure of Wright et al. (2016) with a slight difference [11]. Briefly, the data imported into MATLAB software were initially rectified and filtered with a 12 Hz second-order low-pass Butterworth filter. A normalized reference variable was calculated on the basis of the participants’ trial results; that is, the mean vGRF in the last 2 sec of each trial (8–10 seconds) was ascertained. Subsequently, three standard deviations from the mean for a range of normal variations were calculated for each participant in all the trials. An unbounded third ordinal polynomial with a rectified force of 10 seconds after landing was fitted for each participant and each trial. The vTTS was defined as the point at which the unbounded third ordinal polynomial exceeded the range of variation occurring in the first trial.

Statistical analysis

The normality of the data was obtained using the Shapiro-Wilk test. Multiple analysis of variance (MANOVA) was conducted to statistically analyse the CoP and vTTS which followed by Bonferroni’s post hoc test. The relationship between vTTS and CoP parameters was determined by using Pearson correlation coefficient. Interpreting of correlations’ results executed based on Evans’ classification (1996) which indicated the values of r respectively between 0.00–0.19 (very weak), 0.20–0.39 (weak), 0.40–0.59 (moderate), 0.60–0.79 (strong), and 0.80–1.00 as a very strong correlation [22].

An alpha of 0.05 was used for all statistical test. All the analyses were performed using the Statistical Package for the Social Sciences (Version 21; SPSS Inc., Chicago, IL). In the statistical analysis of vTTS, the right foot of the soccer and basketball groups (injured-dominant foot) was compared with the right foot of the control group (dominant foot). Also, the left foot was compared in three groups.

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