This study was designed as a controlled laboratory study with a pre-post intervention trial. According to G. Power software, with a power of 0.95, an effect size of 0.45, and an alpha level of 0.05, 30 subjects in age 18 to 25 years were selected in this study (G*Power, Franz Faul University of Kiel, Germany), which were divided into two groups: healthy (n = 15) and GVD (n = 15). Participants were invited to the present study through the board of the university and local sporting clubs. Inclusion criteria were: exercise at least three times a week and continuously, age between 18–25 years, body mass index (BMI) between 18 and 24, and the distance between the ankle medial malleolus should be more than 5 cm in the static position; evaluation by a corrective exercise specialist. Participants were excluded if they: had any musculoskeletal injury in the previous two months or lower-extremity injury in the previous six months, had a lower limb surgery or fractures within the past one year, had any neurological and pathological conditions, and inability to perform the exercise due to lack of understanding of the relevant movement. Before performing the test, all subjects provided informed consent. Ethical approval was granted by the ethical committee of the Allameh Tabataba’i University ethic board (IR.ATU.REC.1399.016).
In the present study, participants were referred to the laboratory once time and completed a 1-hour test session. They were asked to wear comfortable sports clothing without shoes aiming to prevent the influence of footwear differences. In general, Gmed, TFL, and QL activity were recorded while participants performing PD in three different positions of hip rotation (neutral, 15° internal, 20° external rotation) with and without isometric external load. In order to prevent possible fatigue, they were given rest for one minute between different positions of hip rotation and 4 minutes between different load conditions. It is noteworthy that prior to performing the PD, participants pedaled with a cycle ergometer at a self-selected speed and low resistance for 5 minutes as a warm-up. To determine the different positions of hip rotation, the plate was used to adjust these positions, and the neutral, 15° internal rotation (IR), and 20° external rotation (ER) were drawn (Figure 1). Then, the participants stood on the plate in a position so that the middle line passed posteriorly through the central calcaneus and anteriorly through the second toe. For hip rotation, the participants’ feet were placed on the angled lines as described above. Also, the EMG data were collected in the Gmed and TFL muscles of the dominant limb (support limb) and QL muscles of the opposite side.
To perform the PD task, the participants were asked to stand on their dominant leg on a 15-cm step, and lower the non-dominant heel toward the floor while maintaining the extension of both knees, then return the leg back to the first position (Figure 1). Also, they were asked to lightly touch their heel to the floor aiming to ensure standardization and consistency between them and to ensure that adequate depth of the exercise was achieved each time.
To create the isometric hip external rotation during PD, by a tensiometer cable connected to a dynamometer, a force in the back direction (posteriorly direction force) was applied to the outer part of the non-stance side of the pelvic (Figure 1). The amount of force applied is determined using a pilot test, such that the balance of individuals is not disturbed and does not change the standing position. Participants are given verbal feedback during the process to prevent compensatory movement patterns such as trunk lateral flexion and rotation (Schmitz., 2002). Also, a bar was placed horizontally in front of the body so that the bar passes over both anterior superior iliac. Participants were asked not to move their bodies away from the bar during the test.
Surface wireless EMG (Myon m320RX, Schwarzenberg, Switzerland) was used to quantify the TFL, Gmed, and QL activation. Raw EMG signals were recorded at the sampling frequency of 1000 Hz, full-wave rectified, and then data noise was filtered at the 20–490 Hz band-pass and smoothed by the symmetrical moving RMS filter. Muscles activity analyses were conducted for the pre and post-intervention during PD task in different positions of hip rotation. The EMG data were normalized to the maximal voluntary isometric contractions (MVIC). The mean activation for each muscle was divided by the corresponding MVIC, and the average EMG data during PD were expressed as a percentage of the MVIC. Eventually, the average of the 2 sets was used for statistical analysis. Also, all EMG data were processed using Matlab software (Mathworks, Natick, MA).
Based on SENIAM recommendation , the electrodes were placed in the direction of muscle fibers and on the dominant leg, which was defined as the preferred one for kicking a soccer ball . Before placing the electrodes, the skin surface is shaved, abraded, and cleaned with 75% alcohol to reduce skin resistance. MVIC were used as a standard protocol to normalize the EMG data. To perform MVIC, the standard manual muscle-test was used in specific positions for each muscle . The MVIC of each muscle were performed for three trials of five seconds with one minute of rest between repetitions. To measure the MVIC for the TFL, subjects lying on their side and the lower extremity were placed at 45 ° hip flexion and 30 ° hip abduction, and the knee was extended. A leather or nylon band was tied around the ankle, individuals were asked to apply a force diagonally between the sagittal and frontal plane at an angle of approximately 45. To measure the MVIC for the Gmed, the subjects are lying on his side, so that the dominant leg is upwards and the whole body is in one direction. The dominant hip is located without abduction/adduction and internal/external rotation. To better perform the move, it is best if the leg below is flexed. The investigator applied a downward force to the ankle While the subjects were asked to resist the applied force isometrically . To measure the MVIC for the QL, the subject lying on her side with an extension in the knee and places her hands on the opposite shoulder. A nylon or leather strap is tied around the ankle to prevent abduction and by placing a resistor on the person's shoulder, the subjects were asked to give lateral flexion to the trunk. for all muscles, verbal stimuli were given to maximize the motivational effect of subjects . The highest mean peak value of these three repetitions was recorded as the MVIC of each muscle from the relevant muscle manual test. Also to calculate the activity ratio of Gmed / TFL and Gmed / QL muscles, the normalized mean EMG of the Gmed was be divided by the normalized mean of the EMG of the TFL, and the normalized mean EMG of the Gmed was be divided by the normalized mean of the EMG of the QL respectively. A ratio greater than 1 indicates that the Gmed activity is greater than that of the TFL and QL activity.
Descriptive analysis (mean ± SD) was performed on all the variables. Given the normality of the data distribution based on the Shapiro–Wilk test, the one-way repeated measures ANOVA followed by post hoc Bonferroni test was used to compare the muscle activities in different positions of the hip rotation for each group separately. A paired t-test was used to compare the pre-post load intervention in each angle. Also, an independent t-test statistical test was used to compare the differences between the healthy and GVD groups.
Statistical analyses were performed using SPSS software Version 22 (Microsoft Corp., Redmond, WA) at the significant level of p≤0.05.