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Participants
This study was designed as a cross-sectional study. According to G. Power software version 3.1.0 (Franz Faul, University of Kiel, Germany), based using a Pearson statistical test and assuming a power of 0.80, an effect size of 0.7 [30], and two-tailed alpha level of 0.05, twenty recreational male athletes were required for this study (Age, 23.53 ± 2.23 years; Mass, 75.51 ± 10.98 kg; Height, 179.86 ± 8.92 cm) who were selected based on certain criteria. The faculty’s board of physical education and Sports Sciences invited people to join the study for one month starting from January 10, 2024. The inclusion criteria were to be physically active, between the ages of 18 and 25, and have a body mass index between 18 and 24. Participants were excluded if they had a musculoskeletal injury within the past two months or a lower-extremity injury within the past six months, had any neurological or pathological conditions, had a lower-limb surgery or fracture within the past year, or were unwilling to cooperate.
The ethical committee of Allameh Tabataba’I university obtained approval before the test, and all participants gave their consent in writing.
Procedures
In this study, participants were referred to the sports biomechanics laboratory on one occasion and completed a one-hour testing session. In order to prevent the influence of footwear differences, they were instructed to wear comfortable sports clothing without shoes. The evaluation focused on the participants’ ankle joint stiffness and landing stability, as measured by the DPSI and TTS indexes during the SLL task. Each participant performed the single-leg landing (SLL) on a force plate (60 cm × 50 cm, Kistler, 9260AA6, Switzerland) for two trials, with a one-minute rest interval between trials; the average of the two trials was used for analysis. Simultaneous recordings of kinematic data (120 Hz) and force plate data (1200 Hz) were collected during each trial to calculate the Ankle DF-stiffness. Prior to the test, participants were instructed to perform a 5-min warm-up consisting of general lower extremity stretching and weight-bearing exercises under the supervision of a corrective exercise expert.
The ankle dorsiflexion range of motion (Ankle DF-ROM) measurement
The weight-bearing lunge test was conducted to functionally assess the ankle dorsiflexion range of motion in a weight-bearing position (Fig. 1); First, the subject was instructed to assume a lunge position with the evaluated leg in front, one palm placed against the wall, and perpendicular to the floor. Next, they were asked to bring their knee closer to the wall without lifting their heel off the ground. If the knee did not touch the wall or if the heel lifted, the subject was directed to move their leg forward and repeat the test until their knee made contact with the wall without lifting the heel. Finally, the examiner measured the distance between the wall and the big toe using a tape measure. It is important to note that each centimeter was considered equivalent to 2° of ankle dorsiflexion range of motion [31].
The dynamic postural stability measurement based on DPSI and TTS indexes
The dynamic postural stability of the subjects was assessed during SLL task. The participants were instructed to stand with their feet shoulder-width apart on a 30-cm-high step, positioned at a distance equal to half their height from the force plate. They were then instructed to land on the force plate using an arm swing with their dominant leg while maintaining balance (Fig. 2); The dominant leg was defined as the preferred landing leg, typically chosen unconsciously.
The dynamic postural stability index (DPSI) and its directional components were calculated using a custom MATLAB script (v9.9.0, Natick, Massachusetts). This method assesses mean square deviations to evaluate fluctuations around zero in the ground reaction force (GRF) across three directions. The DPSI integrates elements from the APSI, MLSI, and VSI. The calculation of the Dynamic Postural Stability Index (DPSI) score along with its three directional components, was performed by analyzing the initial three seconds of the ground reaction force following the point of initial contact, defined as the moment when the vertical ground reaction force exceeded 5% of the participant’s body weight [32].
These ground reaction forces, measured in Newton units, were normalized with respect to the subject’s body weight. It is important to note that elevated values of stability indices and DPSI scores are indicative of poorer dynamic postural stability. Previous research endeavors have highlighted the robustness of this methodology, showing favorable test–retest reliability as evidenced by Intraclass Correlation Coefficients (ICCs) ranging between 0.86 and 0.90, and Standard Error of Measurement (SEM) values falling within the range of 0.028 to 0.06 [9, 33].
Also, the TTS was calculated from the time of initial landing contact until they stabilized within 5% of their bodyweight for 2 s. For instance, if a subject’s initial contact occurred at 1.7 s and stabilization to within 5% of their bodyweight occurred at 2.4 s, TTS of 0.7 s was recorded [34, 35].
The ankle dorsiflexion stiffness (Ankle DF-stiffness) Measurement
In general, stiffness is defined as the ratio of changes in force to changes in length. In the human body, it reflects the potential to resist deformation caused by the ground reaction force. In the present study, ankle joint stiffness was calculated using the torsional-spring model, which is the ratio of the peak sagittal plane joint moment (i.e., the joint rotatory force) to the peak sagittal plane joint angular displacement between the initial landing contact and maximum joint flexion during SLL task [36]: Kjoint (Nm/ θ) = ΔMjoint/ Δθjoint.
In the current formula, ΔM_joint represents the change in joint torque from the initial contact of the foot with the force plane to the maximum knee flexion, while Δθ_joint denotes the change in angular displacement of the joint between the initial landing contact and maximum knee flexion.
Statistical analysis
Based on the Shapiro–Wilk test, the data distribution was found to be non-normal. Consequently, a Spearman correlation coefficient statistical test was employed to assess the relationships among ankle DF-ROM, ankle DF-stiffness, DPSI, and TTS during the SLL task. All analyses were conducted using SPSS software Version 22.0 (Microsoft Corp., Redmond, WA), and the significance level was set at 0.05.
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